Diseases and health traits that involve multiple genes and complex patterns of inheritance are generally described as exhibiting multifactorial transmission. The term multifactorial means many factors. By contrast, simple genetic traits like cystic fibrosis, which are caused by a mutation in a single gene, tend to be inherited in predictable patterns that are either autosomal or sex-linked, and they are either dominant or recessive. Multifactorial traits, however, may involve genes on both autosomes and sex-chromosomes at the same time. Some of the genes may be dominant acting, and others recessive. There may be significant interaction with nongenetic factors like nutrition or other environmental exposures. Traits influenced by multiple genes that do not involve any significant interactions environmental factors are called polygenic traits. When environmental and genetic factors both are involved, the degree of influence attributable to genetics is called heritability. Careful study of identical and fraternal...