Phenylketonuria (PKU) is an autosomal recessive disorder where afflicted individuals are incapable of metabolizing the amino acid phenylalanine, and it accumulates in nerve cells of the brain. What is the probability that a man and a woman who are both phenotypically normal, but are carriers, will have a child with PKU?

The correct answer would be -  25% chances to have a child with PKU.

Explanation:

PKU or phenylketonuria is a genetic problem that has an autosomal recessive disorder that is caused by the defect in the PAH gene. It causes the incapability of breakdown of the amino acid phenylalanine.

If both parents are phenotypically normal but are a carrier for the effected gene there are 25% chances of the child with PKU condition as there are only 25% chances to form the zygote with the gametes with carrier gene copies.

Thus, the correct answer is - 25%.