Polygenic Inheritance

Polygenic inheritance describes a pattern of inheritance in which a single observable phenotypic character is controlled not by just one gene, as in every example discussed so far in this chapter, but by the combined, CUMULATIVE effect of TWO OR MORE different genes acting together — genes that are typically located at different loci, often on entirely different chromosomes, each of which individually makes only a small, partial contribution to the character's final overall expression. Because the effects contributed by the various alleles at these different loci simply ADD together, rather than one allele completely dominating or masking another the way T dominates t in a simple Mendelian character, polygenic traits do not sort neatly into a small number of sharply distinct, easily countable classes the way Mendel's tall-versus-dwarf pea plants did. Instead, a polygenic character shows continuous, graded variation across a smooth range of possible values, with the majority of individuals in a population clustering around some intermediate value and progressively fewer individuals found towards either extreme of the range — producing, when the numbers of individuals showing each value are plotted, the familiar smooth, bell-shaped (approximately normal) distribution curve that is the statistical signature of a polygenic character, quite unlike the small number of sharply separated bars a Mendelian monohybrid character would produce on the same kind of plot.

Human skin colour is the example most commonly used to introduce polygenic inheritance, and with good reason: it is governed by the combined action of at least three separate gene pairs (located at different loci) acting together, each of which contributes, in an additive fashion, to the total amount of the pigment melanin that is deposited in the skin. A person's overall skin colour, on this model, essentially reflects the cumulative "dose" of darker-pigment-producing alleles they happen to carry summed ACROSS all three (or more) contributing genes together, rather than being determined by any single gene pair on its own — someone carrying mostly darker-pigment alleles across all the contributing loci shows a darker skin tone, someone carrying mostly lighter-pigment alleles shows a lighter skin tone, and the great majority of people, carrying some mixture of both types of allele across the different loci, show intermediate skin tones — which is exactly why, across any large human population sampled worldwide, skin colour is observed to vary in a smooth, continuous gradation from very light through many intermediate shades to very dark, rather than falling into just two or three sharply distinguishable categories. …