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Mendel Laws Of Inheritance

Mendel's Laws of Inheritance

Inheritance can be defined as the process of how a child receives genetic information from the parent. The whole process of heredity is dependent upon inheritance and it is the reason that the offsprings are similar to the parents. This simply means that due to inheritance, the members of the same family possess similar characteristics.

It was only during the mid 19th century that people started to understand inheritance in a proper way. This understanding of inheritance was made possible by a scientist named Gregor Mendel, who formulated certain laws to understand inheritance known as Mendel’s laws of inheritance.

Table of Contents

Mendel’s Laws of Inheritance

Why was pea plant selected for mendel’s experiments, mendel’s experiments, conclusions from mendel’s experiments, mendel’s laws, key points on mendel’s laws.

Between 1856-1863, Mendel conducted the hybridization experiments on the garden peas. During that period, he chose some distinct characteristics of the peas and conducted some cross-pollination/ artificial pollination on the pea lines that showed stable trait inheritance and underwent continuous self-pollination. Such pea lines are called true-breeding pea lines.

Also Refer: Mendel’s Laws of Inheritance: Mendel’s Contribution

He selected a pea plant for his experiments for the following reasons:

The pea plant can be easily grown and maintained.
They are naturally self-pollinating but can also be cross-pollinated.
It is an annual plant, therefore, many generations can be studied within a short period of time.
It has several contrasting characters.

Mendel conducted 2 main experiments to determine the laws of inheritance. These experiments were:

Monohybrid Cross

Dihybrid cross.

While experimenting, Mendel found that certain factors were always being transferred down to the offspring in a stable way. Those factors are now called genes i.e. genes can be called the units of inheritance.

Mendel experimented on a pea plant and considered 7 main contrasting traits in the plants. Then, he conducted both experiments to determine the inheritance laws. A brief explanation of the two experiments is given below.

In this experiment, Mendel took two pea plants of opposite traits (one short and one tall) and crossed them. He found the first generation offspring were tall and called it F1 progeny. Then he crossed F1 progeny and obtained both tall and short plants in the ratio 3:1. To know more about this experiment, visit Monohybrid Cross – Inheritance Of One Gene .

Mendel even conducted this experiment with other contrasting traits like green peas vs yellow peas, round vs wrinkled, etc. In all the cases, he found that the results were similar. From this, he formulated the laws of Segregation And Dominance .

In a dihybrid cross experiment, Mendel considered two traits, each having two alleles. He crossed wrinkled-green seed and round-yellow seeds and observed that all the first generation progeny (F1 progeny) were round-yellow. This meant that dominant traits were the round shape and yellow colour.

He then self-pollinated the F1 progeny and obtained 4 different traits: round-yellow, round-green, wrinkled-yellow, and wrinkled-green seeds in the ratio 9:3:3:1.

Check Dihybrid Cross and Inheritance of Two Genes to know more about this cross.

After conducting research for other traits, the results were found to be similar. From this experiment, Mendel formulated his second law of inheritance i.e. law of Independent Assortment.

The genetic makeup of the plant is known as the genotype. On the contrary, the physical appearance of the plant is known as phenotype.
The genes are transferred from parents to the offspring in pairs known as alleles.
During gametogenesis when the chromosomes are halved, there is a 50% chance of one of the two alleles to fuse with the allele of the gamete of the other parent.
When the alleles are the same, they are known as homozygous alleles and when the alleles are different they are known as heterozygous alleles.

Also Refer: Mendelian Genetics

The two experiments lead to the formulation of Mendel’s laws known as laws of inheritance which are:

Law of Dominance
Law of Segregation
Law of Independent Assortment

This is also called Mendel’s first law of inheritance. According to the law of dominance, hybrid offspring will only inherit the dominant trait in the phenotype. The alleles that are suppressed are called the recessive traits while the alleles that determine the trait are known as the dominant traits.

The law of segregation states that during the production of gametes, two copies of each hereditary factor segregate so that offspring acquire one factor from each parent. In other words, allele (alternative form of the gene) pairs segregate during the formation of gamete and re-unite randomly during fertilization. This is also known as Mendel’s third law of inheritance.

Also known as Mendel’s second law of inheritance, the law of independent assortment states that a pair of traits segregates independently of another pair during gamete formation. As the individual heredity factors assort independently, different traits get equal opportunity to occur together.

The law of inheritance was proposed by Gregor Mendel after conducting experiments on pea plants for seven years.
Mendel’s laws of inheritance include law of dominance, law of segregation and law of independent assortment.
The law of segregation states that every individual possesses two alleles and only one allele is passed on to the offspring.
The law of independent assortment states that the inheritance of one pair of genes is independent of inheritance of another pair.

Frequently Asked Questions

What are the three laws of inheritance proposed by mendel.

The three laws of inheritance proposed by Mendel include:

Which is the universally accepted law of inheritance?

Law of segregation is the universally accepted law of inheritance. It is the only law without any exceptions. It states that each trait consists of two alleles which segregate during the formation of gametes and one allele from each parent combines during fertilization.

Why is the law of segregation known as the law of purity of gametes?

The law of segregation is known as the law of purity of gametes because a gamete carries only a recessive or a dominant allele but not both the alleles.

Why was the pea plant used in Mendel’s experiments?

Mendel picked pea plants in his experiments because the pea plant has different observable traits. It can be grown easily in large numbers and its reproduction can be manipulated. Also, pea has both male and female reproductive organs, so they can self-pollinate as well as cross-pollinate.

What was the main aim of Mendel’s experiments?

The main aim of Mendel’s experiments was:

To determine whether the traits would always be recessive.
Whether traits affect each other as they are inherited.
Whether traits could be transformed by DNA.

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Mendel’s experiments.

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Mendel is known as the father of genetics because of his ground-breaking work on inheritance in pea plants 150 years ago.

Gregor Johann Mendel was a monk and teacher with interests in astronomy and plant breeding. He was born in 1822, and at 21, he joined a monastery in Brünn (now in the Czech Republic). The monastery had a botanical garden and library and was a centre for science, religion and culture . In 1856, Mendel began a series of experiments at the monastery to find out how traits are passed from generation to generation. At the time, it was thought that parents’ traits were blended together in their progeny .

Studying traits in peas

Mendel studied inheritance in peas ( Pisum sativum ). He chose peas because they had been used for similar studies, are easy to grow and can be sown each year. Pea flowers contain both male and female parts, called stamen and stigma , and usually self-pollinate. Self-pollination happens before the flowers open, so progeny are produced from a single plant.

Peas can also be cross-pollinated by hand, simply by opening the flower buds to remove their pollen-producing stamen (and prevent self-pollination) and dusting pollen from one plant onto the stigma of another.

Traits in pea plants

Mendel followed the inheritance of 7 traits in pea plants, and each trait had 2 forms. He identified pure-breeding pea plants that consistently showed 1 form of a trait after generations of self-pollination.

Mendel then crossed these pure-breeding lines of plants and recorded the traits of the hybrid progeny. He found that all of the first-generation (F1) hybrids looked like 1 of the parent plants. For example, all the progeny of a purple and white flower cross were purple (not pink, as blending would have predicted). However, when he allowed the hybrid plants to self-pollinate, the hidden traits would reappear in the second-generation (F2) hybrid plants.

Dominant and recessive traits

Mendel described each of the trait variants as dominant or recessive Dominant traits, like purple flower colour, appeared in the F1 hybrids, whereas recessive traits, like white flower colour, did not.

Mendel did thousands of cross-breeding experiments. His key finding was that there were 3 times as many dominant as recessive traits in F2 pea plants (3:1 ratio).

Traits are inherited independently

Mendel also experimented to see what would happen if plants with 2 or more pure-bred traits were cross-bred. He found that each trait was inherited independently of the other and produced its own 3:1 ratio. This is the principle of independent assortment.

Find out more about Mendel’s principles of inheritance .

The next generations

Mendel didn’t stop there – he continued to allow the peas to self-pollinate over several years whilst meticulously recording the characteristics of the progeny. He may have grown as many as 30,000 pea plants over 7 years.

Mendel’s findings were ignored

In 1866, Mendel published the paper Experiments in plant hybridisation ( Versuche über plflanzenhybriden ). In it, he proposed that heredity is the result of each parent passing along 1 factor for every trait. If the factor is dominant , it will be expressed in the progeny. If the factor is recessive, it will not show up but will continue to be passed along to the next generation. Each factor works independently from the others, and they do not blend.

The science community ignored the paper, possibly because it was ahead of the ideas of heredity and variation accepted at the time. In the early 1900s, 3 plant biologists finally acknowledged Mendel’s work. Unfortunately, Mendel was not around to receive the recognition as he had died in 1884.

Useful links

Download a translated version of Mendel’s 1866 paper Experiments in plant hybridisation from Electronic Scholarly Publishing.

This apple cross-pollination video shows scientists at Plant & Food Research cross-pollinating apple plants.

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Microbe Notes

Mendel’s 3 Laws (Segregation, Independent Assortment, Dominance)

In the 1860s, an Austrian monk named Gregor Mendel introduced a new theory of inheritance based on his experimental work with pea plants.
Mendel believed that heredity is the result of discrete units of inheritance, and every single unit (or gene) was independent in its actions in an individual’s genome.
According to this Mendelian concept, the inheritance of a trait depended on the passing-on of these units.
For any given trait, an individual inherits one gene from each parent so that the individual has a pairing of two genes. We now understand the alternate forms of these units as ‘alleles’.
If the two alleles that form the pair for a trait are identical, then the individual is said to be homozygous and if the two genes are different, then the individual is heterozygous for the trait.
The breeding experiments of the monk in the mid‐1800s laid the groundwork for the science of genetics.
He studied peas plant for 7 years and published his results in 1866 which was ignored until 1900 when three separate botanists, who also were theorizing about heredity in plants, independently cited the work.
In appreciation of his work he was considered as the “Father of Genetics”.
A new stream of genetics was established after his name as Mendelian genetics which involves the study of heredity of both qualitative (monogenic) and quantitative (polygenic) traits and the influence of environment on their expressions.
Mendelian inheritance while is a type of biological inheritance that follows the laws originally proposed by Gregor Mendel in 1865 and 1866 and re-discovered in 1900.

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Interesting Science Videos

Mendel’s Experiment

Mendel carried out breeding experiments in his monastery’s garden to test inheritance patterns. He selectively cross-bred common pea plants ( Pisum sativum ) with selected traits over several generations. After crossing two plants which differed in a single trait (tall stems vs. short stems, round peas vs. wrinkled peas, purple flowers vs. white flowers, etc), Mendel discovered that the next generation, the “F1” (first filial generation), was comprised entirely of individuals exhibiting only one of the traits. However, when this generation was interbred, its offspring, the “F2” (second filial generation), showed a 3:1 ratio- three individuals had the same trait as one parent and one individual had the other parent’s trait.

Mendel’s Laws

I. Mendel’s Law of Segregation of genes (the “First Law”)

Image Source: Encyclopædia Britannica .

The Law of Segregation states that every individual organism contains two alleles for each trait, and that these alleles segregate (separate) during meiosis such that each gamete contains only one of the alleles.
An offspring thus receives a pair of alleles for a trait by inheriting homologous chromosomes from the parent organisms: one allele for each trait from each parent.
Hence, according to the law, two members of a gene pair segregate from each other during meiosis; each gamete has an equal probability of obtaining either member of the gene.

II. Mendel’s Law of Independent Assortment (the “Second Law”)

Mendel’s second law. The law of independent assortment; unlinked or distantly linked segregating genes pairs behave independently.
The Law of Independent Assortment states that alleles for separate traits are passed independently of one another.
That is, the biological selection of an allele for one trait has nothing to do with the selection of an allele for any other trait.
Mendel found support for this law in his dihybrid cross experiments. In his monohybrid crosses, an idealized 3:1 ratio between dominant and recessive phenotypes resulted. In dihybrid crosses, however, he found a 9:3:3:1 ratios.
This shows that each of the two alleles is inherited independently from the other, with a 3:1 phenotypic ratio for each.

