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 $x$?

The correct option is C $15$
In polygenic inheritance, a particular trait is controlled by 3 different genes. The given problem is of polygenic inheritance. In this, phenotypic ratio is 1:6:15:20:15:6:1 instead of 27:9:9:9:3:3:3:1 in F${}_2$ generation. In the given example, the genotype of parents is AaBbCc and AaBbCc. They will produce gametes ABC, ABc, Abc, abc, aBC, aBc, abC, AbC. The phenotypic ratio will be 1:6:15:20:`15:6:1.

Thus, the correct answer is '15.'