A child with phenylketonuria was born in a family of healthy parents. What is the probability of having a healthy child.

If a sick child is born to healthy parents, it is obvious that the mother and father are heterozygous for this trait (phenylketonuria), and the disease itself is inherited as a recessive autosomal trait (otherwise the text would contain a disclaimer that the child is male).

Let allele A – healthy, allele a – phenylketonuria. Then both the mother and the father have genotypes Aa, the child – aa.

With this genotype, gametes A, a are formed in each of the parents.

Parents: Aa x Aa.

Gametes: A; a x A; but.

Children: AA, 2 Aa, aa.

In total, 4 genotypes are possible in children, only 1 of them (aa) corresponds to the disease.
4 – 1 = 3 healthy genotypes.

3/4 = 75% chance of having a healthy baby.