10.3 Polygenic Inheritance

Polygenic Inheritance
     Certain characteristics are controlled by two or more genes. Many genes will play a part on the final characteristic of a trait. Polygenic inheritance usually follows a normal bell-shaped distribution curve with common characteristics in the middle and extreme characteristics at each side of the bell curve. With a greater number of genes controlling a trait, the number of phenotype combinations will also increase.
     An example of a characteristic that is controlled by multiple genes would be skin color. Human skin color changes depending on the amount of melanin present. The more melanin, the darker the skin. It has been discovered that at least 4 genes are involved in melanin production, where one allele of each gene will code for melanin production while the other allele will not. This combination of melanin producing alleles with non melanin producing alleles can result in a continuous variation of skin pigmentation.
     Similarly, the grain color in wheat also depends on multiple genes. The color of grain covers a range from white to dark red depending on the amount red pigment they contain. There are three genes with two alleles each that either code for the pigment or the lack of pigments. The most common genotype would have equal numbers of each allele. The most extreme genotypes would either have all pigment producing alleles, or all no pigment producing alleles.