Mendel's Experiment

1.0Gregor Johann Mendel

Mendel was an Austrian monk and scientist who first worked on heredity experiments and gave theory in 1866 but people of that time couldn't understand him and rejected his theory. Later in 1900, three different scientists from 3 different places also found the same result.

They were Hugo de Vries, Tschermak and Correns. They rediscovered the theory of heredity proposed by Mendel. So, Mendel is called “Father of genetics”.

Mendel said that each character is controlled by a pair of factors which is now known as gene.

2.0Mendelian Experiments

In 1856-57, he started his historical experiments of heredity on pea (Pisum sativum) plant. 

Advantages of Pisum Sativum

Pisum sativum is a unique plant selected by Mendel for his experiments which had following importance:

Before starting his experiments, he obtained the pure breed. A pure breed is the one in which a plant is crossed with same allelic type and results in all of its offspring of same kind. Tall plant only results in tall plant for any number of generations and so does other characters too.

Pairs of Allelic Characters Found In Garden Pea Plant

3.0Monohybrid Cross

It is a cross in which only one character is studied at a time. 

In his first monohybrid cross, Mendel crossed a pure breed tall plant (TT) and a pure breed dwarf plant (tt). 

And he found that in F₁ generation only tall plant were born out of these seeds, no intermediate character or mixing or blending was observed by him. Mendel termed tallness as dominant and dwarfness as a recessive character.

He repeated the same experiment taking other characters too and always found same results.

Mendel’s Postulates

To explain the experiment, he said that each character is controlled by a pair of factors.

During gamete formation, the paired factors got segregated and moved to different gametes. 

When these gametes fused together, they used to restore paired state again.

In heterozygous conditions, out of the two alleles only one allele is able to express its effect and it is called dominant allele and other which does not show its effect in heterozygous condition is called recessive allele.

When F₁ plants are grown and allowed to self-pollinate he got many seeds and when they were grown, he found that the phenotypic  ratio of tall plants and dwarf plants was 3:1 respectively. 

The dwarfness which was not visible in F₁ generation reappeared in F₂ generation.

4.0Dihybrid Cross

It is a cross in which 2 characters are simultaneously studied. 

In Mendel's first dihybrid cross, he crossed a pure breed plant having yellow and round (RRYY) seeds with another pure breed plant having green and wrinkled (rryy) seeds. 

F₁ generation had plants with yellow and round seeds.

It means yellow and round seed characters are dominant over green and wrinkled seeds.

He allowed these plants produced in F₁ generation to self-pollinate and he expected the same 3 : 1 in F₂ generation.

But to his surprise the ratio was all new 9 : 3 : 3 : 1 in which beside 2 parental varieties, 2 new varieties were also produced. So, the ratio was 9 yellow and round ; 3 yellow and wrinkled; 3 green and round ; 1 green and wrinkled.

This could be explained only when beside the parental combination of factors (RY) and (ry), two new combinations of factors are also produced as follows (rY) and (Ry). 

Phenotypic Ratio

The occurrence of four types of plants in the F₂ generation of dihybrid cross shows that during gamete formation in F₂ generation, the two factors of a character are independently assorted and get randomly rearranged in the offspring.  So, four types of gametes are formed as RY, rY, Ry, ry from each parent. When they are randomly fused, they resulted following combinations. So, this is how the 9 : 3 : 3 : 1 ratio could be understood.

5.0Mendel's Laws of Inheritance

Human beings were always interested to know more about the law of inheritance. On the basis of Mendel's work, 3 basic laws of inheritance were proposed.

(i) Law of Dominance

(ii) Law of Segregation

(iii) Law of Independent Assortment

Law of Dominance

In crossing between organisms pure for contrasting characters of a pair, only one character of the pair appears in the F₁ generation. This character is termed as dominant while the one which does not express itself in F₁ generation is termed as recessive.

Law of Segregation 

Different alleles or genes of a character remain together in an individual and segregate randomly at the time of gamete formation. This is also known as the Law of purity of gametes.

Law of Independent Assortment

This law states that when individuals differing in two or more than two pairs of contrasting characters are crossed, the inheritance of any one pair is not affected by the presence of the other.

E.g., The inheritance of seed shape character is not related to the seed colour character. Rather, the two characters inherit independently of each other.

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