Tom's brother suffers from phenylketonuria (pku), a recessive disorder. the brothers' parents do not have pku. what is the chance that tom, who is normal for this trait, is a carrier of pku?

There are many possible outcomes.

I think, there are many possible outcomes here.

Normal Gene: N
PKU Carrier gene; P

If their one of their parents have pku, but deactivated, their genotype might be like this; NP (Carrier Parent) × NN (Normal Parent). If the carrier parent do not have PKU, the PKU gene is recessive and it is impossible for someone to be acquire the disease. If we do this in a punnet square, the Genotypic ratio will be; 0 NN, 0 PP, and 4 NP

But, if both parents do have PKU recessive in their genes, their genotype will be like this; NP (Carrier parent) × NP (Carrier parent) it is possible that one child will not be a PKU carrier, one child with PKU, and 2 carriers of PKU. If we do that in a punnet square and find their Genotypic ratio, it will be 1 NN, 1 PP, and 2 PN.

So, if we base the final data on the second setup, the percentage will be ;
25% Normal
25% With disease
50% PKU carrier

that's all I know. have a great day :)