Rules for the Inheritance of Traits - Mendel's Contributions

Inheritance can be defined as the process of passing the traits or genetic information from parents to their offspring, this can be done either by sexual reproduction or asexual reproduction. The process of heredity is dependent upon inheritance and it is the reason that the offspring are similar to the parents, due to this the members of the same family possess similar characteristics.

Read Also: How do Organisms Reproduce?

Gregor Mendel, also known as the ‘father of genetics’ studied the patterns of inheritance in mid-19th century between year 1856 and 1863, with his famous pea plant experiment. After his Pea Plant experiment, he established several rules of heredity, which are now known as Known as Mendelian’s inheritance.

He carried out his experiments on inheritance on pea plant taking 7 features of the plant which included 

Due to following reasons Mendel selected pea plant for his experiment : -

1. The pea plant is easy to grow and maintain.
2. Pea Plant naturally self-pollinate but can also be cross-pollinated.
3. Pea Plant is an annual plant; therefore, many generations can be studied within a short period of time.
4. Pea Plant has several contrasting characters.

To determine the law of inheritance, 2 experiments were conducted by Mendel:

1. Inheritance of One gene or Monohybrid Cross Experiment
2. Inheritance of two genes or Dihybrid cross Experiment.

Read More Difference between Monohybrid and Dihybrid

Inheritance of One Gene/Monohybrid Cross

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In this experiment, one trait or one character was considered, Mendel took two pea plants of different traits (one short and one tall) and crossed them. After crossing it was found that the first-generation offspring were tall and he called it F1 progeny or generation. This visible tall feature was called as dominant trait and the nonvisible short feature was termed as recessive trait.

Then this F1 progeny was allowed to self-replicate and obtained both tall and short plants however short plants were obtained in very less number. Every 3 plants which showed the dominant trait whilst 1 plant showed the recessive trait, therefore the ratio came out to be 3:1.

Parental             TT X tt

F1 Generation       Tt Tt Tt tt

This experiment was conducted with other contrasting traits of pea plant such as green peas vs yellow peas, round vs wrinkled, etc. In every case, the ratio was found to be similar.

Inheritance of Two Genes or Dihybrid Cross Experiment

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In this experiment, Mendel considered two traits or two contrasting characters, each having two alleles. He crossed wrinkled (rr)-green seed (yy) and round (RR)-yellow (YY) seeds and observed that all the first-generation progeny (F1 progeny) were round (RR)-yellow (YY). This meant that dominant traits were the round shape and yellow colour.

Mendel crossed the first generation (F1) of plants all came out to be heterozygous. It can be represented in the following manner.

RRYY x rryy

He then self-pollinated the F1 progeny and obtained 4 different traits wrinkled-yellow, round-yellow, wrinkled-green seeds and round-green in the ratio 9:3:3:1. The genotype of the F1 generation plants was RrYy. These were called di-hybrids.

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

1. Law of Dominance
2. Law of Segregation
3. Law of Independent Assortment

Law of Dominance

Also known as the Mendel’s first law of inheritance, when two parents of two contrasting characters are crossed then the dominant trait shows in the next generation while the recessive trait is suppressed. The alleles that are suppressed are called as the recessive traits while the alleles that determine the trait are known as the dominant traits.

Law of Segregation

Also known as Mendel’s Second law of inheritance, states that for any trait each pair of alleles of a gene split and one gene passes from each parent to the offspring. So, during the production of gametes, two copies of each hereditary factor segregate so that offspring acquire one characteristic from each parent. 

Law of Independent Assortment

Also known as Mendel’s third law of inheritance, according to this law of independent assortment completely different pairs of alleles are passed to the offspring independent of each other. So, we can also say that according to independent assortment a pair of trait segregates independently of another pair during gamete formation. As the individual heredity factors group independently, different traits get equal opportunity to occur together.