Suppose that tail length in horses is controlled by a number of additive genes, each of which has 2 alleles. When a true-breeding female with a 12-inch tail is mated to a true-breeding male with a 36-inch tail, all F1 offspring have the same tail length. When the F1 offspring are mated to each other, 7 different tail lengths are seen in the F2 offspring. How many genes contribute to tail length? (Note: Assume that the genes segregate independently and that environmental influence on tail length is negligible.) nothing genes

3 genes

Explanation:

This is a case of polygenic trait with additive alleles which means that the phenotype is in a continuous spectrum and depends on the number of dominant alleles present. Here there are 7 phenotypes produced in F2 generation which means that there are 7 phenotypic classes.

In case of additive genes,

number of phenotypic classes =  2n + 1 where, n = number of genes involved

Here,

2n + 1 = 7

so n = (7 -1)/2 = 3

Hence, 3 genes are responsible for tail length. Out of the seven phenotypes, three will be nearer to the shorter tail length, one will be the average intermediate length and other three will be nearer to the longer tail length.