Human features like height, eye color, and hair color come in lots of slightly different forms because they are controlled by many genes, each of which contributes some amount to the overall phenotype. For example, there are two major eye color genes, but at least 14 other genes that play roles in determining a person’s exact eye color^3
3
cubed.
Looking at a real example of a human polygenic trait would get complicated, largely because we’d have to keep track of tens, or even hundreds, of different allele pairs (like the 400 involved in height!). However, we can use an example involving wheat kernels to see how several genes whose alleles "add up" to influence the same trait can produce a spectrum of phenotypes^{1,4}
1,4
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In this example, there are three genes that make reddish pigment in wheat kernels, which we’ll call A, B, and C. Each comes in two alleles, one of which makes pigment (the capital-letter allele) and one of which does not (the lowercase allele). These alleles have additive effects: the aa genotype would contribute no pigment, the Aa genotype would contribute some amount of pigment, and the AA genotype would contribute more pigment (twice as much as Aa). The same would hold true for the B and C genes^{1,4}
1,4
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64-square Punnett square illustrating the phenotypes of the offspring of an _AaBbCc_ x _AaBbCc_ cross (in which each uppercase allele contributes one unit of pigment, while each lowercase allele contributes zero units of pigment).
Of the 64 squares in the chart:
1 square produces a very very dark red phenotype (six units of pigment)
6 squares produce a very dark red phenotype (five units of pigment)
15 squares produce a dark red phenotype (four units of pigment).
20 squares produce a red phenotype (three units of pigment)
15 squares produce a light red phenotype (two units of pigment)
6 squares produce a very light red phenotype (one unit of pigment)
1 square produces a white phenotype (no units of pigment)
64-square Punnett square illustrating the phenotypes of the offspring of an AaBbCc x AaBbCc cross (in which each uppercase allele contributes one unit of pigment, while each lowercase allele contributes zero units of pigment).
Of the 64 squares in the chart:
1 square produces a very very dark red phenotype (six units of pigment)
6 squares produce a very dark red phenotype (five units of pigment)
15 squares produce a dark red phenotype (four units of pigment).
20 squares produce a red phenotype (three units of pigment)
15 squares produce a light red phenotype (two units of pigment)
6 squares produce a very light red phenotype (one unit of pigment)
1 square produces a white phenotype (no units of pigment)
Diagram based on similar diagram by W. P. Armstrong^5
5
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Now, let’s imagine that two plants heterozygous for all three genes (AaBbCc) were crossed to one another. Each of the parent plants would have three alleles that made pigment, leading to pinkish kernels. Their offspring, however, would fall into seven color groups, ranging from no pigment whatsoever (aabbcc) and white kernels to lots of pigment (AABBCC) and dark red kernels. This is in fact what researchers have seen when crossing certain varieties of wheat^{1,4}
1,4
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This example shows how we can get a spectrum of slightly different phenotypes (something close to continuous variation) with just three genes. It’s not hard to imagine that, as we increased the number of genes involved, we’d be able to get even finer variations in color, or in another trait such as height.
gene chart below
