Question

In genetic crosses, showing recessive epistasis such as mice coat colour, F${}_2$ phenotypic ratio is

• A. 9 : 3 : 4
• B. 9 : 6 : 1
• C. 12 : 3 : 4
• D. 13 : 3

Answer 1

Answer: (A)

9 : 3 : 4

A gene is said to be epistatic gene when its suppresses the effect of another gene on a different locus. In the terms of Bateson, a gene which masks the effect of another gene is called as an epistatic gene. There are only three different phenotypes for hair colour: agouti, black and albino. The individual with genotype recessive for both traits, i.e., aabb, has the same albino phenotype as the aaBB and aaBb individuals due to epistasis. The albino phenotype masks any phenotype that might be caused by the recessive homozygous, recessive bb genotype. AA or Aa mice are agouti. Mice with genotype aa are albino because all pigment production is blocked, regardless of the phenotype at the second locus. At the second locus, the B allele (agouti coat) is dominant to the b allele (black coat). It thus, gives a ratio of 9:3:4 (agouti : black : albino).