III. Mendel’s Law of Dominance (the “Third Law”)

The genotype of an individual is made up of the many alleles it possesses.
An individual’s physical appearance, or phenotype, is determined by its alleles as well as by its environment.
The presence of an allele does not mean that the trait will be expressed in the individual that possesses it.
If the two alleles of an inherited pair differ (the heterozygous condition), then one determines the organism’s appearance and is called the dominant allele; the other has no noticeable effect on the organism’s appearance and is called the recessive allele.
Thus, the dominant allele will hide the phenotypic effects of the recessive allele.
This is known as the Law of Dominance but it is not a transmission law: it concerns the expression of the genotype.
The upper case letters are used to represent dominant alleles whereas the lowercase letters are used to represent recessive alleles.
Verma, P. S., & Agrawal, V. K. (2006). Cell Biology, Genetics, Molecular Biology, Evolution & Ecology (1 ed.). S .Chand and company Ltd.
Gardner, E. J., Simmons, M. J., & Snustad, D. P. (1991). Principles of genetics. New York: J. Wiley.
https://www.cliffsnotes.com/study-guides/biology/plant-biology/genetics/mendelian-genetics
http://kmbiology.weebly.com/mendel-and-genetics—notes.html
http://knowgenetics.org/mendelian-genetics/
https://en.wikipedia.org/wiki/Mendelian_inheritance
https://www.acpsd.net/site/handlers/filedownload.ashx?moduleinstanceid=40851&dataid=33888&FileName=Mendelian%20Genetics.pdf

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Sagar Aryal

2 thoughts on “Mendel’s 3 Laws (Segregation, Independent Assortment, Dominance)”

Good to know when one works with plants like me.

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Mendel’s Laws of Inheritance | Mendel’s Experiments

Mendel’s law of inheritance states that offspring inherited from their parents that results in similar characteristics of parents and offspring. This law of inheritance depends upon three other laws including the law of dominance, the law of segregation, law of independent assortment. Gregor Mendel was an Austrian monk who conducted groundbreaking experiments on pea plants in the mid-1800s. Mendel’s experiments focused on the inheritance of certain traits, such as seed color, pod shape, and flower color, and he discovered that these traits are passed down predictably.

In this article, we will learn about Mendel’s Laws of Inheritance, the Characteristics of Mendel experiments, and the Conclusion of the experiments.

Table of Content

Mendel’s Law of Inheritance

What are mendel’s experiments, characteristics of mendel experiments , why was pea plant selected for mendel’s experiments, conclusion of mendel’s experiments, key points of mendel’s laws of inheritance, modern applications of mendel’s laws of inheritance.

Gregor Johann Mendel (1822-1884) was an Austrian scientist, teacher, and Augustinian prelate who lived in the 1800s. He was educated in a monastery and went on to study science and mathematics at the University of Vienna. Mendel blended his knowledge of science and mathematics and became the first one to keep count of individuals exhibiting a particular trait in each generation. This helped him to raise the laws of inheritance .

Inheritance is the process by which genetic information is transferred from the parent to the offspring. Inheritance is the main reason that family members possess the same characteristics. Mendel’s experiments focused on inheriting certain traits, such as seed color, pod shape, and flower color.

Mendel had given three laws of inheritance after observing his experiments. These are:

Law of Dominance
Law of Independent Assortment

Law of Segregation

1. law of dominance.

The law of dominance states that the expression of only one of the forms of the parental trait in the F1 hybrid. In heterozygous condition i.e. different alleles, the dominant allele get expressed. only one is dominant and will be expressed when two different alleles are present. F1 generation expresses dominant alleles. The suppressed allele is known as the recessive allele or trait.

TT × tt (parents) ——> Tt; F1 generation

Law of Independent Assortment

The law of independent assortment is also the second law of Mendels. It states that completely different pairs of alleles are passed on to the offspring independently of each other that is during gametes formation, two genes segregate independently of each other as well as of the other trait. The inheritance of one gene does not affect the inheritance of any other gene.

The law of segregation is the third law of Mendel. The law of segregation states that for any trait, each pair of alleles of a gene segregate, and one gene passes from each parent to an offspring. Two alleles do not mix when they come together in a hybrid pair and are independent of each other.

Related Articles: Mendel and the Principles of Inheritance Law of Segregation And Law of Dominance – Mendel’s Law

Mendel worked on inheritance. Inheritance is genetic qualities that transfer from parent to offspring. Mendel took pea plants with different characteristics example-tall/short plants, white/violet flowers, etc. A gene that expresses itself in the presence of its contrasting gene in a hybrid is termed a dominant gene. A recessive gene is that whose expression is suppressed in the presence of a dominant gene e.g. in a hybrid (Tt) tall plant, the t gene for dwarfness is recessive and T gene for tallness is dominant.

Filial generation – The generation of offspring is termed filial generation. First Filial generation (F1) – The first generation of offspring produced from the parent generation. Second Filial generation (F2) – The second generation of offspring.

Mendel explains the concept of dominant and recessive alleles. The following table shows each of the traits and which traits are dominant and which are recessive.

Also Read, Incomplete Dominance & Mendel’s Experiment

Mendel selected the pea plant (Pisum sativum) because of the following reasons:

Many varieties were available with observable alternative forms for a trait or characteristics.
Peas are normally self-pollinated; as their corolla completely encloses the reproductive organs until pollination is completed. But cross-pollination also be done.
Peas are easily available.
Peas have contrasting characters. The trait was seed color, pod color, pod shape, flower shape, the position of the flower, seed shape, and plant height.
Its life cycle was short and produced a large number of offspring.
The plant is grown easily annually plant and does not require care except at the time of pollination .

Monohybrid Cross

It is a cross in which only one character is considered at a time, e.g. in a cross between a tall and dwarf plant, the size of the stem is considered. Mendel made a cross between a pure tall (TT) and a pure dwarf (tt) pea plant. He obtained all tall (hybrid) plants in the F1 generation. On self, these plants produced tall and dwarf in the ratio 3:1 The genotypic ratio of 1:2:1 and the phenotypic ratio of 3:1 is termed the monohybrid ratio. It is a single cross between two organisms of a species that is made to study the inheritance of single pairs of genes or factors. Monohybrid cross helps to study the principle of dominance given by Mendel.

Dihybrid Cross

It is a cross between two individuals taking two contrasting traits at a time. It helps to study the inheritance of two pairs of alleles. The genotypic ratio in the F2 generation is 1:2:2:4:1:2:1:2:1 and the phenotypic ratio in the F2 generation is 9:3:3:1 This cross helps to study the principle of Independent assortment given by Mendel. For example – the cross between pea plants having yellow wrinkled seeds with those having green round seeds is a dihybrid cross .

After multiple crosses Mendel concludes the following points:

Genes are transferred from parent to new generation in pairs known as alleles.
The genetic composition is known as genotype and the physical appearance of any organism is known as phenotype .
Genes are independent at the time of segregation.
Genes have 2 pairs of alleles if both of them are the same known as homozygous and of a difference then alleles are called heterozygous alleles.

Also Read: Difference between Homozygous and Heterozygous

Mendel proposed 3 laws of inheritance after doing observation from its different crosses on Pea Plant.
Mendel’s third law i.e., the Law of Segregation states that at the time of gametogenesis, both copies of gametes segregate so that the offspring get one copy of each gene from both the parents.
Mendel’s Law of Independent Assortment states that at the time of gametes segregation, gametes segregate independently.

Below are the modern applications and examples of Mendel’s Laws of Inheritance: Farmers and breeders use Mendelian principles to selectively breed plants and animals with desired traits. This has led to the development of crops with improved yield, resistance to diseases, and other desirable characteristics.

Understanding Mendelian inheritance is crucial in medical genetics. It helps in predicting the likelihood of genetic disorders and diseases in individuals based on their family history. Genetic counseling often involves explaining Mendelian patterns to individuals or families at risk.
In genetic engineering, scientists manipulate genes to produce organisms with desired traits. Mendel’s laws guide the understanding of how genes segregate and assort, providing a basis for the design of genetically modified organisms (GMOs).
Mendelian principles are applied in pharmacogenetics, where researchers study how genetic variations influence an individual’s response to drugs. This information is used to tailor drug treatments based on a person’s genetic makeup.
Mendelian genetics is fundamental to the study of population genetics, which explores how gene frequencies change over time in populations. This has applications in evolutionary biology and understanding the genetic diversity within species.
Mendelian laws play a role in forensic genetics, where DNA analysis is used to identify individuals based on their genetic profiles. Understanding inheritance patterns is essential for interpreting genetic data in forensic investigations.
In the study of cancer genetics, Mendelian principles are used to understand the inheritance of genetic mutations that may predispose individuals to certain types of cancer. This knowledge informs cancer risk assessments and preventive measures.

FAQs on Mendel’s Laws of Inheritance

1. state the names of mendel’s laws of inheritance.

Law of Dominance Law of Independent Assortment Law of Segregation

1. How did Mendel control pollination in pea plants?

To avoid self-pollination, models remove the anthers of some plants and breed them by the pollens of their desired characters.

2. What are the three different Laws of Mendel?

Mendel proposed 3 laws based on his experiments: Law of Dominance Law of Segregation Law of Independent Assortment

3. Which is the universally accepted law of inheritance?

The law of Segregation is the universally accepted law. The law of Indepent assortment has a drawback i.e. crossing over.

4. Why Mendels chosse Pea Plant?

Mendel opted for pea plants in his experiments due to their beneficial traits, including a short life cycle, easy breeding, diverse traits for studying inheritance, and the ability to undergo both self-pollination and cross-pollination conduct controlled mating.

5. What is Mendel’s law of dominance?

Mendel’s law of dominance asserts that when an organism carries two different alleles for a particular trait (heterozygote), one allele will overshadow the expression of the other. Instead of a combined influence on the phenotype, only the dominant allele will manifest its characteristics.

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Linkage And Recombination - Principles Of Inheritance And Variation Class 12 NCERT CBSE Class 12- Principles Of Inheritance And Variation- Linkage And Recombination: Linkage and recombination are the phenomena that describe the inheritance of genes. Linkage and Recombination both are related to the genetic information inherited from parents to offspring. Linkage is the tendency of 6 min read
What is Polygenic Inheritance? Polygenic inheritance is a type of inheritance in which multiple genes control the phenotype of an organism. The phenotypes or traits can be height, skin color, the color of the eyes, etc. This type of inheritance is also known as quantitative inheritance or multifactorial inheritance. Such traits a 7 min read
Pleiotropy - An Overview and Related Disorders 'PLEIOTROPY' term refers to the phenomenon in which a single locus affects two or more apparently unrelated phenotypic traits and is frequently identified as a single mutation affecting two or more wild-type traits. Pleiotropy comes from the Greek words pleio, which means "many," and tropic, which m 5 min read
Sex Determination Genetic information is transferred to the offspring from their parents by the means of asexual or sexual reproduction. Transferring traits from parents to their offspring is called heredity and it is also known as biological inheritance or inheritance. Natural selection is the way in which the speci 7 min read
Mutation The human body might be visualized as a simple organism. But it is the combination of different complex processes. From the outside, a human body might resemble a very simple one. A body that has two arms, two legs & one head for monitoring purposes. But from the inside of the body, there are ma 15+ min read
Pedigree Analysis As a rule, including different plant and creature species, researchers utilize family investigation to break down the legacy of aggregates, or characteristics, utilizing mating tests called crosses. Mendel's analyses uncovered that the 'factors', what we know as qualities, are liable for the legacy 7 min read
Mendelian Disorder in Human Mendelian disorders are a form of genetic disorder that is caused by the inheritance of single or multiple mutant genes from one or both parents. The function of the mutant gene determines how severe a Mendelian disorder is. The condition is typically severe if the gene controls a crucial function. 6 min read
Chromosomal Disorders: Principles of Inheritance And Variation Class12 CBSE Class-12 Principles Of Inheritance And Variation - Chromosomal Disorders: The chromosomes are thread-like structures that are mainly present in the nucleus which carries the hereditary information of genes that are passed from the parents to the offspring. Due to some irregularities of cell div 5 min read

