Continuous variation in a polygenic trait.

Skin colour is a polygenic trait that can display a continuum of phenotypes. The bell curve on the left (solid line) shows the range of skin pigmentation in a hypothetical human population. The bar graph below the curve shows the additive effects of three genes that affect pigment production in this population; each bar shows the fraction of people with a particular number of dark alleles (AD, BD, and CD) and light alleles (AL, BL, and CL). The bell curve on the right (dashed line) represents the expected range of phenotypes if the same population was raised in a sunnier environment.