Question

In complementary genes, the dihybrid ratio of 9 : 3 : 3 : 1 is modified to

• A. 9 : 7
• B. 12 : 3 : 1
• C. 15 : 1
• D. 13 : 3

Answer 1

The correct option is A 9 : 7

When one gene inhibits the expression of other; the dominant alleles of one gene (A) in homozygous and heterozygous (AA, Aa) condition and homozygous recessive alleles bb of another gene (B) produce the same phenotype. Here, the rest genotypes produce contrasting phenotype; thus only two phenotypes. The resultant F${}_2$ ratio is 13: 3. The non-allelic gene interaction in which the trait is governed by duplicate genes for which presence of the single dominant gene is enough to produce the dominant trait, the dihybrid ratio becomes 15 : 1. Here, all genotypes carrying a single/multiple dominant genes show dominant phenotype. Dominant epistasis is the non-allelic gene interaction where the presence of one dominant gene masks the expression of another dominant gene, dihybrid ratio becomes 12 : 3 : 1. Complementary genes are the non-allelic genes which have their independent effect but produce a new trait when their dominant forms are present together. A dihybrid cross of complementary genes gives 9 : 7 phenotypic ratio. For example, flower colour in sweet pea is purple under double dominant genotypes (C-P-), white in all other genotypic combinations. Thus, the correct answer is option A.