Pleiotropy

A single gene typically influences one trait, but that is not always the rule. When one gene affects multiple, seemingly unrelated phenotypic traits, the phenomenon is called pleiotropy, and the gene responsible is a pleiotropic gene.

The mechanism behind pleiotropy is usually metabolic. A gene codes for an enzyme that participates in a metabolic pathway. That pathway may produce products that influence several different body systems or developmental processes. A defect in that single gene therefore disrupts the pathway and causes a cascade of effects, each appearing as a separate symptom.

A classic human example is the genetic disorder phenylketonuria (PKU). The underlying cause is a mutation in the single gene that codes for the enzyme phenylalanine hydroxylase. This enzyme normally converts the amino acid phenylalanine into tyrosine. When the enzyme is defective, phenylalanine accumulates to toxic levels in the body.

This single-gene defect does not produce just one symptom. Instead, it manifests through multiple phenotypic expressions:

• Mental retardation — caused by the toxic effect of accumulated phenylalanine on the developing brain.
• Reduction in hair and skin pigmentation — because tyrosine, the product of the normal reaction, is a precursor for melanin pigment. With less tyrosine, melanin production drops. …