Chapter 5: Molecular Basis Of Inheritance

Evolution Notes for Class 12 Chapter 6 Evolutionary biology is the study of the evolutionary processes that produced the diversity of life on Earth. Earth came into existence sometime between 4 and 5 billion years ago. Life evolved on planet Earth about 3.5 billion years ago. Since then, approximately 15 million different species of orga 11 min read
Molecular Basis of Inheritance Notes Class 12 CBSE Class 12 Molecular Basis of Inheritance: Inheritance is transmitted by certain molecules that Mendel termed as ‘factors’, but their nature was discovered later with the development of various scientific techniques. The molecules which govern the inheritance are called genes and it is of two typ 15+ min read
DNA: Structure, Types, and Functions DNA structure is made of nucleotide base pairs (other than RNA). DNA is the hereditary material that is possessed by all the organisms found on the Earth except certain virus species. DNA functions involve the transfer of genetic information from generation to generation. The full form of DNA is Deo 11 min read
Packaging of DNA Helix: Histones & Importance DNA packaging refers to the process through which DNA molecules are tightly compacted into a smaller volume so that they can fit into the nucleus of a cell. DNA packaging is important because the length of DNA molecules is much greater than the size of the cell nucleus, and therefore, if the DNA wer 5 min read
Search For Genetic Material The search for genetic material has been important in understanding inheritance and evolution. Scientists have explored various models and experiments to identify the substance responsible for transmitting hereditary traits. From Griffith's transformation experiments to Avery, MacLeod, and McCarty's 5 min read
Difference Between DNA and RNA The difference Between DNA and RNA lies in their structure, function, and location within cells, with DNA typically double-stranded, storing genetic information in the nucleus, while RNA is generally single-stranded, involved in protein synthesis, and present in various cellular compartments. DNA (D 6 min read
RNA - Definition, Structure, Types and Functions RNA is a ribonucleic acid that helps in the synthesis of proteins in our body. This nucleic acid is responsible for the production of new cells in the human body. It is usually obtained from the DNA molecule. RNA resembles the same that of DNA, the only difference being that it has a single strand u 11 min read
DNA Replication DNA replication is a fundamental biological process by which a cell duplicates its entire DNA. DNA is a self-replicating structure and the replication is catalyzed by enzymes. Through DNA Replication, genetic information is passed on from one generation of cells to the next during cell division. It 8 min read
The Experimental Proof Of DNA Replication The process by which cells duplicate their genetic material during cell division—the replication of DNA—was still largely a mystery. This sparked a race to understand how DNA replication happens among several well-known experts. The experimental evidence of DNA replication, which showed that DNA rep 5 min read
Transcription of DNA Transcription of DNA is a cellular process where the genetic information encoded in DNA is converted into RNA. It initiates with RNA polymerase binding to the DNA at a specific promoter region. Then, the enzyme unwinds the DNA and synthesizes a complementary RNA strand by following the DNA template. 6 min read
Genetic Code - Molecular Basis of Inheritance CBSE Class12- Molecular Basis Of Inheritance- Genetic Code: The sequence of nucleotides in deoxyribonucleic acid and ribonucleic acid which determines the amino acids sequence of proteins is known as Genetic code. DNA consists of information for protein sequences. RNA consists of four nucleotides: a 5 min read
Genetic Code and Mutations Genetic code and mutations are important to understand and explain the central dogma of biology. The set of rules governing how DNA sequences are translated into proteins is the genetic code. The four nucleotide bases adenine (A), thymine (T), guanine (G), and cytosine (C), which are organized in pa 5 min read
tRNA - the Adapter Molecule tRNA is also known as transfer RNA is a subtype of RNA, tRNA help in the protein synthesis process. tRNA carries the amino acid to the ribosome, which is the molecular machine that assembles the protein, and ensures that the amino acid is incorporated into the growing protein chain in the correct or 5 min read
RNA Translation The Central Dogma, claims that once "information" has transferred into protein, it cannot be retrieved. In greater detail, information transmission from nucleic acid to the nucleic acid or nucleic acid to protein may be conceivable, but transfer from protein to protein or protein to nucleic acid is 15+ min read
Lac Operon Lac operon consists of the genes that are required for the metabolism of lactose in a bacterium E. coli and some other enteric bacteria. The name Lac operon actually stands for lactose operon. Lac operon works only when the nutrient source lacks glucose and has only lactose as it takes more steps to 7 min read
Human Genome Project Human Genome Project was the world’s largest collaborative biological project that gave us the ability to examine the full genetic manual for creating a human being in nature. HGP was international scientific research that mainly aims to determine the base pairs that make human DNA, as well as the i 9 min read
What is DNA Fingerprinting? DNA Fingerprinting is a technique used to identify individuals by analyzing their unique DNA patterns. Studying the DNA Fingerprinting steps and process helps in understanding genetic relationships, solving crimes, and identifying individuals based on their unique DNA profiles. In this article, we w 10 min read

Chapter 6: Evolution

Origin of Life The origin of life on earth is one of the mysteries to mankind. According to a common man, life is gifted by god whereas scientists believe that life has originated from non-living matter by natural means. This mystery of whether life originated from non-living matter was solved by scientists Pirie. 4 min read
Evolution Of Life Forms – A Theory Evolution is a process of gradual changes in the heritable characteristics of a biological population, over successive generations, over a long period. (Population: - It is a group of individuals of the same species who live in the same area and can interbreed) Theories of EvolutionTill now, several 5 min read
Understanding Adaptive Radiation: Evolutionary Diversification Explained Adaptive radiation is a phenomenon observed in evolutionary biology, that involves the rapid diversification of species into various forms to exploit new ecological niches. This process leads to the exposure of multiple species with distinct adaptations, enhancing their survival in diverse environme 4 min read
Hardy-Weinberg Principle A system of guidelines for genetic inheritance is known as mendelian inheritance. A monk by the name of Gregor Mendel made the initial discoveries of genetics in the 1850s, and his findings were first published in 1866. People have been aware of how qualities are passed on from parents to their offs 13 min read
Evolution Of Humans - History, Stages, Characteristics, FAQs Humans, or Homo sapiens, are a species of upright-walking beings known for their cultural diversity, inhabiting the Earth's surface. Believed to have originated in Africa around 315,000 years ago, human evolution is a complex process involving the development of traits such as bipedalism and languag 6 min read

Chapter 7: Human Health and Disease

NCERT Notes on Class 12 Biology Chapter 7 - Human Health and Disease NCERT Chapter 7 of Class 12 Notes on Human Health and Disease: According to the World Health Organisation, health can be defined as a state of complete physical, mental, and social well-being and not merely the absence of disease and infirmity. Good health has many benefits like it helps to keep us 15+ min read
Common Diseases In Humans Disease: - A disease is a physiological condition in which the human body fights against the external or internal causes of infection. On the basis of externally caused diseases, various examples are present, ranging from bacteria, viruses, protozoans, helminths, and many more. Pathogen: - The patho 5 min read
Immunity - Definition, Types and Vaccination Immunity is a defense mechanism of the body that is provided by the immune system and helps in fighting disease-causing organisms. There are two immunity types: innate and acquired immunity. Immunity-enhancing foods help boost the body's immune system Vaccination also enhances immunity by exposing t 11 min read
Innate And Acquired Immunity The immune system fights against germs and foreign substances on the skin, in the body's tissues, and in bodily fluids such as blood. The overall ability of the host to fight the disease-causing organisms conferred by the immune system is called Immunity. The immune system can be broadly categorized 5 min read
What are HIV and AIDS? AIDS (Acquired Immune Deficiency Syndrome) is a sexually transmitted disease caused by HIV (Human immunodeficiency virus). HIV HIV (human immunodeficiency infection) is an infection that assaults cells that assist the body with battling contamination, making an individual more powerless against diff 8 min read
Difference Between Vaccination And Immunization The difference between vaccination and immunization is that vaccination has to be given externally from outside the body. The vaccination process involves the introduction of a vaccine into the individual whereas immunization involves producing antibodies against vaccines containing weak pathogens. 5 min read
What is Cancer? Introduction, Types, Stages, Treatment Health is "a condition of total physical, mental, and social well-being and not only the absence of disease or disability," according to the World Health Organization. Over time, several definitions have been employed for various objectives. Healthy behaviors can be encouraged, such as regular exerc 14 min read
Alcohol and Drug Abuse Prevention Control As opposed to the normal thoughts pervasive in general society, substance use is very far-reaching. So is substance misuse. It's anything but a little issue, confined to the domain of the feeble and detestable. The utilization of medications rises above race, orientation, age, or financial status. T 10 min read
Adolescence and Drug Abuse Adolescence is a critical period of development during which individuals experience physical, emotional, and social changes. However, it is also a time when many young people may be exposed to drugs and alcohol, which can have serious and long-lasting effects on their health and development. In this 5 min read
Addiction And Dependence Human health refers to a person's overall well-being and physical and mental health. It includes various aspects such as physical, emotional, social, and mental well-being and can be influenced by factors such as genetics, lifestyle, environment, and access to medical care. This includes healthy beh 7 min read

Chapter 8: Microbes In Human Welfare

Microbes in Human Welfare Notes CBSE Class 12 Chapter 8 Microbes in Huaman Welfare: Microbes are the smallest living organisms that can only be seen under the microscope. Microbes are found everywhere. Examples- are air, water, soil, inside and outside the bodies of plants and animals, thermal vents (1000 degree Celsius), under th 6 min read
Microbes In Human Welfare Microbes are microscopic organisms, that can be classified under protozoa, bacteria, fungi, and microscopic plants viruses, viroid, and prions (proteinaceous infectious agents). They are present everywhere– in soil, water, and air, inside our bodies, animals, and plants. Not only in life forms, but 6 min read
Biofertilizers Biofertilizers are biologically active substances that help in enriching the soil's fertility. Biofertilizers are microbes or microbial products. It helps to reduce the use of chemical fertilizers. Reducing the use of chemical fertilizers from the environment biofertilizers helps to protect the ecos 8 min read

Chapter 9: Biotechnology _ Principles And Processes

NCERT Notes Biology Class 12 Chapter 9 Biotechnology: Principles and Processes NCERT CBSE Class 12th Science Notes Chapter 9 Biotechnology: Principles and Processes: Biotechnology Principles and Processes is an important part of Class 12 Science Notes for quick revision. They will benefit from having challenging study material to use in preparing for the exam. Students can get 15+ min read
Restriction Enzymes Restriction enzyme is a bacterial protein that cleaves DNA at particular locations, these sites are called restricted sites. The restriction enzymes guard against bacteriophages in living bacteria. They identify the bacteriophage and cleave it at its restriction sites, destroying its DNA. Important 8 min read
Competent Host in Recombinant DNA Competent Host - For Transformation With Recombinant DNA: Competent Host refers to a living organism, such as bacteria or yeast that has been modified or treated in such a way that it can uptake and express foreign DNA molecules. These competent hosts are commonly used in the process of genetic tran 3 min read
Recombinant DNA Technology DNA is a collection of molecules that is in charge of transporting and passing genetic information from parents to offspring. DNA is the genetic material of a cell that carries information from generation to generation. It is essential for the survival of the cell. For the betterment of an individua 10 min read

