Complex traits are important not only in humans, but also in agricultural plants and animals. We will examine human height in some detail because it has been widely studied, but equally well known complex traits are number of eggs laid by hens, milk production in dairy cows, and yield per acre of grain (Fig. 18.1).

Many common human diseases, including high blood pressure, obesity, diabetes, and depression, are complex traits (Fig. 18.2). High blood pressure, for example, affects about one-third ofthe U.S. population, and obesity another third. Type 2 diabetes affects around 8% of the U.S. population, and an estimated 15% will suffer at least one episode of severe depression in the course of a lifetime. Taken together, about 200 million Americans—two-thirds of the entire population—suffer from one or more of these common disorders. None of these traits shows single-gene Mendelian inheritance.

In many complex traits, the phenotype of an individual is determined by measurement: Human height is measured in inches, milk yield by the gallon, grain yield by the bushel, egg production by the number of eggs, blood pressure by millimeters of mercury, and blood sugar by millimoles per liter. Because the phenotype of complex traits such as these is measured along a continuum with only small intervals between similar individuals, complex traits like these are often called quantitative traits. By contrast, single-gene traits often appear in one of two or more different phenotypes, such as round versus wrinkled seeds, or green versus yellow seeds.

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