Principles of Inheritance and Variation

Laws of Inheritance


Gregor Johann Mendel was a scientist who is recognized as the Father and Founder of genetics. Mendel conducted many experiments on the pea plant (Pisum sativum) between 1856 and 1863. He studied the results of the experiments and deducted many observations. Thus, laws of inheritance or Mendel’s laws of inheritance came into existence. Before learning about Mendel’s laws of inheritance, it is important to understand what the experiments performed by Mendel were.

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Mendel’s Experiments on Pea Plant

Mendel after carefully study selected the pea plant for many reasons:

  • The pea plants were easy to grow and maintain
  • It has many clearly distinct and contrasting characters.
  • The pea plant is an annual plant and so many generations of the plant can be studied in a short period of time.
  • Peas are naturally self-pollinating but can also be cross-pollinated.

Mendel made a list of contrasting characters which he studied:

Laws of Inheritance

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Mendel structured his experiments in a way that he would observe one pair of contrasting characters at one time. He began his experiments using purebred lines for contrasting characters.

He cross-pollinated two pure lines for contrasting characters and the resultant offsprings were called F1 generation(also called the first filial generation). The F1 generations were then self-pollinated which gave rise to the F2 generation of second filial generation.

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Results of Mendel’s Experiments

Let us look at the results of Mendel’s experiments on crossing a pure tall pea plant with a pure short pea plant.

  • In the F1 generation, Mendel observed that all plants were tall. there were no dwarf plants.
  • In the F2 generation, Mendel observed that 3 of the offsprings were tall whereas 1 was dwarf.
  • Similar results were found when Mendel studied other characters.
  • Mendel observed that in the F1 generation, the characters of only one parent appeared whereas, in the F2 generation, the characters of the other parent also appeared.
  • The characters that appear in the F1 generation are called dominant traits and those that appear for the first time in the F2 generation are called recessive traits.

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  • The genes that are passed from the parents to the offsprings exist in pairs. These pairs are called alleles.
  • When the two alleles are the same, they are called homozygous. When both the alleles are different, they are called as heterozygous.
  • Dominant characters are described using capital letters and recessive using small letters. For example, the dominant genes for tallness in a pea plant are written as TT and recessive genes as tt. The heterozygous genes are written as Tt where the plant appears tall has the recessive gene which might express itself in the future generations.
  • The appearance of the plant is known as the phenotype whereas the genetic makeup of the plant is called the genotype. So, a plant with Tt genes appears tall phenotypically but has a recessive gene.
  • During gametogenesis, when the chromosomes become half in the gametes, there is a 50% chance of either of the alleles to fuse with that of the other parent to form a zygote.

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Based on these observations, Mendel proposed three laws.

Laws of Inheritance

Mendel proposed three laws:

  • Law of Dominance
  • The Law of Segregation
  • Law of independent assortment

Law of Dominance

Laws of Inheritance

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This law states that in a heterozygous condition, the allele whose characters are expressed over the other allele is called the dominant allele and the characters of this dominant allele are called dominant characters. The characters that appear in the F1 generation are called as dominant characters. The recessive characters appear in the F2 generation.

Law of Segregation

Laws of Inheritance

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This law states that when two traits come together in one hybrid pair, the two characters do not mix with each other and are independent of each other. Each gamete receives one of the two alleles during meiosis of the chromosome.

Mendel’s law of segregations supports the phenotypic ratio of 3:1 i.e. the homozygous dominant and heterozygous offsprings show dominant traits while the homozygous recessive shows the recessive trait.

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 Laws of Inheritance

Law of Independent Assortment

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This means that at the time of gamete formation, the two genes segregate independently of each other as well as of other traits. Law of independent assortment emphasizes that there are separate genes for separate traits and characters and they influence and sort themselves independently of the other genes.

This law also says that at the time of gamete and zygote formation, the genes are independently passed on from the parents to the offspring.

Solved Example for You

Q1: What is the genotype of an individual?

  1. Physical appearance
  2. Genetic makeup
  3. Nature of the individual
  4. Blended characteristics of the individual

Sol. The correct answer is the option ”b”. The genetic makeup of the individual is known as the genotype whereas the physical appearance of the individual is known as the phenotype.

FAQ’s for You

Q1. State the three Mendel’s laws of inheritance

Answer: Mendel’s Laws of inheritance can be described as;
1. The Law of Dominance: The offspring always exhibits a dominant trait. From the two alleles received from parents, the only dominant allele is expressed.
2. The Law of Segregation: The two copies of each chromosome will be separated from each other, causing the two distinct alleles located on those chromosomes to segregate from one another.
3. The Law of Independent Assortment: The traits inherited through one gene will be inherited independently of the traits inherited through another gene because the genes reside on different chromosomes that are independently assorted into daughter cells during meiosis.

Q2. Enlist Mendel’s law of Inheritance.

Answer: Mendelian inheritance is a type of biological inheritance that follows the laws originally proposed by Gregor Mendel in 1865 and 1866 and re-discovered in 1900. Between 1856 and 1863, Mendel cultivated and tested some 5,000 pea plants. From these experiments, he induced two generalizations which later became known as Mendel’s Principles of Heredity or Mendelian inheritance. Mendel discovered that, when he crossed purebred white flower and purple flower pea plants (the parental or P generation), the result was not a blend. Rather than being a mix of the two, the offspring (known as the F1 generation) was purple-flowered. When Mendel self-fertilized the F1 generation pea plants, he obtained a purple flower to white flower ratio in the F2 generation of 3 to 1. Mendel’s law of inheritance are as follows:
Law of segregation: During gamete formation, the alleles for each gene segregate from each other so that each gamete carries only one allele for each gene.
Law of independent assortment: Genes for different traits can segregate independently during the formation of gametes.
Law of dominance: Some alleles are dominant while others are recessive; an organism with at least one dominant allele will display the effect of the dominant allele.

Q3. Short / Long answer type questions.
Discuss Mendels laws of inheritance. Which one of these laws you consider the most important and why?

Answer: Mendel during his study on pea plants stated three laws of inheritance. These were:
1. Law of dominance: A dominant gene will express itself over the recessive gene.
2. Law of segregation: Parental genes are randomly separated to the germ cells such that each germ cell receives only one gene from each pair.
3. Law of independent assortment: Genes for different traits are sorted separately from one another such that the inheritance of one trait is not dependent on the inheritance of the other trait.
Out of these three laws, the law of segregation is the most important law because it has no exceptions and is universally accepted.

Q4. Mendel’s law of inheritance composed of?

Answer: Mendel proposed the law of inheritance of traits from the first generation to the next generation. Law of inheritance is made up of three laws: Law of segregation, law of independent assortment and law of dominance.

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