Chapter 10: Biotechnology and Its Application

CBSE Class 12 Biology Biotechnology And Its Application Revision Notes CBSE Class 12 Chapter 10 Biotechnology and Its Applications: Biotechnology refers to the production of biopharmaceuticals and biologicals on a large scale, which involves using genetically modified organisms such as microbes, fungi, plants, and animals. Biotechnology has various applications, includ 11 min read
Application of Biotechnology Biotechnology is an applied branch of science and Biotechnologists use a living organism and its systems, or its products, to improve the quality of life of people. It is a highly advanced branch in today’s world that utilizes genetic, molecular, and cellular processes to bring about signific 7 min read
Genetically Engineered Insulin Genetically Designed Insulin, commonly known as recombinant insulin, is a type of insulin created using genetic engineering techniques. Insulin is a hormone that regulates blood sugar levels in the body and is vital for diabetics who cannot make enough insulin on their own. Prior to the development 6 min read
Biotechnology And Its Application- Gene Therapy BiotechnologyBiotechnology is focused on the large-scale production of biopharmaceuticals, including microorganisms, fungi, plants, and animals that have undergone genetic modification. Its applications include the fields of medicine and diagnostics as well as food processing, GM crops for agricultu 10 min read
Molecular Diagnosis Biotechnology is the application of biological processes, organisms, cells, and molecular biology to technology, engineering, and medicine. It involves using biological systems and techniques to develop new products and processes that can improve human health, food production, and the environment. B 7 min read
Transgenic Animals Notes - Biotechnology And Its Application CBSE Class 12- Biotechnology And Its Application - Transgenic Animals: When a foreign gene inserts into the genome of the animals to alter its DNA or the animals with a modified genome are Known as Transgenic Animals. It is a method that helps to improve the genetic traits of targeted animals. Trans 4 min read
Ethical Issues Related to Genetically Modified Organisms Genetically Modified Organisms (GMOs) are used in laboratories for research to know the organism and its function in a better way. Biotechnology is the field of study that involves the application of biological systems, organisms, or cells to make products that benefit human beings. Genetically Modi 7 min read

Chapter 11: Organisms And Populations

Organism and Population Notes Class 12 Biology Chapter 11 CBSE Class 12 Organisms and Population: The study of organisms and populations is an important area of biology known as ecology. An organism is a single living individual that is capable of carrying out all basic life processes. Organisms can be unicellular, or multicellular. Whereas population refe 9 min read
Responses To Abiotic Factors - Organisms And Populations Responses to abiotic factors are the ways in which living organisms react and adapt to changes in the non-living components of their habitat. There are majorly four abiotic factors, namely, Temperature, Water, Light and Soil which affect living organisms. Living organisms show responses to these abi 6 min read
What is Adaptation? Adaptation refers to a change in an organism's structure and function as a result of a natural process that makes the organism more suited to endure and proliferate in a given environment. Adaptation occurs in plants and animals, allowing them to adjust well within a given environment. E.g. Dessert 9 min read
Population Attributes - Overview Notes- Class 12 Population Attributes are the characteristics used to define a population. Population attributes play an essential role in understanding the dynamics and characteristics of a population. These attributes are measured for a population, not for an organism. There are five important attributes of a pop 6 min read
Population Growth - CBSE Class 12 Population Growth: Population growth refers to the increase in the number of people in a given area in a particular period of time. It is the main cause of this world because the population of human beings is not a static factor. Population growth depends on various factors such as weather, food ava 6 min read
Population Interactions Population interaction in the ecosystem occurs between the populations which interact with one another living in a community. Population interactions are divided into several types. There are two kinds of factors- biotic and abiotic factors. Different kinds of population interaction affect a lot to 4 min read
Predators One species completely depends on the other in this connection for food and survival. The species that are fed upon is known as the prey, while the species that feeds on another species is known as the predator. Predation is the name given to the entire relationship. Because the predator is typicall 6 min read
What Is Parasitism? Definition, Types and Examples Parasitism is a kind of symbiosis, a close and continuous long-term biological relationship between two species, where one organism, the parasite, lives on or inside another organism, the host. The parasite may have a negative impact on the host's health and is adapted structurally to this way of li 7 min read
Commensalism A commensal symbiosis is a symbiotic relationship between two species in which one species benefits from association, while the other species neither benefits nor harms. What is Commensalism? In other words, an organism that benefits from the relationship has a positive effect on its survival or rep 7 min read
Mutualism Mutualism is a type of symbiotic relationship in which both species involved are benefited from interaction. In mutualism, each species provides something of value to the other. This type of relationship is critical to the survival of one or both species and plays a role in shaping ecosystems and in 5 min read

Chapter 12: Ecosystem

Ecosystem Notes Class 12 Biology Chapter 12 Class 12 CBSE Biology Chapter 12- Ecosystem: Living organisms interact with one another and their physical surroundings in a functioning ecosystem. The size of ecosystems can vary, from little ponds to enormous forests or seas. According to some ecologists, the entire biosphere is made up of all the 13 min read
What is Ecosystem? Definition, Structure, Types, and Functions The ecosystem term was first coined by an ecologist Arthur Tansley in 1935. The ecosystem is a balance or equilibrium between living and non-living factors of the ecosystem where they tend to interact with each other. All living things, including plants, animals, and microorganisms, depend on non-li 12 min read
Energy Flow of Ecosystem The energy flow of ecosystem means the pathway energy takes to move from one organism to another in an ecosystem. The energy flow of an ecosystem is a fundamental concept of ecological studies. The direction of flow of energy in an ecosystem is unidirectional and is typically in the form of food ene 8 min read
Ecological Pyramid - Definition, Types, Importance, Limitations An ecological pyramid is a graphical representation of the relationship that every living creature present at different levels of the ecosystem shares with each other. Ecological Pyramids represent the different forms of bio-productivity of an ecosystem i.e. how much biomass, energy, or number of in 8 min read
Ecological Succession - Definition, Types, Characteristics, Causes Ecological succession is the process by which the structure and composition of a biological community change over time. Each of the ecological succession stages is characterized by different species compositions and environmental conditions. Understanding ecological succession and its types helps in 7 min read
What is Nutrient Cycling? To survive, organisms need nutrients. The natural recycling process is called the nutrient cycle. From one organism to the next, an element travels in a circular pattern. Recycling is the ecological process that supports and makes additional contributions to human welfare. Nutrient CycleThe term "nu 7 min read
Carbon Cycle The Carbon cycle is a type of Biogeochemical Cycle. The carbon cycle definition states that it is a natural process of a continuous cycle of carbon on the planet. The carbon cycle steps maintain the balance of carbon within the environment. It is a complex web of interconnected processes that involv 8 min read
Phosphorus Cycle The phosphorus cycle is a natural phenomenon by which phosphorus cycles through the three components of the biosphere which are the hydrosphere, lithosphere, and atmosphere. The phosphorus cycle is a very gradual process. The phosphorus cycle steps include weathering, release of phosphates into soil 7 min read
Types of Ecosystem Services - CBSE Class 12- Ecosystem Types of Ecosystem Services: Ecosystem Services is an effort sponsored by the UN in order to study and analyze the impact of human actions on the ecosystem because humans directly interact with the ecosystem and derive a number of benefits from the ecosystem and these actions of humans also lead to 5 min read

Chapter 13: Biodiversity and Its Conservation

Biodiversity and Conservation Notes Class 12 Chapter 13 CBSE Class 12 Science Notes Chapter 13 Biodiversity and Conservation are an important part of Class 12 Science Notes for quick revision. They will benefit from having challenging study material to use in preparing for the exam. Students can get CBSE Class 12th Science Notes Chapter 13 Biodiversity a 11 min read
How Many Species Are There On Earth And How Many In India? Biodiversity refers to the variety of life on earth, including the different species of plants and animals, the ecosystems they inhabit, and the genetic variation within each species. Biodiversity plays a critical role in maintaining the balance of our planet's ecosystem and the survival of life on 4 min read
Pattern of Biodiversity The word "biodiversity" refers to the variety of life on Earth at all its levels, from genes to ecosystems, and can cover the evolutionary, ecological, and cultural processes that support life. The term "biodiversity" refers to a wide range of living things, from people to microorganisms, fungi, and 6 min read
In-Situ and Ex-Situ Conservation of Biodiversity In-situ and ex-situ conservation of biodiversity are two approaches to the conservation of biodiversity. In-situ conservation mainly focuses on protecting the organism in its natural habitat whereas ex-situ conservation mainly focuses on protecting the organism by relocating it into an ideal protec 8 min read

NCERT Solutions

NCERT Solutions for CBSE Class 12 Biology CBSE Class 12th Biology NCERT Solutions 2023-24 is available here. These solutions will help students in their preparation for Class 12th CBSE Boards exams for the year 2023-2024. NCERT Solutions gives a detailed explanation of questions in the NCERT textbooks. By studying these solutions you will a 11 min read
Sexual Reproduction in Flowering Plants NCERT Solutions *As per the revised curriculum of CBSE Syllabus 2023-24, this chapter, previously known as Chapter 2, has now been renumbered as Chapter 1. Stay updated with the latest changes in the curriculum. Sexual Reproduction in Flowering Plants Class 12 NCERT Solutions is all about the process of Sexual Repr 12 min read
NCERT Solutions for Class 12 Chapter 2 Human Reproduction As per the revised curriculum of CBSE Syllabus 2023-24, this chapter, previously known as Chapter 3, has now been renumbered as Chapter 2. Stay updated with the latest changes in the curriculum. NCERT Solutions for Class 12 Chapter 2 Human Reproduction is all about the process of sexual reproduction 12 min read
NCERT Solutions for Class 12 Biology Chapter 3 Reproductive Health As per the revised curriculum of CBSE Syllabus 2023-24, the Reproductive health chapter, previously known as Chapter 4, has now been renumbered as Chapter 3. Stay updated with the latest changes in the curriculum. NCERT Solutions for Class 12 Biology Chapter 3 Reproductive Health is all about the im 15 min read
NCERT Solutions Class 12 Biology Chapter 4 Principles of Inheritance and Variation As per the revised curriculum of CBSE Syllabus 2023-24, this chapter, previously known as Chapter 5, has now been renumbered as Chapter 4. Stay updated with the latest changes in the curriculum. Principles of Inheritance and Variation Class 12 NCERT Solution is all about the process and principle of 15+ min read
NCERT Solutions of Class-12 Biology Chapter-5: Molecular Basis of Inheritance Molecular Basis of Inheritance Class 12 NCERT Solution is all about the process of inheritance at the molecular level. These NCERT Solutions are prepared by our Top Biology Experts in order to take care of all Important Topics that might be asked in the upcoming examination 2023. So, Students can al 9 min read
NCERT Solutions for Class 12 Biology Chapter 6 Evolution *As per the revised curriculum of CBSE Syllabus 2023-24, this chapter, previously known as Chapter 7, has now been renumbered as Chapter 6. Stay updated with the latest changes in the curriculum. Evolution Class 12 NCERT Solution is all about the process of Evolution, the benefits of evolution reaso 8 min read
NCERT Solutions for Class 12 Biology Chapter 7 Human Health and Disease As per the revised curriculum of CBSE Syllabus 2023-24, this chapter, previously known as Chapter 8, has now been renumbered as Chapter 7. Stay updated with the latest changes in the curriculum. Human Health and Disease Class 12 NCERT Solutions are all about human health and human-related disease. T 10 min read
NCERT Solutions for Class 12 Biology Chapter 8 As per the revised curriculum of CBSE Syllabus 2023-24, this chapter, previously known as Chapter 10, has now been renumbered as Chapter 8. Stay updated with the latest changes in the curriculum. Microbes in Human Welfare Class 12 NCERT Solution is all about microbes and their usage in daily life. T 8 min read
NCERT Solutions Class-12 Chapter 9 Biotechnology: Principles and Processes *As per the revised curriculum of CBSE Syllabus 2023-24, this chapter, previously known as Chapter 11, has now been renumbered as Chapter 9. Stay updated with the latest changes in the curriculum. Biotechnology: Principles and Processes Class 12 NCERT Solution is all about the process of Biotechnolo 8 min read
NCERT Solutions for Class 12 Biology Chapter 10 Biotechnology and Its Applications *As per the revised curriculum of CBSE Syllabus 2023-24, this chapter, previously known as Chapter 12, has now been renumbered as Chapter 10. Stay updated with the latest changes in the curriculum. Biotechnology and Its Applications Class 12 NCERT Solution is all about the process of Biotechnology a 7 min read
CBSE Solutions for Class 12 Biology Chapter 11 Organisms and Populations *As per the revised curriculum of CBSE Syllabus 2023-24, this chapter, previously known as Chapter 13, has now been renumbered as Chapter 11. Stay updated with the latest changes in the curriculum. Organism and Populations Class 12 NCERT Solution is all about the basics of organisms, populations, an 9 min read
CBSE Solutions for Class 12 Biology Chapter 13 Biodiversity and Conservation *As per the revised curriculum of CBSE Syllabus 2023-24, this chapter, previously known as Chapter 15, has now been renumbered as Chapter 13. Stay updated with the latest changes in the curriculum. Biodiversity and Conservation Class 12 NCERT Solution is all about biodiversity its importance and dif 7 min read
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Gregor Mendel and the Principles of Inheritance

