1. A cross which involves two pairs of alleles is called a dihybrid cross. A phenotypic ratio of 9 : 3 : 3 : 1 obtained in the F generation of a dihybrid cross is called a dihybrid ratio.
(2) Thus for example, when we cross a true breeding pea plant bearing round and yellow seeds with a true breeding pea plant bearing wrinkled and green seeds we get pea plants bearing round and yellow seeds in the F generation.
(3) When F1 plants are selfed, we get a ratio of 9 : 3 : 3 : 1 in the F2 generation, where 9 plants bear yellow round seeds, 3 plants bear yellow wrinkled seeds, 3 plants bear green round seeds and 1 plant bears green wrinkled seeds.
(4) Parents (P1) : RRYY × rryy
Gametes of P1 RY and ry
F1 generation : RrYy(Yellow round)
On selfing F1 : RrYy × RrYy
Gametes of F1 : RY, Ry, rY, ry
P2 generation:
Round Yellow : 9 Round green : 3 Wrinkled
yellow : 3 Wrinkled green : 1
Phenotypic ratio : 9 : 3 : 3 : 1
Genotypic ratio : 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1
