Major Mendelian genes versus minor polygenes as predisposers

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When "genetic diseases" are discussed, the usual focus is on the major single-gene disorders, such as cystic fibrosis, phenylketonuria and Huntington's chorea. In these disorders the affected individual is at very high, perhaps certain, probability of developing the disorder, no matter what. In such

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Figure 11.2 Joint risk of developing Alzheimer's disease as a function of apolipoprotein-E genotype and atherosclerosis score (from Hofman et at, 1997).

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Figure 11.2 Joint risk of developing Alzheimer's disease as a function of apolipoprotein-E genotype and atherosclerosis score (from Hofman et at, 1997).

cases, the interest in environment is either negligible, as an aetiological influence, or is special, as in the case of phenylketonuria where avoidance of phenylalanine in the diet can preclude occurrence of the metabolic disorder.

Most genetic influences on disease risks, however, occur in polygenic situations where any single contributory gene may shift the probability by no more than a few percentage points. Further, it is likely that it is the interacting set of alleles that is most relevant in determining the level of innate susceptibility of an individual to developing a disorder or disease in response to a particular environmental exposure. In recent years it has become apparent that there are multiple genetic loci which contribute to the occurrence of hypertension, a tendency to rapid weight gain, coronary heart disease, colon cancer, lung cancer and so on. This is hardly surprising. After all, the complex metabolism and physiology of the mammalian organism is, fundamentally, under genetic control, in that all proteins and other active molecules are genetically coded for, and the resultant slight interindividual variations in the molecular structure of proteins affect their biological activity.

In light of this, it is important to realize that, for the great majority of individuals, any underlying genetic susceptibility to any particular disease is polygenic. ''Brave New World'' ideas of genetic ''correction'' in this context are premature, if not misplaced. The more important inference is that, by minimizing the potentially hazardous environmental exposure, the typically modest impact of those predisposing genes will be largely averted. That, of course, requires knowledge of the specific environmental triggers!

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