Heredity

The process by which the genetic code of parents is passed on to their children.

There are certain traits that parents pass on to their children, including eye color, hair color, height, and other physical characteristics. The coding for these traits are contained inside DNA molecules that are present within all human cells. Since the discovery of DNA by James Watson (1928-) in the 1950s, the science of genetics has focused on the study of DNA and the ways in which physical traits are passed on from generation to generation. Within genetics, a special branch of DNA science—called quantitative, or biometrical, genetics— has emerged, which studies the heritability of such traits as intelligence, behavior, and personality. This branch focuses on the effects of polygenes in the creation of certain phenotypes. Polygenes, as the name implies, refer to the interaction of several genes; and phenotypes are certain variable characteristics of behavior or personality. Quantitative geneticists, therefore, study the effects of groups of genes on the development of personality and other abstract variables. They rarely, it should be noted, are able to pinpoint a behavior's genesis to a specific gene. Specific genes have been found to cause a small number of diseases, however, such as Huntington's disease and other degenerative disorders.

In studying personality traits and intelligence, the latest research in quantitative genetics suggests that the heritability rate for many characteristics hovers around 50 percent. In 1988 a study of twins reared apart revealed the heritability of 11 common character traits. The findings, published in the Journal of Personality and Social Psychology, reported that social potency is 61% influenced by genes; traditionalism, 60%; stress reaction, 55%; absorption (having a vivid imagination), 55%; alienation, 55%; well-being, 54%; harm avoidance (avoiding dangerous activities), 51%; aggression, 48%; achievement, 46%; control, 43%; and social closeness, 33 percent.

Other recent studies have compiled lists of traits most influenced by heredity. Physical characteristics that are most genetically determined include height, weight, tone of voice, tooth decay, athletic ability, and age of death, among others. Intellectual capabilities include memory, IQ scores, age of language acquisition, reading disabilities, and mental retardation. Emotional characteristics found to be most influenced by heredity were shyness, extroversion, neuroses, schizophrenia, anxiety, and alcohol dependence. It is important to note that these are tendencies and not absolutes. Many children of alcoholics, for instance, do not become alcoholics themselves. Many social and cultural factors intervene as humans develop, and the child of an alcoholic, who may be genetically vulnerable to acquiring the disease, may avoid drinking from witnessing the devastation caused by the disease. (For a fuller discussion of the role of environment, see Nature-Nurture Controversy.)

Recent work has shown that genes can both be influenced by the environment and can even influence the environments in which we find ourselves. A 1990 study found that animals raised in environments requiring significant motor activity actually developed new structures in the brain that were significantly different from the brain structures of animals raised in environments lacking motor stimuli. Observations from such experiments have revealed that complex environments actually "turn on" sets of genes that control other genes, whose job it is to build new cerebral structures. Therefore, living in an environment that provides challenges can genetically alter a person's makeup. Additionally, a genetic predisposition to introversion can cause people to isolate themselves, thus changing their environment and, in the process, altering their development of social skills. This, then, contributes further to their genetic predisposition to introversion.

There also appears to be universal, inherited behavior patterns in humans. Common behaviors across diverse cultures include the patterns of protest among infants and small children at being separated from their mothers. A study conducted in 1976 found that separation protests emerge, peak, and then disappear in nearly identical ways across five widely diverse cultures. Other studies have found universal facial expressions for common emotions, even among pre-literate hunter-gatherer cultures that have had no exposure to media. It used to be thought that the human smile was learned through observation and imitation, but a 1975 study found that children who had been blind from birth began smiling at the same age as sighted children. Many of these behaviors are thought to be instinctual. Aside from the infant/developmental behaviors already mentioned, other inherited behavior patterns in humans include sex, aggression, fear, and curiosity/exploration.