If two people who are heterozygous for the pku allele have a child with pku, what is the likelihood their next child will have pku?

The correct answer is 25% likely.

Explanation:

Phenylketonuria (PKU) is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell must have mutations to express the disease.  

If we let the allele causing PKU be “p” and the normal allele be “P”, the affected individual must have “pp” combination. If both parents are heterozygous, this means the have “Pp” combination and therefore the result in their progeny will be the following:

¼: PP (25%)

¼: pp (25%)

½: Pp (50%)

This pattern will repeat itself every time they try to have a child.