Expression of complex traits is notoriously susceptible to lifestyle choices and other environmental factors. Inadequate nutrition is linked to slow growth rate and short stature in adults. Salt intake is associated with an increased likelihood of high blood pressure and is therefore an environmental risk factor for this common disorder. An environmental risk factor is a characteristic in a person’s surroundings that increases the likelihood of developing a particular disease. For example, a junk-food diet high in fat and carbohydrates is an environmental risk factor for obesity and diabetes.

Environmental effects are also important in agriculture. Farmers are well aware that adequate nutrition is essential to normal growth of chicks, lambs, piglets, and calves, and that continued high-quality feed is necessary for high egg production and milk yield when the animals reach adulthood. In crop plants like grains, adequate soil moisture and nutrients are necessary for sustained high yields.

Environmental factors not only affect the average phenotype for complex traits, but they also affect the variation in phenotype from one individual to the next. For example, most fields of corn you see planted along the roadside come from seeds that are genetically identical to one another, yet there is phenotypic variation in complex traits like plant height (Fig. 18.3). Because the plants are genetically identical, the differences in phenotype result from differences in the environment. Some parts of the field may receive more sunlight than others, and some parts may have better water drainage. No matter how uniform an environment may seem, there are always minor differences from one area to the next, and these differences can result in variation in complex traits. In the example in Fig. 18.3, the plants in the foreground are shorter than the others because that corner of the field has poorer drainage, and the plants growing there have to compete for nutrients with the grass growing nearby.

366

Environmental effects on complex traits in animals are evident in true-breeding, homozygous strains like those used by Mendel in his experiments with pea plants. Such true-breeding strains are called inbred lines, and they are often used for research. Even though all animals in any inbred line are genetically identical and are caged in the same facility and fed the same food, there is variation from one animal to the next. In one study of cholesterol levels in an inbred line of mice, for example, average serum cholesterol was 120 mg/dl (milligrams per deciliter), but the range was 60–180 mg/dl. Because the mice are genetically identical, the variation in serum cholesterol resulted entirely from variation in the environment.