The probability that their baby will be a carrier of the gene for PKU is (A) __________. The probability that their baby will be affected by PKU is (B) __________. If both genotypes were Pp, the probability that their baby would be affected by PKU would be (C) _________.

Question

my answer for first one is 3/4 third answer is 1/4 i cant figured the second answer.

Answer 1

I am not a biology expert, but here are some mathematical aspects of the problem.

If both parents are heterozygous (contains one PKU allele, or Pp), the outcomes are:
PP has PKU (probability 1/4)
Pp carrier
pP carrier
pp non-carrier (probability 1/4)

Probability of being a carrier is therefore (1/4+1/4)=1/2.

The probability of inheriting a PKU allele is 1 in 50 in the US general population.

Assuming random distribution, the probability of having both parents heterozygous is (1/50)*(1/50)=1/2500.
Out of these parents, there is a 1/4 probability that the child will have PKU, making a 1/10,000 probability in the general population.

The question is not clear if we assume that both parents are heterozygous or not. So you may have a better context for the correct answers.