Q. A polygenic trait is controlled by 3 genes A, B and C. In a cross AaBbCc$\times AaBbCc$, 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
Explanation
Polygenes show polygenic inheritance or quantitative inheritance. As genes are present on different chromosomes they will segregate independently during meiosis. Number of individuals in $F_{2} = 4^{n}$ $n = number\ of\ genes\ present$ $= 4^{3} = 64\ individuals$ 1+6+2x+20+6+1=64 2x=64-34 $x = \frac{30}{2} = 15$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.