Your cousin learns that she is a carrier for phenylketonuria (PKU). Her husband's biological mother has PKU. She
asks you to explain what this could mean if she and her husband have children and asks what, if anything, could be done
to best protect any child that may have. What would you tell her?
What will be an ideal response?
ANSWER: First, I would tell her that PKU is easily treated by dietary adjustments, and that with proper
guidance and behavior, people with PKU are able to lead full, productive, and essentially
normal lives. I would tell her that PKU demonstrates autosomal recessive inheritance;
therefore, her husband must be a carrier. He received a normal PKU allele from his father,
and the mutant PKU allele from his mother. The normal copy from his father prevents him
from being affected. I would explain to her that, as a carrier, she is in the same situation--she
has one normal allele and one abnormal allele. Since the odds of passing on any allele are
essentially the same as flipping a coin, any child that she has will have a 50% chance of
receiving a normal allele and a 50% chance of receiving an abnormal allele. The same will
apply for her husband. Since having PKU requires inheriting an abnormal allele from both
parents, each of their children will have a 25% chance of having PKU. Then, I would tell her
that nearly all hospitals in the United States screen newborn infants for PKU, and she should
ensure any child she has is tested. If the results of the test indicate the child has PKU, she
should follow the dietary instructions for the child closely to protect the child's brain
development.
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