Polygenic Traits Exhibit Continuous Variation in Phenotype

In the case of a single incomplete dominant trait, we observe three phenotypic classes in a 1:2:1 ratio (figure 12.2.A). In the above case of two genes, wherein each gene contributes equally to the trait, the nine different genotypic classes fall into five different phenotypic classes. These phenotypic classes are represented in a 1:4:6:4:1 ratio (figure 12.2.B). A similar situation, with more genes, will result in more phenotypic classes. The ratios for the phenotypic classes are shown in Figure 12.2.C and D. If we continued this process further for more genes, in which each gene provides an equal contribution to the phenotype, we would get a bell-shaped curve, or normal distribution, of many

Figure 12.2 Bar Graphs of Phenotypic Classes of Polygenic Traits. In these examples, the uppercase forms of the genes each contribute equally to the trait. Graphs depict the percentage of individuals in each phenotypic category. The categories represent the total number of upper-case genes, from none on the left to eight on the right, for a trait determined by up to four different genes. A. A graph depicting a single incomplete-dominance gene trait. B. A graph depicting the two-gene trait shown in figure 12.1. C. A graph depicting a three-gene trait. D. A graph depicting an example of a four-gene trait

Figure 12.2 Bar Graphs of Phenotypic Classes of Polygenic Traits. In these examples, the uppercase forms of the genes each contribute equally to the trait. Graphs depict the percentage of individuals in each phenotypic category. The categories represent the total number of upper-case genes, from none on the left to eight on the right, for a trait determined by up to four different genes. A. A graph depicting a single incomplete-dominance gene trait. B. A graph depicting the two-gene trait shown in figure 12.1. C. A graph depicting a three-gene trait. D. A graph depicting an example of a four-gene trait more phenotypic classes. In other words, phenotypes would no longer be sharply definable.

In the examples given above of flower color, each gene contributes equally to the phenotype, but this is probably not the case in real life examples of polygenic traits, where many genes control the phenotype. You can imagine that with a potential for varied genetic contribution, even with a few genes, rather than observing three or five distinct phenotypic classes, we would see an almost continuous variation. This continuous variation in phenotype is the hallmark of a polygenic trait. If we graphed the number of people of different heights, we would get a graph similar to a normal distribution: most people would be of average height, and there would be fewer people that are taller or shorter and few that were very tall or very short. Weight or skin color in humans, size of fruit in plants, and oil content of canola seeds are all polygenic traits that exhibit continuous variation in phenotype. Thus, without knowing anything about the genes underlying a trait, if there is a continuous variation in the trait in the population, we can safely assume that many genes determine the expression of that trait.

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