How does both the environment and genetics influence genetic traits such as phenylketonuria?

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Answer: All traits depend both on genetic and environmental factors. Genetics and environment interact with each other to produce their effects. This means that the way genes act depends on the environment in which they act. Phenylketonuria (PKU) is an excellent example of interaction of genetics and environmental factors. PKU is a t of mental retardation caused by abnormal breakdown of the essential amino acid, phenylalanine, which is found in all protein. The enzyme responsible for breaking down phenylalanine becomes defective, so it accumulates and breaks down abnormally. So in PKU, a single gene can dramatically affect behavior: it is clearly a genetically influenced process. But the effect of that defective gene expression depends on the environment in which it occurs. A low phenylalanine containing diet have little toxic effects result.. The abnormal gene is still there, but because little phenylalanine is present in the diet, little toxic effects result. This defective gene was no longer expressed in mental retardation, when phenylalanine was greatly reduced in the food the child with the defective gene ate.