Question

In a genetic cross having recessive epistasis, ${{F}_{2}}$ phenotypic ratio would be
A. 9:6:1
B. 15:1
C. 9:3:4
D. 12:3:1

Answer 1

Hint: The external characteristic appearance of the individual is referred to as phenotype. Depending upon the mutations of a gene, the phenotype of an individual is decided. In epistasis, one gene controls the other gene.

Complete answer: In genetics, the phenomenon in which the effect of gene mutation is dependent on the absence or presence of mutations in one or more genes is called epistasis. These genes are called the modifier genes as they modify the genetic information by mutation. A mutation refers to the alteration of the nucleotide sequence of the DNA. The genetic background in which the mutations occur determines the effect of the mutation. In epistasis, the phenotype of one gene’s allele is masked by the other gene’s allele. Different types of epistasis are recessive epistasis and dominant epistasis. In recessive epistasis, the recessive allele of one gene masks the effect of the other allele of a gene. Phenotypic ratio of recessive epistasis in ${{F}_{2}}$ is 9:3:4. The progeny produced shows incomplete dominance. Example of recessive epistasis is the dihybrid cross of mouse genes A (agouti) and C (colour) which gives phenotypic ratio for mouse coat colours black, albino, and agouti. In dominant epistasis, the dominant allele of one gene masks the effect of the allele of another gene. The phenotypic ratio of dominant epistasis is 12:3:1. Example of dominant epistasis is the colour of the summer squash fruit. Therefore, in a genetic cross having recessive epistasis, ${{F}_{2}}$ phenotypic ratio would be 9:3:4.
So, option C is the correct answer.

Note: The phenotype is not necessarily dependent on the genotype of an organism. The phenotype of an individual depends on the expression of the genotype. Expressivity is the intensity with which a particular genotype is expressed in a phenotype in a given individual. The individuals which show all the phenotypes have 100% penetrance.