Gene and Environment

Human phenotypes are often classified as either simple (or Mendelian) or complex. A "simple" or Mendelian phenotype is one that demonstrates a recognizable inheritance pattern (such as autosomal dominant or recessive, or X-linked). A Mendelian phenotype is caused by a particular genetic variant, or allele, of a gene. The expression of Mendelian phenotypes may vary by age, but, in general, the effect of a single gene is sufficient to cause the phenotype.

In contrast, "complex" phenotypes do not adhere to simple Mendelian laws, and they are influenced by several factors (either genetic or environmental) acting independently or together. In complex phenotypes, alleles of particular genes increase the probability that the phenotype will develop, but do not determine with certainty whether a person will have the phenotype. They are neither necessary nor sufficient to cause the phenotype. Genes that act in this fashion are called susceptibility genes. The complex interaction of susceptibility genes with other genetic and environmental risk factors determines whether or not a person will develop a complex phenotype.