Polygenic Inheritance in Wheat (Kernel Colour)

Polygenic inheritance is the pattern in which several genes together contribute additively to a single trait, and it is what allows Mendelian genetics to explain continuously varying (quantitative) traits rather than only sharply either/or ones. Swedish geneticist H. Nilsson-Ehle provided the first experimental demonstration of polygenic inheritance in 1909, using kernel (grain) colour in wheat. He crossed a pure-breeding dark-red wheat variety (R1R1R2R2) with a pure-breeding white variety (r1r1r2r2); kernel colour here is governed by two genes, each with two alleles, with red dominant over white at each locus. The F1 (R1r1R2r2) had medium-red kernels, and produced four kinds of gametes (R1R2, R1r2, r1R2, r1r2). In the F2, the intensity of red colour tracked simply with how many 'R' alleles (of either gene) a plant carried: four R alleles gave dark red, three gave medium-dark red, two gave medium red, one gave light red, and zero gave white - each R allele adding red pigment in an additive, dose-dependent way. Plotting the number of individuals against colour intensity in F2 produces a bell-shaped curve, the classic signature of a continuously distributed, polygenically controlled trait. A third gene affecting wheat kernel colour was later discovered by other researchers, and with three independently assorting gene pairs involved, Nilsson-Ehle's F2 data actually splits into seven distinguishable shades in the ratio 1 dark red : 6 : 15 : 20 : 15 : 6 : 1 white, summing to the familiar 63 red : 1 white extreme-class ratio. Crucially, the reappearance of the original pure dark-red and white parental phenotypes among the F2 individuals shows that blending inheritance was not taking place even here - only the visible phenotype forms a smooth gradient …