Traits are passed down in families in different patterns. Pedigrees can illustrate these patterns by following the history of specific characteristics, or phenotypes, as they appear in a family. For example, the pedigree in Figure 1 shows a family in which a grandmother (generation I) has passed down a characteristic (shown in solid red) through the family tree. The inheritance pattern of this characteristic is considered dominant , because it is observable in every generation. Thus, every individual who carries the genetic code for this characteristic will show evidence of the characteristic. In contrast, Figure 2 shows a different pattern of inheritance, in which a characteristic disappears in one generation, only to reappear in a subsequent one. This pattern of inheritance, in which the parents do not show the phenotype but some of the children do, is considered recessive . But where did our knowledge of dominance and recessivity first come from?

Gregor Mendel’s Courage and Persistence

Mendel was curious about how traits were transferred from one generation to the next, so he set out to understand the principles of heredity in the mid-1860s. Peas were a good model system, because he could easily control their fertilization by transferring pollen with a small paintbrush. This pollen could come from the same flower (self-fertilization), or it could come from another plant's flowers (cross-fertilization). First, Mendel observed plant forms and their offspring for two years as they self-fertilized, or "selfed," and ensured that their outward, measurable characteristics remained constant in each generation. During this time, Mendel observed seven different characteristics in the pea plants, and each of these characteristics had two forms (Figure 3). The characteristics included height (tall or short), pod shape (inflated or constricted), seed shape (smooth or winkled), pea color (green or yellow), and so on. In the years Mendel spent letting the plants self, he verified the purity of his plants by confirming, for example, that tall plants had only tall children and grandchildren and so forth. Because the seven pea plant characteristics tracked by Mendel were consistent in generation after generation of self-fertilization, these parental lines of peas could be considered pure-breeders (or, in modern terminology, homozygous for the traits of interest). Mendel and his assistants eventually developed 22 varieties of pea plants with combinations of these consistent characteristics.

Mendel not only crossed pure-breeding parents, but he also crossed hybrid generations and crossed the hybrid progeny back to both parental lines. These crosses (which, in modern terminology, are referred to as F 1 , F 1 reciprocal, F 2 , B 1 , and B 2 ) are the classic crosses to generate genetically hybrid generations.

Understanding Dominant Traits

Understanding recessive traits.

When conducting his experiments, Mendel designated the two pure-breeding parental generations involved in a particular cross as P 1 and P 2 , and he then denoted the progeny resulting from the crossing as the filial, or F 1 , generation. Although the plants of the F 1 generation looked like one parent of the P generation, they were actually hybrids of two different parent plants. Upon observing the uniformity of the F 1 generation, Mendel wondered whether the F 1 generation could still possess the nondominant traits of the other parent in some hidden way.

To understand whether traits were hidden in the F 1 generation, Mendel returned to the method of self-fertilization. Here, he created an F 2 generation by letting an F 1 pea plant self-fertilize (F 1 x F 1 ). This way, he knew he was crossing two plants of the exact same genotype . This technique, which involves looking at a single trait, is today called a monohybrid cross . The resulting F 2 generation had seeds that were either round or wrinkled. Figure 4 shows an example of Mendel's data.

When looking at the figure, notice that for each F 1 plant, the self-fertilization resulted in more round than wrinkled seeds among the F 2 progeny. These results illustrate several important aspects of scientific data:

Multiple trials are necessary to see patterns in experimental data.
There is a lot of variation in the measurements of one experiment.
A large sample size, or "N," is required to make any quantitative comparisons or conclusions.

In Figure 4, the result of Experiment 1 shows that the single characteristic of seed shape was expressed in two different forms in the F 2 generation: either round or wrinkled. Also, when Mendel averaged the relative proportion of round and wrinkled seeds across all F 2 progeny sets, he found that round was consistently three times more frequent than wrinkled. This 3:1 proportion resulting from F 1 x F 1 crosses suggested there was a hidden recessive form of the trait. Mendel recognized that this recessive trait was carried down to the F 2 generation from the earlier P generation .

Mendel and Alleles

As mentioned, Mendel's data did not support the ideas about trait blending that were popular among the biologists of his time. As there were never any semi-wrinkled seeds or greenish-yellow seeds, for example, in the F 2 generation, Mendel concluded that blending should not be the expected outcome of parental trait combinations. Mendel instead hypothesized that each parent contributes some particulate matter to the offspring. He called this heritable substance "elementen." (Remember, in 1865, Mendel did not know about DNA or genes.) Indeed, for each of the traits he examined, Mendel focused on how the elementen that determined that trait was distributed among progeny. We now know that a single gene controls seed form, while another controls color, and so on, and that elementen is actually the assembly of physical genes located on chromosomes. Multiple forms of those genes, known as alleles , represent the different traits. For example, one allele results in round seeds, and another allele specifies wrinkled seeds.

One of the most impressive things about Mendel's thinking lies in the notation that he used to represent his data. Mendel's notation of a capital and a lowercase letter ( Aa ) for the hybrid genotype actually represented what we now know as the two alleles of one gene : A and a . Moreover, as previously mentioned, in all cases, Mendel saw approximately a 3:1 ratio of one phenotype to another. When one parent carried all the dominant traits ( AA ), the F 1 hybrids were "indistinguishable" from that parent. However, even though these F 1 plants had the same phenotype as the dominant P 1 parents, they possessed a hybrid genotype ( Aa ) that carried the potential to look like the recessive P 1 parent ( aa ). After observing this potential to express a trait without showing the phenotype, Mendel put forth his second principle of inheritance: the principle of segregation . According to this principle, the "particles" (or alleles as we now know them) that determine traits are separated into gametes during meiosis , and meiosis produces equal numbers of egg or sperm cells that contain each allele (Figure 5).

Dihybrid Crosses

Mendel had thus determined what happens when two plants that are hybrid for one trait are crossed with each other, but he also wanted to determine what happens when two plants that are each hybrid for two traits are crossed. Mendel therefore decided to examine the inheritance of two characteristics at once. Based on the concept of segregation , he predicted that traits must sort into gametes separately. By extrapolating from his earlier data, Mendel also predicted that the inheritance of one characteristic did not affect the inheritance of a different characteristic.

Mendel tested this idea of trait independence with more complex crosses. First, he generated plants that were purebred for two characteristics, such as seed color (yellow and green) and seed shape (round and wrinkled). These plants would serve as the P 1 generation for the experiment. In this case, Mendel crossed the plants with wrinkled and yellow seeds ( rrYY ) with plants with round, green seeds ( RRyy ). From his earlier monohybrid crosses, Mendel knew which traits were dominant: round and yellow. So, in the F 1 generation, he expected all round, yellow seeds from crossing these purebred varieties, and that is exactly what he observed. Mendel knew that each of the F 1 progeny were dihybrids; in other words, they contained both alleles for each characteristic ( RrYy ). He then crossed individual F 1 plants (with genotypes RrYy ) with one another. This is called a dihybrid cross . Mendel's results from this cross were as follows:

315 plants with round, yellow seeds
108 plants with round, green seeds
101 plants with wrinkled, yellow seeds
32 plants with wrinkled, green seeds

Thus, the various phenotypes were present in a 9:3:3:1 ratio (Figure 6).

Next, Mendel went through his data and examined each characteristic separately. He compared the total numbers of round versus wrinkled and yellow versus green peas, as shown in Tables 1 and 2.

Table 1: Data Regarding Seed Shape

Table 2: Data Regarding Pea Color

The proportion of each trait was still approximately 3:1 for both seed shape and seed color. In other words, the resulting seed shape and seed color looked as if they had come from two parallel monohybrid crosses; even though two characteristics were involved in one cross, these traits behaved as though they had segregated independently. From these data, Mendel developed the third principle of inheritance: the principle of independent assortment . According to this principle, alleles at one locus segregate into gametes independently of alleles at other loci. Such gametes are formed in equal frequencies.

Mendel’s Legacy

More lasting than the pea data Mendel presented in 1862 has been his methodical hypothesis testing and careful application of mathematical models to the study of biological inheritance. From his first experiments with monohybrid crosses, Mendel formed statistical predictions about trait inheritance that he could test with more complex experiments of dihybrid and even trihybrid crosses. This method of developing statistical expectations about inheritance data is one of the most significant contributions Mendel made to biology.

But do all organisms pass their on genes in the same way as the garden pea plant? The answer to that question is no, but many organisms do indeed show inheritance patterns similar to the seminal ones described by Mendel in the pea. In fact, the three principles of inheritance that Mendel laid out have had far greater impact than his original data from pea plant manipulations. To this day, scientists use Mendel's principles to explain the most basic phenomena of inheritance.

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"Experiments in Plant Hybridization" (1866), by Johann Gregor Mendel

During the mid-nineteenth century, Johann Gregor Mendel experimented with pea plants to develop a theory of inheritance. In 1843, while a monk in the Augustian St Thomas’s Abbey in Brünn, Austria, now Brno, Czech Repubic, Mendel examined the physical appearance of the abbey’s pea plants ( Pisum sativum ) and noted inconsistencies between what he saw and what the blending theory of inheritance, a primary model of inheritance at the time, predicted. With his experiments, which he recored in “Versuche über Pflanzenhybriden” (“Experiments in Plant Hybridization”) in 1865, Mendel discredited the blending theory of inheritance, and from them he proposed laws for inheritance patterns. Despite the fact that Mendel’s work did not define all aspects of inheritance, his ideas and laws contributed to later concepts of traits, specifically that offspring inherit traits from their parents via genes, that an offspring has at least two genetic factors for any given qualitative trait, and that the offspring inherits the genetic factors in equal proportion from both parents.

In 1856 Mendel noticed that plants in the same species had different physical appearances, including colors, heights, and seed shapes. At the time, many biologists held that all offspring were a mixture of parental traits that could never be separated back into the original parental traits. Consequently, all traits would eventually blend together and result in a homogenous amalgamation of the parental characters. This idea of a blended inheritance conflicted with what Mendel noted in many of the abbey’s plants. Mendel investigated these phenomena by experimentally mating pea plants and observing the results.

Mendel encountered a number of benefits in using the pea plant for his experiments on heredity. Specifically, the Pisum sativum plant reproduces and matures quickly, has easily observable physical traits, and can be easily artificially fertilized. As Mendel sought to trace the heredity of physical characteristic’s through generations, he needed to fertilize plants from one generation with others from the same generation. With controlled fertilization, Mendel bred generations of pea plants with the confidence that there was little or no contamination from plants of other generations. Mendel managed mating by removing the reproductive organ of a flower (piston) from one plant and pollinating another plant of his choice. He repeated his tests with thousands of plants in a relatively short time.

