Single-Gene and Polygenic Traits

What determines the number of phenotypes for a given trait?

Genes control phenotype in different ways. In some cases, a single gene controls a trait. Other times, several genes interact to control a trait. The number of phenotypes produced for a trait depends on how many genes control the trait.

Single-Gene Traits In the species of snail shown below, some snails have dark bands on their shells, and other snails don't. The presence or absence of dark bands is a single-gene trait—a trait controlled by only one gene. The gene that controls shell banding has two alleles. The allele for a shell without bands is dominant over the allele for a shell with dark bands. All genotypes for this trait have one of two phenotypes—shells with bands or shells without bands. Single-gene traits may have just two or three distinct phenotypes.

The bar graph in Figure 17–4 shows the relative frequency of phenotypes for this single gene in one population of snails. This graph shows that the presence of dark bands on the shells may be more common in a population than the absence of bands. This is true even though the allele for shells without bands is the dominant form. In populations, phenotypic ratios are determined by the frequency of alleles in the population as well as by whether the alleles are dominant or recessive.

FIGURE 17–4 Two Phenotypes In this species of snail, a single gene with two alleles controls whether or not a snail's shell has bands. The graph shows the percentages, in one population, of snails with bands and snails without bands.

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