Question

The phenotypic ratio obtained in quantitative inheritance of a dihybrid cross is

• A. 1 : 2 : 1
• B. 1 : 4 : 6 : 4 : 1
• C. 1 : 6 : 15 : 20 : 15 : 6 : 1
• D. 9 : 3 : 3 : 1

Answer 1

The correct option is B 1 : 4 : 6 : 4 : 1

Quantitative traits are those whose expression is controlled by two or more genes, each of which affects the phenotype in the same way. These genes are called quantitative genes, or polygenes. Inheritance for each of these genes is incompletely dominant, so that the heterozygote is intermediate between the two homozygotes. The ratio of phenotypes in polygenic inheritance depends on the number of genotypes that produce a particular phenotype. Further, even though genotypes represent different combinations of alleles from the F${}_1$ parents, they would all have the same phenotype and would all be equally likely to occur (1/16), so the chance of having one or the other of them would be 4/16. More genetic combinations (6/16) would produce some more phenotypes. Thus the phenotypic ratio obtained from the quantitative inheritance of trait controlled by two pairs of genes of a dihybrid cross will be 1:4:6:4:1.