Polygenic Traits Help
In the early days of Mendelian genetics it was thought that there was a fundamental difference in the essence of qualitative and quantitative traits. One of the classical examples that helped to bridge the gap between these two kinds of traits is the multiple-gene model developed around 1910 by the Swedish geneticist Nilsson-Ehle to explain kernel color in wheat. When he crossed a certain red strain to a white strain he observed that the F1 was all light red and that approximately 1/16 of the F2 was as extreme as the parents; i.e., 1/16 was white and 1/16 was red. He interpreted these results in terms of two genes, each with a pair of alleles exhibiting cumulative effects. In the following explanation, the use of capital and lowercase letters does not imply dominant and recessive allelic interactions, but rather additive gene action in which each R gene makes an equal contribution to redness and each r allele contributes nothing to color of the wheat kernel. Note that the phenotype of the F1 is intermediate between the two parental types and that the average phenotype of the F2 is the same as that of the F1 but is a much more variable population; i.e., the F2 contains many more phenotypes (and genotypes) than in the F1. Note that the F2 distribution is an expansion of the binomial (a+b)4, where a = "active alleles" R1 or R2, b = "inactive alleles" r1 or r2, and a = b = 1/2 in the F1.
In this model situation, each of the "active" alleles R1 or R2 adds an equal amount of red to the phenotype, so that the genotype of white contains neither of these alleles and a fully red genotype contains only R1 and R2 alleles. These results are plotted as histograms in Fig. 8-1.
Certain other strains of wheat with dark-red kernels when crossed to whites exhibit an F1 phenotype intermediate between the two parental types, but only 1/64 of the F2 is white. In this case, the F1 is probably segregating for three pairs of genes and only the genotype r1r1r2r2r3r3 produces white. Of course, there would be more shades of red exhibited in the F2 here than in the previous case where only two genes are segregating. Even if the environment does not modify these color phenotypes (which it probably does to some extent), the ability of the eye to measure subtle differences in shading would probably cause difficulties with this many phenotypes and color discernment would become impossible if four or five genes were each contributing to kernel color.
A rough estimate of the number of gene loci contributing to a polygenic trait can be obtained by determining the fraction of the F2 (resulting from selfing the F1 hybrid between two pure varieties) that is as extreme in its phenotype as that of one of the pure parental strains.
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