Question

A polygenic trait is controlled by 3 genes A,B and C. In a cross AaBbCc×AaBbCc, the phenotypic ratio of the offsprings was observed as 1:6:x:20:x:6:1 What is the possible value of x?

• A. 3
• B. 9
• C. 15
• D. 25

Answer 1

The correct option is C

15

The correct option is C.

Explanation for correct option:

1. In polygenic inheritance, a particular character is controlled by three different genes. In general, phenotypic ratio is 1:6:15:20:15:6:1 instead of 27:9:9:9:3:3:3:1 in second filial​ generation.
2. Here, the genotype of parents is AaBbCc and AaBbCc.
3. Gametes formed are ABC, ABc, Abc, abc, aBC, aBc, abC, AbC.
4. The phenotypic ratio would be 1:6:15:20:15:6:1.
5. Hence, x will be 15.

Final answer: In a cross AaBbCc×AaBbCc, the phenotypic ratio of the offsprings was observed as 1:6:x:20:x:6:1, so the value of x=15.