Phenylketonuria (PKU) is a genetic disease caused by mutation of the PAH gene, which normally produces the enzyme phenylalanine hydroxase. This enzyme is essential for the conversion of the amino acid phenylalanine into tyrosine. Without the enzyme, phenylalanine builds up in the blood and tissues of various body regions. It can produce severe skin discolorations and foul odor, but also intellectual disabilities and behavioral problems. Some people may experience only one type of PKU symptoms and not the others. PKU is an example of
A. incomplete penetrance.
B. pleiotropy.
C. genetic heterogeneity.
D. complete penetrance.
Answer: B
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