Why does research on genes and disease so often involve specific racial and ethnic groups?
As we have seen, many diseases vary in prevalence among populations, which are sometimes defined as ethnic groups or races. This variation reflects population differences in genetic and non-genetic factors, which tend to be shared within populations. For example, the foods eaten in Germany are quite different from those eaten in Japan, Afghanistan, Ghana, or Papua New Guinea. Similarly, population differences are seen in typical leisure activities, occupations, and exposure to harmful substances such as tobacco smoke. Accordingly, it is often illuminating to compare the frequencies of diseases and susceptibility factors among populations. An instructive example is given by Alzheimer disease (AD), the most common cause of dementia in older members of many populations. A major genetic risk factor for AD is the ε4 allele of the apolipoprotein E locus. Among Europeans, a large study showed that individuals who inherit two copies of the ε4 allele have a 15-fold elevation in the risk of deve
