Complex Traits

Most of the inherited disorders mentioned so far are monogenic—that is, they are determined by a single gene, and their expression is usually not greatly influenced by the environment. However, most if not all characteristics and disorders reflect input from the environment as well as genes.

Traits determined by more than one gene are termed polygenic. Usually, several genes each contribute to differing degrees toward molding the overall pheno-type, which may vary greatly among individuals. Such a trait, with many degrees of expression because of the input of several genes, is said to be continuously varying. Height, skin color, and eye color are polygenic traits (figs. 24.8, 24.9A and 24.10).

Although the expression of a polygenic trait is continuous, we can categorize individuals into classes and calculate the frequencies of the classes. When we do this and plot the frequency for each phenotype class, a bell-shaped curve results. This curve indicating continuous variation of a polygenic trait is strikingly similar for different characteristics, such as fingerprint patterns, height, eye color, and skin color. Even when different numbers of genes are involved, the curve is the same shape.

Eye color illustrates how interacting genes can contribute to a single trait. The colored part of the eye, the iris, becomes darker as melanocytes produce the pigment melanin. Blue eyes have just enough melanin to make the color opaque. People with dark blue or green, brown or black eyes make increasingly more melanin in the iris. Unlike melanin in skin melanocytes, the pigment in the eye tends to stay in the cell that produces it.

Two genes, with two alleles each, can interact addi-tively to account for five distinct eye colors—light blue, deep blue or green, light brown, medium brown, and dark brown/black. (It seems that manufacturers of mascara follow this two-gene scheme too!). If each dominant allele contributes a certain amount of pigment, then the greater the number of such alleles, the darker the eye color. If eye color is controlled by two genes A and B, each of which comes in two allelic forms A and a and B

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Essentials of Human Physiology

Essentials of Human Physiology

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