Polygenic inheritance Topic 10.3 Polygenic inheritance

10.3.1 Define Polygenic inheritance Polygenic Inheritance – When a single trait is influenced by many genes. Each allele of a polygenic character often contributes only a small amount to the over all phenotype. This makes studying the individual alleles difficult. In addition environmental effects smooth out the genotypic variation to give continuous distribution curves. Occurs when there are more than one gene that controls the phenotype. • The more chromosomes involved contributes to more variation. • Human skin color seems to be controlled by three or four chromosomes. • Human hair color is also controlled by various genes.

Polygenic Inheritance of skin color At least four loci and possibly more genes are involved. Example 3 loci: each has two possible alleles: A,a B,b C,c, each capital allele adds one unit of darkness each lower case allele adds nothing Parents with intermediate tone aabbcc AABBCC Offspring can have tone darker or lighter than either parent

Polygenic Inheritance of skin color Most examples of polygenic inheritance involve more than two genes with codominant alleles. As the number of genes involved increases, the number of posible phenotypes increases. Eventually, it becomes impossible to divide individuals into discrete groups-the variation is continuous. 1 2 3 4 5 6 Number of ‘darker’ alleles