Complex sentence: Pleiotropy occurs when one gene influences two or more seemingly unrelated phenotypic traits, an example being phenylketonuria, which is a human disease that affects multiple systems but is caused by one gene defect.
Split into simpler sentences: Pleiotropy occurs when one gene influences two or more seemingly unrelated phenotypic traits. An example is phenylketonuria, a human disease that affects multiple systems but is caused by one gene defect.