Question

Dominant epistasis ratio is ______.
a. 9:7
b. 9:3:3:1
c. 9:3:4
d. 12:3:1
e. 9:6:1

Answer 1

Epistasis is a natural phenomena that is concerned with transfer and management of genes designing phenotype of the organism. It reflects a kind of bond shared between the two types of genes (that is, dominant and recessive genes).

Complete Answer
The above given ratios are standard values of various types of Epistasis designs. Following is a detailed description of such designs:
9:7 is a representative of Complementary gene ratio. In this kind of gene design, two independent genes that are capable enough of representing similar kinds of traits form an all new kind of dominating trait by interacting with each other such that the new trait formed is generally a combination of the latter two. Conventionally, the dihybrid cross represents phenotypes in ratio 9 : 3 : 3 : 1 but in this case the last three alleles combine to form new and single, dominating allele patterns due to which only two phenotypes are produced. Thus, the ratio turns out to be 9:7. Therefore, option 'a' cannot be the correct answer.
9:3:3:1 is a representative of Dihybrid phenotypic gene ratio. This is the most common type of phenotypic representation of a dihybrid cross in which four individual genes are selected to produce the desired progeny. It produces four different types of phenotypes in its progeny. Therefore, option 'b' cannot be the correct answer.
9:3:4 is representative of the Recessive epistasis gene ratio. In this genetic design, two genes work in collaboration that are interdependent of each other. Therefore, three phenotypes are produced. Thus, option 'c' cannot be the correct answer.
12:3:1 is representative of the Dominant epistasis ratio. In this genetic design, Dominant epistasis is a natural phenomena in which a dominant allele covers all other alleles of another gene such that none of them are able to express themselves. They also produce three types of phenotypes but in this (12:3:1) ratio.

Thus, option 'd' is the correct answer.

Note:
In a dihybrid cross, four genotypes are crossed that leads to formation of four different types of phenotypes for progenies. Also, 12 different allele patterns are formed following the pattern of 9:3:3:1 such that 1 represents the least occuring trait and 9 represents the most.