Many characters that display continuous variation are controlled by genes at many loci.
Characters influenced by a group of genes are called polygenic characters. The number of genes involved in the control of some trait is commonly around 10, but may be as large as 100 or so in some cases.
Figure: characters involved by many genes often show continuous variation. In (a) the phenotypic character, such as size, is controlled by one locus with two alleles (A and a), where A is dominant to a. There are two discrete phenotypes in the population. (b) The character is controlled by two loci with two alleles each (A and a, B and b); there are three discrete phenotypes. In (c) and (d) it is shown that as the number of loci increase, the phenotype frequency distribution becomes increasingly continuous.