Many characters, such as height, weight, hair color, and skin color, are polygenic. Often, the environment strongly influences polygenic characters.
Genes controlling human traits show many patterns of inheritance. Some of these genes cause genetic disorders. Genetic disorders are diseases or disabling conditions that have a genetic basis.
Single genes having two or more alleles can determine traits, such as blood type or cystic fibrosis. Geneticists have learned, however, that most human characteristics are polygenic (PAHL-ee-JEHN-ik) characters: they are influenced by several genes. Polygenic characters show many degrees of variation, as seen in Figure 12-10. Skin color, for example, results from the additive effects of three to six genes. These genes control the amount of the brownish-black pigment called melanin in the skin. The more melanin skin cells produce, the darker the skin. Each of three to six genes has an allele that produces low amounts of melanin and another allele that makes high amounts of melanin. The final amount of melanin in a person's skin that is not exposed to sunlight comes from the number of high-melanin alleles among these few skin-color genes. Eye color, height, and hair color are also polygenic characters.
Many human conditions are complex characters—characters that are influenced strongly both by the environment and by genes. Skin color is both polygenic and complex. Exposure to sunlight generally causes the skin to become darker, no matter what the skin-color genotype is. Human height is another polygenic character that is controlled by an unknown number of genes that influence the growth of the skeleton. Height, however, is also influenced by environmental factors, such as nutrition and disease.
Other complex characters play a role in diseases or conditions such as breast cancer, diabetes, heart disease, stroke, and schizophrenia. Most breast cancer, for example, occurs in people with no familial history of the disease. But breast cancer also runs in some families.
Geneticist Marie-Claire King studied the genetics of breast cancer in families in which several individuals had the disease at younger ages than the average breast cancer patient does. Figure 12-11 shows the pedigrees of two of the families she studied. In Family A, each affected person has an affected parent, which is the inheritance pattern of a dominant trait. Family B demonstrates that additional genetic and environmental factors can influence whether or not a person expresses a trait. In Family B, a female, individual III-A, does not develop breast cancer herself but has a child who goes on to develop breast cancer. Notice also, individual III-B in Family B is a male who has breast cancer. This again shows that many factors in addition to the central gene influence the onset of a genetic disease.
Biologists hope that by identifying the environmental components that contribute to a disease, they can educate people in ways that minimize their risk of developing the disease. For breast cancer, for example, non-genetic risk factors include a diet high in saturated fat.
These diagrams show the pedigrees of families in which there is hereditary breast cancer.
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