Biology (Year 12) - Genetics
Polygenic inheritance is the inheritance of more than one gene which affects the inheritance of single characteristic. For one characteristic, two or more genes and therefore two or more sets of alleles contributes to a phenotype. A characteristic controlled by more than one gene is known as a polygenic characteristic. Polygenes are genes that have a small additive effect on phenotype. A key example of a polygenic trait in humans is height. Humans have a range of heights, from one extreme to another. According to the Australian Bureau of Statistics' Health Survey of 2011-12, the average adult height in Australia was 161.8cm for women and 175.6cm for men. The condition of showing a range of phenotypes is called continuous variation. Traits which show continuous variation are controlled by two or more genes. The greater the number of genes and the greater the influence of environmental factors, the greater the expected distribution of all phenotypes for height and the smaller the difference between any two individuals ordered along the spectrum.
The table shows continuous variation, which occurs in skin tone due to the additive effects of at least three melanin-producing genes, represented above as A, B, and C. The alleles a, b, and c do not produce melanin.