one gene leading to many different phenotypic expressions. An excellent example of a gene with pleiotropic effects is the gene for myotonic dystrophy. Affected individuals can have one or more of a range of signs and symptoms including characteristic Christmas-tree like cataracts, myotonia, narcolepsy, testicular atrophy, frontal balding, mental retardation, and cardiac abnormalities, among others.
Pleiotropy occurs when a single gene influences multiple phenotypic traits. Consequently, a new mutation in the gene will have an effect on all traits simultaneously. This can become a problem when selection on one trait favours one specific mutant, while the selection at the other trait favours another mutant.