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(D) Polygenic inheritance involves the control of a single trait by multiple genes. For a trait controlled by $n$ pairs of genes,the phenotypic ratio

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$1:6:15:20:15:6:1$

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(D) Polygenic inheritance involves the control of a single trait by multiple genes. For a trait controlled by $n$ pairs of genes,the phenotypic ratio in the $F_2$ generation of a cross between trihybrids (heterozygous for all genes) follows the binomial expansion of $(a + b)^{2n}$. Here,$n = 3$ (three pairs of genes). The expansion is $(a + b)^{2 	imes 3} = (a + b)^6$. Using Pascal's triangle or binomial coefficients for the power of $6$,the coefficients are $1, 6, 15, 20, 15, 6, 1$. Therefore,the phenotypic ratio is $1:6:15:20:15:6:1$.

In a case of polygenic inheritance,a trait is controlled by $3$ pairs of genes. If an individual is heterozygous for all $3$ alleles,what will be the phenotypic ratio produced by self-crossing?

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