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

Here crossing is AaBbCc x AaBbCc

As it is trihybrid cross,

we know, Number of zygotes formed =  4 n , where n = 3

4 3 = 64

Given ratio of offsprings = 1 : 6 : m : 20 : m : 6 : 1

Now,  1 + 6 + m + 20 + m + 6 + 1 = 64

34 + 2m = 64

2m = 30

=> m = 15.