Question

A man with the autosomal recessive disorder phenylketonuria (PKU) and a woman without PKU have a son named Peter, who does not have PKU. Peter is curious about whether his mother is a carrier for PKU. What facts would allow him to know?

Answer 1

The only fact that would allow him to know is if either of his maternal grandparents has PKU

Step-by-step explanation:

The two alleles for a trait are gotten from the two parents. Hence, for someone to have an autosomal trait, one of the alleles must have come from the father and the other from the mother.

For autosomal recessive traits, a carrier or an heterozygous individuals always appear normal. For Peter's mother to be a carrier for PKU, she must have to inherit the allele from either of her parent. The only way for Peter to know if her mother is a carrier is if either of his grandparents is affected for the disorder. That is, either of them have two affected alleles for the trait where it is certain that Peter's mother will inherit one.