Mendel used pea plants that, within a lineage, displayed only one physical characteristic, like a specific pod color or a specific seed shape, for many generations. He then crossed those plants with those from a different lineage that had displayed a different physical characteristic for many generations. He chose to cross pea plants with seven different characteristics: plant height (tall vs. short), seed color (green vs. yellow), seed shape (smooth vs. wrinkled), seed-coat color (gray vs. white), pod shape (full vs. constricted), pod color (green vs. yellow), and flower distribution (along stem vs. at the end of the stem). Mendel examined the first offspring generation, noted physical appearances and then crossed plants within the first generation to produce a second generation of offspring. By examining each characteristic throughout the generations of offspring, Mendel concluded that individuals in successive generations displayed the original characteristics of their parents.

Mendel noticed that only one of the characteristics for each category was displayed per offspring. For example, pea plants exhibited either green or yellow seeds, but not both colors within the same plant or seed colors that blended yellow and green. In the first generation of hybrids the trait that resulted always mirrored one of the parents. These results discredited the theory of blending between parental traits, as the offspring of a tall pea plant and a short pea plant yielded not a medium pea plant, but only tall pea plants.

From 1856 to 1863, Mendel continued his experiments and noted that the trait of the parent that was missing in an organism from the first generation reappeared in organisms of the second generation. Furthermore, the ratio of these traits within the second generation occurred in roughly a 3:1 proportion, such that out of every four offspring, approximately three possessed the physical trait of one parent and one displayed the physical trait of the other parent. The trait that appeared most often Mendel called the dominant trait, and the other he called recessive. Through his experiments, Mendel determined the dominant traits in pea plants to be: tall plant height, yellow seed color, smooth seed shape, gray seed-coat color, full pod shape, green pod color, and flower distribution along the stem.

Mendel re-tested his experiment from 1856 to 1863 on almost 30,000 plants to verify his results. He proposed that factors (later called genes) determine the appearance of a characteristic and that for each physical character, a factor has two contributing forms (later called alleles). Furthermore, an organism inherits one form from its mother and one form from its father. If, within a factor, the forms are different, for example, a green seed color form via the mother and a yellow seed color form via the father, then one is dominant and determines the physical appearance of a trait in an offspring, while the other is recessive, and doesn’t influence the physical character. Mendel formulated a theory of particulate inheritance around this theory that recessive traits, although not always physically expressed in the offspring of one generation, can reappear in the offspring of subsequent generations. Mendel postulated two laws to explain the results he had obtained.

The law of segregation states that during sex cell formation, each sex cell will receive one factor out of a pair of factors. The law of independent assortment, claims that when each of these sex cells receives a factor, the members of each pair separate into sex cells independently of one another.

Few people noticed Mendel’s experiments for most of the nineteenth century, even after publication of “Versuche über Pflanzenhybriden” in the journal Verhandlungen des naturforschenden Vereins Brünn (Proceedings of the Natural History Society of Brünn) in 1866. Mendel’s article remained untranslated from German. However, Mendel posthumously received credit for his work. In 1899 at the Royal Horticultural Society’s International Conference on Hybridization and Plant Breeding in London, Great Britain, William Bateson revived the papers and findings of Mendel through his own experiments on heredity in the UK.

Furthermore, in 1900, three botanists in Europe, Hugo de Vries, Carl Correns, and Erich von Tschermak-Seysenegg, each performed their own experiments and independently arrived at the same conclusions as Mendel, without knowing Mendel’s work. Repetitions of Mendel’s experiments showed that not all traits exhibited a classic dominance and recessive character. Hybrids, or mixes, appeared and showed that a blending of traits can occur in some cases.

Corcos, Alain F. and Floyd V Monaghan. Gregor Mendel’s Experiments on Plant Hybrids: A Guided Study . New Brunswick, NJ: Rutgers University Press, 1993.
Dodson, Edward O. “Mendel and the Rediscovery of His Work.” The Scientific Monthly 81 (1955): 187–95.
Hartl, Daniel L. and Vitezslav Orel. “What Did Gregor Mendel Think He Discovered?” Genetics 131 (1992): 245–53.
Iltis, Hugo. “Gregor Mendel and His Work.” The Scientific Monthly 56 (1943): 414–23.
Mendel, Gregor Johann. “Versuche über Pflanzen-Hybriden” [Experiments Concerning Plant Hybrids]” [1866]. In Verhandlungen des naturforschenden Vereines in Brünn [Proceedings of the Natural History Society of Brünn] IV (1865): 3–47. Reprinted in Fundamenta Genetica , ed. Jaroslav Kříženecký, 15–56. Prague: Czech Academy of Sciences, 1966. http://www.mendelweb.org/Mendel.html

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Mendel's Experiments: The Study Of Pea Plants & Inheritance

** Gregor Mendel ** was a 19th-century pioneer of genetics who today is remembered almost entirely for two things: being a monk and relentlessly studying different traits of pea plants. Born in 1822 in Austria, Mendel was raised on a farm and attended the University of Vienna in Austria's capital city.

There, he studied science and math, a pairing that would prove invaluable to his future endeavors, which he conducted over an eight-year period entirely at the monastery where he lived.

In addition to formally studying the natural sciences in college, Mendel worked as a gardener in his youth and published research papers on the subject of crop damage by insects before taking up his now-famous work with Pisum sativum, the common pea plant. He maintained the monastery greenhouses and was familiar with the artificial fertilization techniques required to create limitless numbers of hybrid offspring.

An interesting historical footnote: While Mendel's experiments and those of the visionary biologist ** Charles Darwin ** both overlapped to a great extent, the latter never learned of Mendel's experiments.

Darwin formulated his ideas about inheritance without knowledge of Mendel's thoroughly detailed propositions about the mechanisms involved. Those propositions continue to inform the field of biological inheritance in the 21st century.

Understanding of Inheritance in the Mid-1800s

From the standpoint of basic qualifications, Mendel was perfectly positioned to make a major breakthrough in the then-all-but-nonexistent field of genetics, and he was blessed with both the environment and the patience to get done what he needed to do. Mendel would end up growing and studying nearly 29,000 pea plants between 1856 and 1863.

When Mendel first began his work with pea plants, the scientific concept of heredity was rooted in the concept of blended inheritance, which held that parental traits were somehow mixed into offspring in the manner of different-colored paints, producing a result that was not quite the mother and not quite the father every time, but that clearly resembled both.

Mendel was intuitively aware from his informal observation of plants that if there was any merit to this idea, it certainly didn't apply to the botanical world.

Mendel was not interested in the appearance of his pea plants per se. He examined them in order to understand which characteristics could be passed on to future generations and exactly how this occurred at a functional level, even if he didn't have the literal tools to see what was occurring at the molecular level.

Pea Plant Characteristics Studied

Mendel focused on the different traits, or characters, that he noticed pea plants exhibiting in a binary manner. That is, an individual plant could show either version A of a given trait or version B of that trait, but nothing in between. For example, some plants had "inflated" pea pods, whereas others looked "pinched," with no ambiguity as to which category a given plant's pods belonged in.

The seven traits Mendel identified as being useful to his aims and their different manifestations were:

• Flower color: Purple or white. • Flower position: Axial (along the side of the stem) or terminal (at the end of the stem). • Stem length: Long or short. • Pod shape: Inflated or pinched. • Pod color: Green or yellow. • Seed shape: Round or wrinkled. • Seed color: Green or yellow.

Pea Plant Pollination

Pea plants can self-pollinate with no help from people. As useful as this is to plants, it introduced a complication into Mendel's work. He needed to prevent this from happening and allow only cross-pollination (pollination between different plants), since self-pollination in a plant that does not vary for a given trait does not provide helpful information.

In other words, he needed to control what characteristics could show up in the plants he bred, even if he didn't know in advance precisely which ones would manifest themselves and in what proportions.

Mendel's First Experiment

When Mendel began to formulate specific ideas about what he hoped to test and identify, he asked himself a number of basic questions. For example, what would happen when plants that were true-breeding for different versions of the same trait were cross-pollinated?

"True-breeding" means capable of producing one and only one type of offspring, such as when all daughter plants are round-seeded or axial-flowered. A true line shows no variation for the trait in question throughout a theoretically infinite number of generations, and also when any two selected plants in the scheme are bred with each other.

• To be certain his plant lines were true, Mendel spent two years creating them.

If the idea of blended inheritance were valid, blending a line of, say, tall-stemmed plants with a line of short-stemmed plants should result in some tall plants, some short plants and plants along the height spectrum in between, rather like humans. Mendel learned, however, that this did not happen at all. This was both confounding and exciting.

Mendel's Generational Assessment: P, F1, F2

Once Mendel had two sets of plants that differed only at a single trait, he performed a multigenerational assessment in an effort to try to follow the transmission of traits through multiple generations. First, some terminology:

• The parent generation was the P generation , and it included a P1 plant whose members all displayed one version of a trait and a P2 plant whose members all displayed the other version. • The hybrid offspring of the P generation was the F1 (filial) generation . • The offspring of the F1 generation was the F2 generation (the "grandchildren" of the P generation).

This is called a _ monohybrid cross _: "mono" because only one trait varied, and "hybrid" because offspring represented a mixture, or hybridization, of plants, as one parent has one version of the trait while one had the other version.

For the present example, this trait will be seed shape (round vs. wrinkled). One could also use flower color (white vs. purpl) or seed color (green or yellow).

Mendel's Results (First Experiment)

Mendel assessed genetic crosses from the three generations to assess the heritability of characteristics across generations. When he looked at each generation, he discovered that for all seven of his chosen traits, a predictable pattern emerged.

For example, when he bred true-breeding round-seeded plants (P1) with true-breeding wrinkled-seeded plants (P2):

• All of the plants in the F1 generation had round seeds . This seemed to suggest that the wrinkled trait had been obliterated by the round trait. • However, he also found that, while about three-fourths of the plants in the F2 generation has round seeds, about one-fourth of these plants had wrinkled seeds . Clearly, the wrinkled trait had somehow "hidden" in the F1 generation and re-emerged in the F2 generation.

This led to the concept of dominant traits (here, round seeds) and recessive traits (in this case, wrinkled seeds).

This implied that the plants' _ phenotype (what the plants actually looked like) was not a strict reflection of their genotype _ (the information that was actually somehow coded into the plants and passed along to subsequent generations).

Mendel then produced some formal ideas to explain this phenomenon, both the mechanism of heritability and the mathematical ratio of a dominant trait to a recessive trait in any circumstance where the composition of allele pairs is known.

Mendel's Theory of Heredity

Mendel crafted a theory of heredity that consisted of four hypotheses:

1. Genes (a gene being the chemical code for a given trait) can come in different types. 2. For each characteristic, an organism inherits one allele (version of a gene) from each parent. 3. When two different alleles are inherited, one may be expressed while the other is not. 4. When gametes (sex cells, which in humans are sperm cells and egg cells) are formed, the two alleles of each gene are separated.

The last of these represents the ** law of segregation **, stipulating that the alleles for each trait separate randomly into the gametes.

Today, scientists recognize that the P plants that Mendel had "bred true" were homozygous for the trait he was studying: They had two copies of the same allele at the gene in question.

Since round was clearly dominant over wrinkled, this can be represented by RR and rr, as capital letters signify dominance and lowercase letters indicate recessive traits. When both alleles are present, the trait of the dominant allele was manifested in its phenotype.

The Monohybrid Cross Results Explained

Based on the foregoing, a plant with a genotype RR at the seed-shape gene can only have round seeds, and the same is true of the Rr genotype, as the "r" allele is masked. Only plants with an rr genotype can have wrinkled seeds.

And sure enough, the four possible combinations of genotypes (RR, rR, Rr and rr) yield a 3:1 phenotypic ratio, with about three plants with round seeds for every one plant with wrinkled seeds.

