Questions of "nature versus nurture" have been asked of most human traits: Is it our genes, inherited from our parents, that make us the way we are, or is it the environment in which we live? A phenotype is a trait that can be observed and described in a population. Although some phenotypes may be totally controlled by genetic or environmental factors, most are influenced by a complex combination of the two. Genes and environmental factors may work independently, or they may interact with one another to cause the phenotype.

Classes of Human Genetic Phenotypes

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...