Question

In a dihybrid cross, when one pair of alleles show incomplete dominance, genotypic ratio comes to

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

Answer 1

The correct option is B 1:2:2:4:1:2:1:2:1

• Effect of complete or incomplete dominance is exhibited in the phenotype of the organism, the genotype remains unaffected by this. Hence, the genotypic ratio of a dihybrid cross will always remain the same, i.e., 1 : 2: 1: 2: 4: 2: 1: 2: 1.
• Option B is the correct answer. Incomplete dominance is the condition when none of the factors of a gene is dominant, the phenotype of a heterozygous dominant individual blend of dominant and recessive traits. Here, the monohybrid cross between two pure varieties gives 1: 2: 1 phenotypic ratio in F${}_2$ generation which makes option D wrong.
• The phenotypic ratio of F${}_2$ generation of a dihybrid cross is 9 : 3 : 3: 1 provided that the alleles of both follow dominant-recessive relationship which makes option C wrong.
• When alleles of one gene follow dominant and recessive relationship but the alleles of the second gene are codominant, the F${}_2$ phenotypic ratio of a dihybrid cross becomes 3: 6 : 3: 1 : 2: 1. This makes option A wrong.