Question

What is the probability that the son will be a carrier of the recessive allele?

Ectrodactyly (lobster claw) is an inherited physical defect in humans occurring in homozygous recessive individuals. If 2 normal parents had a daughter affected with this condition and a normal son...

Answer 1

`2/3` chance or ~`67%`

Explanation:

First we have to know the genotype of the parents. Lets call the dominant allele for ectrodactyly `color(green)"E"` and the recessive allele `color(red)"e"`.

The disease is homozygous recessive, so the daughter must have two recessive alleles, her genotype is `color(red)"ee"`. This genotype is only possible if both parents are heterozygous (`color(green)"E"color(red)"e"`).

Knowing this we can make a cross table to show all possible genotypes of the first generation offspring:

Since we know that the son is not affected, he is either homozygous dominant (`color(green)"EE"`) or heterozygous (`color(green)"E"color(red)"e"`). The ratio homozygous to heterozygous is 1 : 2.

In this case we are interested in the chance that he is a carrier, which is the case when he is heterozygous. As seen above, this chance is 2 out of 3, which is `2/3` chance or ~`67%`.