Explain polygenic inheritance with example.

Polygenic inheritance occurs when one characteristic is controlled by two or more genes. Often the genes are large in quantity, but small in effect. Examples of human polygenic inheritance are height, skin color, eye color and weight.

Human skin color is a good example of polygenic (multiple gene) inheritance. Assume that three ‘dominant’, capital letter genes (A, B and C) control dark pigmentation because more melanin is produced. The ‘recessive’ alleles of these three genes (a, b and c) control light pigmentation because lower amounts of melanin are produced. The words dominant and recessive are placed in quotation marks because these pairs of alleles are not truly dominant and recessive as in some ofthe garden pea traits that Gregor Mendel studied.

A genotype with all ‘dominant’ capital genes (AABBCC) has the maximum amount of melanin and very dark skin. A genotype with all ‘recessive’ small case genes (aabbcc) has the lowest amount of melanin and very light skin. Each ‘dominant’ capital gene produces one unit of color, so that a wide range of intermediate skin colors are produced, depending on the number of ‘dominant’ capital genes in the genotype. For example, a genotype with three ‘dominant’ capital genes and three small case ‘recessive’ genes (AaBbCc) has a medium amount of melanin and an intermediate skin color. This latter genotype would be characteristic of a mulatto.