The inheritance of a number of birth defects is multifactorial. Most normal human traits are inherited neither as dominant nor as recessive mendelian attributes, but rather in a more complex manner. For example, multifactorial inheritance determines intelligence, height, skin color, body habitus, and even emotional disposition. Similarly, most of the common chronic disorders of adults represent multifactorial genetic diseases and are well known to "run in families." Such maladies include diabetes, atherosclerosis, and many forms of cancer and arthritis, and hypertension. The inheritance of a number of birth defects is also multifactorial (e.g., cleft lip and palate, pyloric stenosis, hypospadias, and congenital heart disease). The concept of multifactorial inheritance is based on the notion that multiple genes interact with various environmental factors to produce disease in an individual patient. Such inheritance leads to familial aggregation that does not obey simple mendelian rules (see choices A, B, D, and E). As a consequence, the inheritance of polygenic diseases is studied by the methods of population genetics, rather than by the analysis of individual family pedigrees.Diagnosis: Hypospadias, multifactorial inheritance