The human disease phenylketonuria has symptoms that include mental problems, chronic skin rashes, and reduced hair and skin pigmentation. Phenylketonuria can be caused by a mutation in a single gene.

Phenlyketonuria is an example of pleiotropy. Based on this, it can be concluded that pleiotropy occurs when

A.
a single gene influences multiple traits.
B.
a trait is influenced by both genes and environment.
C.
a trait is not influenced by genes.
D.
a single trait is influenced by many genes.