Question

Explain with an example the inheritance of the dihybrid cross. How is it different from monohybrid cross?

Answer 1

The dihybrid cross involves the inheritance of two pairs of contrast characteristics, round–yellow seeds and wrinkled–green seeds. When pea plants having round – yellow seeds cross-bred with pea plants having wrinkled – green seeds, in the first generation (F₁), only round yellow seeds were produced.
No wrinkled–green seeds were obtained. Round yellow colour seeds were dominant and wrinkled-green seeds were recessive.

When round–yellow seeds were cross-bred by self-pollination, four types of seeds having different combinations of shape and colour were obtained in the F₂ generation. They were round- yellow, round-green, wrinkled-yellow, and wrinkled – green seeds.

Parents (P) Parental gametes First generation (F₁) 

Round, Yellow - 9 Wrinkled, Yellow - 3

Round, Green - 3  Wrinkled, Green - 1

Second generation (F₂)

Dihybrid Cross

A dihybrid cross produced four types of F₂ offsprings in the ratio of 9 with two dominant traits, 3 with one dominant trait and one recessive trait, 3 with another dominant trait and another recessive trait and one with two recessive traits. The new combinations of traits with round green and wrinkled yellow had appeared in the dihybrid cross (F₂ generation). The ratio of each phenotype of seeds in the F₂ generation is 9 : 3 : 3 : 1. This is known as the Dihybrid ratio.

Difference between a monohybrid cross and dihybrid cross:

Monohybrid cross:

A Monohybrid cross is a genetic cross, that involves a single pair of genes, which is responsible for one trait.
Parents differ by a single trait.
The Monohybrid ratio in F₂ generation is 3 : 1.

Dihybrid cross:

Dihybrid cross is a genetic cross, that involves two pairs of genes, which are responsible for two traits,
The parents have two different independent traits.
The dihybrid ratio in the F₂ generation is 9 : 3 : 3 : 1.