Proteins, which are the products of genes, interact with one another in complex ways to determine the phenotype. Almost every trait we observe, such as height, normal metabolic level, or intelligence, is really the product of many genes. Many traits, however, also reflect the influence of the environment. Such traits are called complex traits, to distinguish them from simple traits that are governed by single genes. While any single gene contributing to a complex trait can be described in terms of dominance or recessiveness, autosomal or sex-linked, or other categories, the gene products interact to make a much more subtle phenotypic picture.
A polygenic trait is a complex trait controlled by the alleles of two or more genes, without the influence of the environment. A multifactorial trait is a complex trait controlled by both genes and the environment. Intelligence is multifactorial, with strong influences from both genes (such as those controlling nerve-cell growth and connectivity) and the environment (such as early childhood nutrition and education).
As the number of influences grows, so too does the number of possible phenotypes. Because of this, complex traits show not just a few phenotypes, but a continuum (as can be seen in the wide range of possible human heights). The distribution in a population will usually be described by a bell-shaped curve, with most people displaying the mid-range phenotype.
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