Mendel’s Law of Inheritance | Genetics

In this article we will discuss about Mendel’s law of inheritance.

Mendel’s Experiment:

Gregor Johann Mendel conducted hybridisation experiments on garden pea (Pisum sativum) for seven years (1856-1863) and proposed the laws of inheritance in living organisms. He is also known as Father of Genetics.

Mendel Experimental Material:

He selected garden pea plant as a sample for following reasons:

(i) Pea is available in many varieties on a large scale to observe alternate traits.

(ii) Peas are self-pollinated and can be cross-pollinated also to prevent self-pollination.

(iii) These are annual plants with a short life cycle. So, several generations can be studied within a short period.

(iv) Pea plants could easily be raised, maintained and handled.

(v) Many varieties are available with distinct characteristics. Which plants provide many easily detectable contrasting characters.

Mendel conducted artificial pollination/cross-pollination experiments using several true-breeding pea lines. A true-breeding line refers to one that have undergone continuous self-pollination and showed stable trait inheritance and expression for several generations. Mendel selected 14 true-breeding pea plant varieties, as pair, which were similar except for one character with contrasting traits.

A List of Contrasting Traits studied by Mendel in Pea Plant

Mendel’s Procedure:

(i) Mendel observed one trait at a time. For example, he crossed tall and dwarf pea plants to study the inheritance of one gene.

(ii) He hybridised plants with alternate forms of a single trait (monohybrid cross). The seeds produced by this cross were grown to develop into plants of Fillial₁ progeny or F₁-generation (F₁-plants).

(iii) He then self-pollinated the tall F₁ -plants to produce plants of Fillial₂ progeny or F₁-generation.

(iv) In later experiments, Mendel also crossed pea plants with two contrasting characters known as dihybrid cross.

Mendel’s Observations:

(i) In F₁generation, Mendel found that all pea plants were tall and none was dwarf.

(ii) He also observed other pair of traits and found that F₁ always resembled either one of its parents and the traits of other parent was not found in this generation.

(iii) In F₂-generation, he found that some of the offsprings were ‘dwarf, i.e., the character which were not seen in F₁-generation was expressed in F₂.

(iv) These contrasting traits (tall/dwarf) did not show any mixing either in F₁ or in F₂-generation.

(v) Similar results were obtained with the other traits that he studied. Only one of the parental traits was expressed in F₁-generation, while at F₂ stage, both the traits were expressed in the ratio of 3:1.

(vi) Mendel also found identical results in dihybrid cross as in monohybrid cross.

(vii) The trait that appeared in the F₁ is called dominant trait, while the other trait is recessive trait.

(viii) In tall/dwarf traits, tallness is dominant over dwarfness that is recessive.

Mendel’s Inferences:

Following inferences were made by Mendel based on his observations:

(i) He proposed that some factors pass down from parent to offsprings through the gametes. Now-a-days these factors are known as genes.

(a) Genes are hence, the units of inheritance.

(b) Genes which code for a pair of contrasting traits are known as alleles, i.e., they are slightly different forms of the same gene.

(ii) Genes occur in pairs in which, one dominates the other called dominant factor and expresses itself, while the other remains hidden and is recessive.

(iii) Allele can be similar in case of homozygote TT or tt and dissimilar in case of heterozygote Tt.

(iv) In a true-breeding tall or dwarf pea variety, the allelic pair of genes for height are identical or homozygous.

(v) TT and tt are called genotype of the plant, while the term tall and dwarf are the phenotype.

(vi) When the tall and the dwarf plant produce gametes by the process of meiosis, the alleles of the parental pair segregate and only one of the allele gets transmitted to a gamete.

Thus, there is only a 50% chance of a gamete containing either allele, as the segregation is a random process.

(vii) During fertilisation, the two alleles, T from one parent and t from other parent are united to produce zygote, that has one T and one t allele or the hybrids have Tt.

(viii) Since, these hybrids contain alleles which express contrasting traits, the plants are heterozygous.

Punnett Square:

It is a graphical representation to calculate the probability of all possible genotypes of off springs in a genetic cross (Fig.5.2). The production of gametes by the parents, the formation of zygote, the F₁ and F₂ – germinations can be explained by Punnett square. It was developed by British geneticist RC Punnett.

Mendel’s Law:

Mendel’s laws of inheritance are based on his observations on monohybrid crosses.

He proposed the following laws of inheritance:

1. Law of Dominance (First Law):

The law of dominance states that when two alternative forms of a trait or character (genes) are present in an organism, only one factor expresses itself in F₁-progeny and is called dominant, while the other that remains masked is called recessive.

This law is used to explain the expression of only one of the parental characters in a monohybrid cross in the F₁ -generation and the expression of both in the F₂-generation. It also explains the proportion of 3:1 obtained in theF₂-generation.

2. Law of Segregation (Second Law):

This law states that the alleles do not show any blending and both the characters are recovered as such in the F₂-generation, though one of these is not seen in the F₁ -generation.

Due to this, the gametes are pure for a character. The parents contain two alleles during gamete formation.

The factors or alleles of a pair segregate from each other such that a gamete receives only one of the two factors.

3. Law of Independent Assortment (Third Law):

This law states that when two pairs of traits are combined in a hybrid, segregation of one pair of character is independent of the other pair of characters at the time of gamete formation.

It also get randomly rearranged in the offsprings producing both parental and new combinations of characters. The law was proposed by Mendel, based on the results of dihybrid crosses, where inheritance of two traits were considered simultaneously.