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Genetic and Environmental Factors

The range in which genes and the environment interact is vast, from clear but not 100% predictable genetic determinism to unclear genetic influence. The diseases that are clearly genetic are generally single gene errors that follow Mendelian patterns of inheritance. The scientist Gregor Mendel, who was the first man to demonstrate how patterns of inheritance can be mathematically described, explained the Mendelian or Gene Theory. Such diseases include muscular dystrophy, cystic fibrosis, and phenylketonuria. There are also genetic diseases that are not inherited per se, but rather are due to direct damage to the genes. The most common example is Down's syndrome. Finally, there are polygenic diseases, meaning that multiple genes are involved in influencing the development of a particular disease. These diseases include cancer, heart disease, diabetes, and most mental illnesses.

Inheritance

The question of inheritance often implies genetic inheritance, entailing a sense of predestination or determinacy. The underlying theme behind the idea of inheritance is that it is beyond one's control, and therefore, the individual is not held responsible for his or her actions. This concept is fraught with ethical and political overtones that have been discussed at some length previously. To say that genes determine a particular outcome is to lead to misconceptions. There are a myriad of influences that genes are subject to both before and after they code for a particular protein that are not immediately determined. Alternatively to say that the environment escapes deterministic notions is equally absurd. People not only inherit their parents' genes, but also their parents' home and culture. They have no more choice about the environment they were born into than they have about their parents' genes.

Nature Versus Nurture

The misconception is that if a set of behaviors is due to a person's genes, then it must be an illness that requires medical treatment, but if the behaviors are due to a poor environment, then the illness must be due to a social, political, or moral problem (see Question 10 again to review the issues surrounding voluntary behavior). The real questions are as follows: What are the various genetic and environmental influences? How much does each contribute to the development of an individual's physical, cognitive, and behavioral makeup, or what are their contributions to a disease? All that can really be done is to examine a population of related individuals and note the variations in both a particular behavior and a group of symptoms and their genetic relatedness. For example, by studying the offspring of alcoholics, we know that of 100 people with alcoholism, 18 will have children who will also become alcoholics, whereas of 100 people who are not alcoholics, only 5 will have children who will become alcoholics. These statistics still do not allow us to specifically predict the 18 who will develop the condition and the 82 who will not. The two biggest studies on alcohol and genes are twin studies and adoption studies. A third study was designed in the laboratory.

The underlying theme behind the idea of inheritance is that it is beyond one's control, and therefore, the individual is not held responsible for his or her actions. This concept is fraught with ethical and political overtones that have been discussed at some length previously.

For example, by studying the offspring of alcoholics, we know that of 100 people with alcoholism, 18 will have children who will also become alcoholics, whereas of 100 people who are not alcoholics, only 5 will have children who will become alcoholics.

Concordance rates the rate at which genetically related individuals share with one another a particular trait.

 
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