Extra fingers in a person is a dominant feature. A normal number of fingers is a recessive sign.

Extra fingers in a person is a dominant feature. A normal number of fingers is a recessive sign. What is the probability of having a six-toed grandchild in a six-toed grandfather if the child’s parents have a normal number of toes?

Let’s designate the gene that causes the development of a six-fingered hand as P, then the gene for a normal five-fingered hand will be p.

Since the six-toed grandfather has a normal son with the pp genotype, it should be concluded that he is a heterozygote – pp.

Since people with a normal five-toed hand are homozygous for the recessive gene, pp, they produce only one type of germ cell, p. All of their children will have a pp genotype and a normal hand structure.

Consequently, the probability of having a six-fingered grandson in a six-fingered grandfather, provided that a son and his wife have a normal five-fingered hand, is 0%.