Dihybridism
Dihybridism is a cross between two different lines that differ in two observed traits. According to Mendel’s law of independent assortment, genes for different traits can segregate independently during the formation of gametes. This applies only to genes localized on various somatic chromosomes.
When crossing, the P generation is either AABB x aabb or AAbb x aaBB. In both these examples there is a crossing between dominant homozygote and recessive homozygote for each gene. The offspring F1 of this cross is uniform and heterozygous for both genes – AaBb.
In F2 generation Mendel has identified phenotypic segregation ratio: (3/4 + 1/4) x (3/4 + 1/4) = 9:3:3:1. There are 9/16 subjects with both dominant phenotypes, 6/16 (3/16+3/16) subjects with one dominant and one recessive phenotype and 1/16 subjects with both recessive phenotypes. The segregation ratio for backcross is 1:1:1:1.
For estimating a number of phenotypes, a good rule of thumb is 2n where n represents number of genes. Similarly, the number of genotypes in F2 is 3n. Phenotypic segregation ratio in F2 is counted as (3/4 + 1/4)n or (1/2 + 1/2)n when backcrossing.