Because all of the P plants were homozygous, RR for the round-seed plants and rr for the wrinkled-seed plants, all of the F1 plants could only have the genotype Rr. This meant that while all of them had round seeds, they were all carriers of the recessive allele, which could therefore appear in subsequent generations thanks to the law of segregation.

This is precisely what happened. Given F1 plants that all had an Rr genotype, their offspring (the F2 plants) could have any of the four genotypes listed above. The ratios were not exactly 3:1 owing to the randomness of the gamete pairings in fertilization, but the more offspring that were produced, the closer the ratio came to being exactly 3:1.

Mendel's Second Experiment

Next, Mendel created _ dihybrid crosses _, wherein he looked at two traits at once rather than just one. The parents were still true-breeding for both traits, for example, round seeds with green pods and wrinkled seeds with yellow pods, with green dominant over yellow. The corresponding genotypes were therefore RRGG and rrgg.

As before, the F1 plants all looked like the parent with both dominant traits. The ratios of the four possible phenotypes in the F2 generation (round-green, round-yellow, wrinkled-green, wrinkled-yellow) turned out to be 9:3:3:1

This bore out Mendel's suspicion that different traits were inherited independently of one another, leading him to posit the ** law of independent assortment **. This principle explains why you might have the same eye color as one of your siblings, but a different hair color; each trait is fed into the system in a manner that is blind to all of the others.

Linked Genes on Chromosomes

Today, we know the real picture is a little more complicated, because in fact, genes that happen to be physically close to each other on chromosomes can be inherited together thanks to chromosome exchange during gamete formation.

In the real world, if you looked at limited geographical areas of the U.S., you would expect to find more New York Yankees and Boston Red Sox fans in close proximity than either Yankees-Los Angeles Dodgers fans or Red Sox-Dodgers fans in the same area, because Boston and New York are close together and both are close to 3,000 miles from Los Angeles.

Mendelian Inheritance

As it happens, not all traits obey this pattern of inheritance. But those that do are called Mendelian traits . Returning to the dihybrid cross mentioned above, there are sixteen possible genotypes:

RRGG, RRgG, RRGg, RRgg, RrGG, RrgG, RrGg, Rrgg, rRGG, rRgG, rRGg, rRgg, rrGG, rrGg, rrgG, rrgg

When you work out the phenotypes, you see that the probability ratio of

round green, round yellow, wrinkled green, wrinkled yellow

turns out to be 9:3:3:1. Mendel's painstaking counting of his different plant types revealed that the ratios were close enough to this prediction for him to conclude that his hypotheses were correct.

• Note: A genotype of rR is functionally equivalent to Rr. The only difference is which parent contributes which allele to the mix.

Scitable by Nature Education: Gregor Mendel and the Principles of Inheritance
Biology LibreTexts: Mendel's Pea Plants
OpenText BC: Concepts of Biology: Laws of Inheritance
Forbes Magazine: How Mendel Channeled Darwin

Cite This Article

Beck, Kevin. "Mendel's Experiments: The Study Of Pea Plants & Inheritance" sciencing.com , https://www.sciencing.com/mendels-experiments-the-study-of-pea-plants-inheritance-13718433/. 8 May 2019.

Beck, Kevin. (2019, May 8). Mendel's Experiments: The Study Of Pea Plants & Inheritance. sciencing.com . Retrieved from https://www.sciencing.com/mendels-experiments-the-study-of-pea-plants-inheritance-13718433/

Beck, Kevin. Mendel's Experiments: The Study Of Pea Plants & Inheritance last modified August 30, 2022. https://www.sciencing.com/mendels-experiments-the-study-of-pea-plants-inheritance-13718433/

Genetics and Inheritance

Mendel’s experiments and the laws of probability, learning objective.

By the end of this section you will be able to:,

Describe the scientific reasons for the success of Mendel’s experimental work
Describe the expected outcomes of monohybrid crosses involving dominant and recessive alleles
Apply the sum and product rules to calculate probabilities

Sketch of Gregor Mendel, a monk who wore reading glasses and a large cross.

Figure 1. Johann Gregor Mendel is considered the father of genetics.

Johann Gregor Mendel (1822–1884) (Figure 1) was a lifelong learner, teacher, scientist, and man of faith. As a young adult, he joined the Augustinian Abbey of St. Thomas in Brno in what is now the Czech Republic. Supported by the monastery, he taught physics, botany, and natural science courses at the secondary and university levels. In 1856, he began a decade-long research pursuit involving inheritance patterns in honeybees and plants, ultimately settling on pea plants as his primary model system (a system with convenient characteristics used to study a specific biological phenomenon to be applied to other systems). In 1865, Mendel presented the results of his experiments with nearly 30,000 pea plants to the local Natural History Society. He demonstrated that traits are transmitted faithfully from parents to offspring independently of other traits and in dominant and recessive patterns. In 1866, he published his work, Experiments in Plant Hybridization , [1] in the proceedings of the Natural History Society of Brünn.

Mendel’s work went virtually unnoticed by the scientific community that believed, incorrectly, that the process of inheritance involved a blending of parental traits that produced an intermediate physical appearance in offspring; this hypothetical process appeared to be correct because of what we know now as continuous variation. Continuous variation results from the action of many genes to determine a characteristic like human height. Offspring appear to be a “blend” of their parents’ traits when we look at characteristics that exhibit continuous variation. The blending theory of inheritance asserted that the original parental traits were lost or absorbed by the blending in the offspring, but we now know that this is not the case. Mendel was the first researcher to see it. Instead of continuous characteristics, Mendel worked with traits that were inherited in distinct classes (specifically, violet versus white flowers); this is referred to as discontinuous variation. Mendel’s choice of these kinds of traits allowed him to see experimentally that the traits were not blended in the offspring, nor were they absorbed, but rather that they kept their distinctness and could be passed on. In 1868, Mendel became abbot of the monastery and exchanged his scientific pursuits for his pastoral duties. He was not recognized for his extraordinary scientific contributions during his lifetime. In fact, it was not until 1900 that his work was rediscovered, reproduced, and revitalized by scientists on the brink of discovering the chromosomal basis of heredity.

Mendel’s Model System

Mendel’s seminal work was accomplished using the garden pea, Pisum sativum , to study inheritance. This species naturally self-fertilizes, such that pollen encounters ova within individual flowers. The flower petals remain sealed tightly until after pollination, preventing pollination from other plants. The result is highly inbred, or “true-breeding,” pea plants. These are plants that always produce offspring that look like the parent. By experimenting with true-breeding pea plants, Mendel avoided the appearance of unexpected traits in offspring that might occur if the plants were not true breeding. The garden pea also grows to maturity within one season, meaning that several generations could be evaluated over a relatively short time. Finally, large quantities of garden peas could be cultivated simultaneously, allowing Mendel to conclude that his results did not come about simply by chance.

Mendelian Crosses

Mendel performed hybridizations, which involve mating two true-breeding individuals that have different traits. In the pea, which is naturally self-pollinating, this is done by manually transferring pollen from the anther of a mature pea plant of one variety to the stigma of a separate mature pea plant of the second variety. In plants, pollen carries the male gametes (sperm) to the stigma, a sticky organ that traps pollen and allows the sperm to move down the pistil to the female gametes (ova) below. To prevent the pea plant that was receiving pollen from self-fertilizing and confounding his results, Mendel painstakingly removed all of the anthers from the plant’s flowers before they had a chance to mature.

Plants used in first-generation crosses were called P 0 , or parental generation one, plants (Figure). Mendel collected the seeds belonging to the P 0 plants that resulted from each cross and grew them the following season. These offspring were called the F 1 , or the first filial ( filial = offspring, daughter or son), generation. Once Mendel examined the characteristics in the F 1 generation of plants, he allowed them to self-fertilize naturally. He then collected and grew the seeds from the F 1 plants to produce the F 2 , or second filial, generation. Mendel’s experiments extended beyond the F 2 generation to the F 3 and F 4 generations, and so on, but it was the ratio of characteristics in the P 0 −F 1 −F 2 generations that were the most intriguing and became the basis for Mendel’s postulates.

$The diagram shows a cross between pea plants that are true-breeding for purple flower color and plants true-breeding for white flower color. This cross-fertilization of the P generation resulted in an F_{1} generation with all violet flowers. Self-fertilization of the F_{1} generation resulted in an F_{2} generation that consisted of 705 plants with violet flowers, and 224 plants with white flowers.$

Figure 2. In one of his experiments on inheritance patterns, Mendel crossed plants that were true-breeding for violet flower color with plants true-breeding for white flower color (the P generation). The resulting hybrids in the F 1 generation all had violet flowers. In the F 2 generation, approximately three quarters of the plants had violet flowers, and one quarter had white flowers.

Garden Pea Characteristics Revealed the Basics of Heredity

In his 1865 publication, Mendel reported the results of his crosses involving seven different characteristics, each with two contrasting traits. A trait is defined as a variation in the physical appearance of a heritable characteristic. The characteristics included plant height, seed texture, seed color, flower color, pea pod size, pea pod color, and flower position. For the characteristic of flower color, for example, the two contrasting traits were white versus violet. To fully examine each characteristic, Mendel generated large numbers of F 1 and F 2 plants, reporting results from 19,959 F 2 plants alone. His findings were consistent.

What results did Mendel find in his crosses for flower color? First, Mendel confirmed that he had plants that bred true for white or violet flower color. Regardless of how many generations Mendel examined, all self-crossed offspring of parents with white flowers had white flowers, and all self-crossed offspring of parents with violet flowers had violet flowers. In addition, Mendel confirmed that, other than flower color, the pea plants were physically identical.

Once these validations were complete, Mendel applied the pollen from a plant with violet flowers to the stigma of a plant with white flowers. After gathering and sowing the seeds that resulted from this cross, Mendel found that 100 percent of the F 1 hybrid generation had violet flowers. Conventional wisdom at that time would have predicted the hybrid flowers to be pale violet or for hybrid plants to have equal numbers of white and violet flowers. In other words, the contrasting parental traits were expected to blend in the offspring. Instead, Mendel’s results demonstrated that the white flower trait in the F 1 generation had completely disappeared.

Importantly, Mendel did not stop his experimentation there. He allowed the F 1 plants to self-fertilize and found that, of F 2 -generation plants, 705 had violet flowers and 224 had white flowers. This was a ratio of 3.15 violet flowers per one white flower, or approximately 3:1. When Mendel transferred pollen from a plant with violet flowers to the stigma of a plant with white flowers and vice versa, he obtained about the same ratio regardless of which parent, male or female, contributed which trait. This is called a reciprocal cross—a paired cross in which the respective traits of the male and female in one cross become the respective traits of the female and male in the other cross. For the other six characteristics Mendel examined, the F 1 and F 2 generations behaved in the same way as they had for flower color. One of the two traits would disappear completely from the F 1 generation only to reappear in the F 2 generation at a ratio of approximately 3:1 (Table 1).

Upon compiling his results for many thousands of plants, Mendel concluded that the characteristics could be divided into expressed and latent traits. He called these, respectively, dominant and recessive traits. Dominant traits are those that are inherited unchanged in a hybridization. Recessive traits become latent, or disappear, in the offspring of a hybridization. The recessive trait does, however, reappear in the progeny of the hybrid offspring. An example of a dominant trait is the violet-flower trait. For this same characteristic (flower color), white-colored flowers are a recessive trait. The fact that the recessive trait reappeared in the F 2 generation meant that the traits remained separate (not blended) in the plants of the F 1 generation. Mendel also proposed that plants possessed two copies of the trait for the flower-color characteristic, and that each parent transmitted one of its two copies to its offspring, where they came together. Moreover, the physical observation of a dominant trait could mean that the genetic composition of the organism included two dominant versions of the characteristic or that it included one dominant and one recessive version. Conversely, the observation of a recessive trait meant that the organism lacked any dominant versions of this characteristic.

