Explain Law of independent assortment by Experiment of hybridization on Pea Plant

Mendel designed experiments in which two characters were examined simultaneously in order to know how two pairs of factors behave in relationship to one another when followed in the same cross. A cross involving two pairs of contrasting traits is called dihybrid cross.

Shape of Pea seeds can be either round or wrinkled. The round factor (R) is dominant to wrinkled (r). Similarly the colour of seeds may be yellow or green. The yellow factor (Y) is dominant to wrinkled (y). When plants homozygous for round seeds are crossed with those having wrinkled seeds, the F₂ generation showed the ratio of 3:1 between rounds and wrinkled. Similar F₂ratio was obtained when seeds with yellow factor were crossed with seeds with green factor.

Mendel crossed a plant with round yellow seeds (RRYY) to a plant with wrinkled green seeds (rryy). All F₁ dihybrids produced round yellow seeds (RrYy). This is a case of dominance and the results were expected. But whether genes for round and wrinkled and yellow and green would stay together when F₁dihybrids are allowed to self pollinate. The most suitable way to check it was to make a test cross between F₁ dihybrid and double recessive parents. A ration of 1: 1: 1: 1 was obtained when the cross was made.

Formation of four classes of gametes suggests that genes for seed shape and seed colour are behaving independently. In addition to old combination the F₁ dihybrids also produce new combinations Ry and rY. 


Thus the factors are free to form new combinations therefore it can be said that the factors assort independently knowing that dihybrid individuals form four different kinds of gametes in equal proportions, 16 possible combination are expected, when Mendel crossed F₁hybrids, he obtained, 315 round yellow (RY), 108 round green (Ry), 101 round wrinkled (rY) and 32 wrinkled green (ry). Statistical analysis shoed a ratio of 9: 3: 3: 1. This is called dihybrid ration when crosses were made for remaining five pairs of genes, they followed to same rule and produced same results i.e. the dihybrid and test cross ratio were established. These results led Mendel to formulate his second law of inheritance, the law of independent assortment. It states that: “The Members of one pair of factory (genes) segregate independently of members of other pairs of gamete formation as pair Rr segregated independent of Yy.