special subjects
Multifactorial (Complex) Inheritance
Introduction
For many traits, expression involves multiple genes. Many such traits (eg, height) distribute as a bell-shaped curve (normal distribution). Normally, each gene adds to or subtracts from the trait independently of other genes. In this distribution, fewer people are at the extremes and many more are in the middle because people are unlikely to inherit multiple factors acting in the same direction. Environmental factors also add to or subtract from the final result. Many relatively common congenital anomalies and familial disorders result from multifactorial inheritance. In an affected person, the disorder represents the sum of genetic and environmental influences. Risk of the occurrence of such a trait is much higher in 1st-degree relatives (siblings, parents, or children who share, on average, 50% of the affected person’s genes) than in more distant relatives, who are likely to have inherited only a few high-liability genes. Common disorders with multifactorial inheritance include Hypertension , Multigenic, multifactorial traits seldom produce clear patterns of inheritance; however, these traits tend to occur more often among certain ethnic and geographic groups or among one sex or the other. (See also Overview of Genetics .) 