So why did Mendel repeatedly obtain 3:1 ratios in his crosses? To understand how Mendel deduced the basic mechanisms of inheritance that lead to such ratios, we must first review the laws of probability.

Probability Basics

Probabilities are mathematical measures of likelihood. The empirical probability of an event is calculated by dividing the number of times the event occurs by the total number of opportunities for the event to occur. It is also possible to calculate theoretical probabilities by dividing the number of times that an event is expected to occur by the number of times that it could occur. Empirical probabilities come from observations, like those of Mendel. Theoretical probabilities come from knowing how the events are produced and assuming that the probabilities of individual outcomes are equal. A probability of one for some event indicates that it is guaranteed to occur, whereas a probability of zero indicates that it is guaranteed not to occur. An example of a genetic event is a round seed produced by a pea plant. In his experiment, Mendel demonstrated that the probability of the event “round seed” occurring was one in the F 1 offspring of true-breeding parents, one of which has round seeds and one of which has wrinkled seeds. When the F 1 plants were subsequently self-crossed, the probability of any given F 2 offspring having round seeds was now three out of four. In other words, in a large population of F 2 offspring chosen at random, 75 percent were expected to have round seeds, whereas 25 percent were expected to have wrinkled seeds. Using large numbers of crosses, Mendel was able to calculate probabilities and use these to predict the outcomes of other crosses.

The Product Rule and Sum Rule

Mendel demonstrated that the pea-plant characteristics he studied were transmitted as discrete units from parent to offspring. As will be discussed, Mendel also determined that different characteristics, like seed color and seed texture, were transmitted independently of one another and could be considered in separate probability analyses. For instance, performing a cross between a plant with green, wrinkled seeds and a plant with yellow, round seeds still produced offspring that had a 3:1 ratio of green:yellow seeds (ignoring seed texture) and a 3:1 ratio of round:wrinkled seeds (ignoring seed color). The characteristics of color and texture did not influence each other.

The product rule of probability can be applied to this phenomenon of the independent transmission of characteristics. The product rule states that the probability of two independent events occurring together can be calculated by multiplying the individual probabilities of each event occurring alone. To demonstrate the product rule, imagine that you are rolling a six-sided die (D) and flipping a penny (P) at the same time. The die may roll any number from 1–6 (D # ), whereas the penny may turn up heads (P H ) or tails (P T ). The outcome of rolling the die has no effect on the outcome of flipping the penny and vice versa. There are 12 possible outcomes of this action (Table 2), and each event is expected to occur with equal probability.

Of the 12 possible outcomes, the die has a 2/12 (or 1/6) probability of rolling a two, and the penny has a 6/12 (or 1/2) probability of coming up heads. By the product rule, the probability that you will obtain the combined outcome 2 and heads is: (D 2 ) × (P H ) = (1/6) × (1/2) or 1/12 (Table). Notice the word “and” in the description of the probability. The “and” is a signal to apply the product rule. For example, consider how the product rule is applied to the dihybrid cross: the probability of having both dominant traits in the F 2 progeny is the product of the probabilities of having the dominant trait for each characteristic, as shown here:

[latex]\frac{3}{4}\times\frac{3}{4}=\frac{9}{16}[/latex]

On the other hand, the sum rule of probability is applied when considering two mutually exclusive outcomes that can come about by more than one pathway. The sum rule states that the probability of the occurrence of one event or the other event, of two mutually exclusive events, is the sum of their individual probabilities. Notice the word “or” in the description of the probability. The “or” indicates that you should apply the sum rule. In this case, let’s imagine you are flipping a penny (P) and a quarter (Q). What is the probability of one coin coming up heads and one coin coming up tails? This outcome can be achieved by two cases: the penny may be heads (P H ) and the quarter may be tails (Q T ), or the quarter may be heads (Q H ) and the penny may be tails (P T ). Either case fulfills the outcome. By the sum rule, we calculate the probability of obtaining one head and one tail as [(P H ) × (Q T )] + [(Q H ) × (P T )] = [(1/2) × (1/2)] + [(1/2) × (1/2)] = 1/2 (Table). You should also notice that we used the product rule to calculate the probability of P H and Q T , and also the probability of P T and Q H , before we summed them. Again, the sum rule can be applied to show the probability of having just one dominant trait in the F 2 generation of a dihybrid cross:

[latex]\frac{3}{16}+\frac{3}{4}=\frac{15}{16}[/latex]

To use probability laws in practice, it is necessary to work with large sample sizes because small sample sizes are prone to deviations caused by chance. The large quantities of pea plants that Mendel examined allowed him calculate the probabilities of the traits appearing in his F 2 generation. As you will learn, this discovery meant that when parental traits were known, the offspring’s traits could be predicted accurately even before fertilization.

Section Summary

Working with garden pea plants, Mendel found that crosses between parents that differed by one trait produced F 1 offspring that all expressed the traits of one parent. Observable traits are referred to as dominant, and non-expressed traits are described as recessive. When the offspring in Mendel’s experiment were self-crossed, the F 2 offspring exhibited the dominant trait or the recessive trait in a 3:1 ratio, confirming that the recessive trait had been transmitted faithfully from the original P 0 parent. Reciprocal crosses generated identical F 1 and F 2 offspring ratios. By examining sample sizes, Mendel showed that his crosses behaved reproducibly according to the laws of probability, and that the traits were inherited as independent events.

Two rules in probability can be used to find the expected proportions of offspring of different traits from different crosses. To find the probability of two or more independent events occurring together, apply the product rule and multiply the probabilities of the individual events. The use of the word “and” suggests the appropriate application of the product rule. To find the probability of two or more events occurring in combination, apply the sum rule and add their individual probabilities together. The use of the word “or” suggests the appropriate application of the sum rule.

Additional Self Check Questions

Describe one of the reasons why the garden pea was an excellent choice of model system for studying inheritance.
How would you perform a reciprocal cross for the characteristic of stem height in the garden pea?
The garden pea is sessile and has flowers that close tightly during self-pollination. These features help to prevent accidental or unintentional fertilizations that could have diminished the accuracy of Mendel’s data.
Two sets of P 0 parents would be used. In the first cross, pollen would be transferred from a true-breeding tall plant to the stigma of a true-breeding dwarf plant. In the second cross, pollen would be transferred from a true-breeding dwarf plant to the stigma of a true-breeding tall plant. For each cross, F 1 and F 2 offspring would be analyzed to determine if offspring traits were affected according to which parent donated each trait.

blending theory of inheritance: hypothetical inheritance pattern in which parental traits are blended together in the offspring to produce an intermediate physical appearance

continuous variation: inheritance pattern in which a character shows a range of trait values with small gradations rather than large gaps between them

discontinuous variation: inheritance pattern in which traits are distinct and are transmitted independently of one another

dominant: trait which confers the same physical appearance whether an individual has two copies of the trait or one copy of the dominant trait and one copy of the recessive trait

F 1: first filial generation in a cross; the offspring of the parental generation

F 2: second filial generation produced when F 1 individuals are self-crossed or fertilized with each other

hybridization: process of mating two individuals that differ with the goal of achieving a certain characteristic in their offspring

model system: species or biological system used to study a specific biological phenomenon to be applied to other different species

P 0: parental generation in a cross

product rule: probability of two independent events occurring simultaneously can be calculated by multiplying the individual probabilities of each event occurring alone

recessive: trait that appears “latent” or non-expressed when the individual also carries a dominant trait for that same characteristic; when present as two identical copies, the recessive trait is expressed

reciprocal cross: paired cross in which the respective traits of the male and female in one cross become the respective traits of the female and male in the other cross

sum rule: probability of the occurrence of at least one of two mutually exclusive events is the sum of their individual probabilities

trait: variation in the physical appearance of a heritable characteristic

Candela Citations

Biology. Authored by : Open Stax. Located at : http://cnx.org/contents/[email protected]:1/Biology . License : CC BY: Attribution
Johann Gregor Mendel, Versuche über Pflanzenhybriden Verhandlungen des naturforschenden Vereines in Brünn, Bd. IV für das Jahr, 1865 Abhandlungen, 3–47. (for English translation see http://www.mendelweb.org/Mendel.plain.html) ↵

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Mendel's Laws of Inheritance
Then he crossed F1 progeny and obtained both tall and short plants in the ratio 3:1. To know more about this experiment, visit Monohybrid Cross - Inheritance Of One Gene. Mendel even conducted this experiment with other contrasting traits like green peas vs yellow peas, round vs wrinkled, etc.
Mendel's experiments
Mendel also experimented to see what would happen if plants with 2 or more pure-bred traits were cross-bred. He found that each trait was inherited independently of the other and produced its own 3:1 ratio. This is the principle of independent assortment. Find out more about Mendel's principles of inheritance. The next generations
Mendel's 3 Laws (Segregation, Independent Assortment, Dominance)
Mendel found support for this law in his dihybrid cross experiments. In his monohybrid crosses, an idealized 3:1 ratio between dominant and recessive phenotypes resulted. In dihybrid crosses, however, he found a 9:3:3:1 ratios. This shows that each of the two alleles is inherited independently from the other, with a 3:1 phenotypic ratio for each.
Mendel's Laws of Inheritance
In this article, we will learn about Mendel's Laws of Inheritance, the Characteristics of Mendel experiments, and the Conclusion of the experiments. ... The genotypic ratio in the F2 generation is 1:2:2:4:1:2:1:2:1 and the phenotypic ratio in the F2 generation is 9:3:3:1 This cross helps to study the principle of Independent assortment given ...
PDF Gregor Mendel's Pea Plant Experiment
In this famous experiment, Mendel purposefully cross-pollinated pea plants based on their different features to make important discoveries on how traits are ... plants, with there being a 3:1 ratio for plants that showed the dominant trait for every plant that showed the recessive trait.
Gregor Mendel and the Principles of Inheritance
When conducting his experiments, Mendel designated the two pure-breeding parental ... the various phenotypes were present in a 9:3:3:1 ratio (Figure 6). Next, Mendel went through his data and ...
"Experiments in Plant Hybridization" (1866), by Johann Gregor Mendel
Furthermore, the ratio of these traits within the second generation occurred in roughly a 3:1 proportion, such that out of every four offspring, approximately three possessed the physical trait of one parent and one displayed the physical trait of the other parent. ... Gregor Mendel's Experiments on Plant Hybrids: A Guided Study. New ...
5.10 Mendel's Experiments and Laws of Inheritance
Figure 5.10.5 Mendel's first experiment with pea plants. Figure 5.10.5 shows Mendel's first experiment with pea plants. The F1 generation results from the cross-pollination of two parent (P) plants, and it contains all purple flowers.
Mendel's Experiments: The Study Of Pea Plants & Inheritance
Mendelian inheritance is a term arising from the singular work of the 19th-century scientist and Austrian monk Gregor Mendel. His experiments on pea plants highlighted the mechanisms of inheritance in organisms that reproduce sexually and led to the laws of segregation and independent assortment.
Mendel's Experiments and the Laws of Probability
Mendel's experiments extended beyond the F 2 generation to the F 3 and F 4 generations, and so on, but it was the ratio of characteristics in the P 0 −F 1 −F 2 generations that were the most intriguing and became the basis for Mendel's postulates. Figure 2. In one of his experiments on inheritance patterns, Mendel crossed plants that ...

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Mendel’s 3 Laws (Segregation, Independent Assortment, Dominance)

Mendel’s Experiment

I. Mendel’s Law of Segregation of genes (the “First Law”)

II. Mendel’s Law of Independent Assortment (the “Second Law”)

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Mendel’s Laws of Inheritance | Mendel’s Experiments

Mendel’s Law of Inheritance

Law of Segregation

Law of Independent Assortment

Monohybrid Cross

Dihybrid Cross

FAQs on Mendel’s Laws of Inheritance

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