A question about inheritance usually focuses on individuals or families. That is not surprising. Individuals are the object of medical concern. Families give us information about the genes they have inherited—and which relatives might, too. But we have already seen how we can learn important information by expanding our perspective to groups of families through pedigrees. Pedigree analysis tells us things about transmission that an individual family with a small number of children might not. New information surfaces. But even single families do not explain many important factors. For example, we cannot quantify the amount of a trait's penetrance from just a single family or even a pedigree of several families. We may be able to find one, or even a few, examples of incomplete penetrance. But that does not tell us how often that event will happen in the population as a whole. Yet, it is on that population frequency that individual predictions depend. We must look at many families in which the trait is segregating. We must use "population thinking."
Every population is highly diverse, but the genetic basis of diversity is not uniformly distributed. Examples are familiar, such as the higher frequency of sickle cell hemoglobin in those with ancestry in some northern African and Mediterranean areas or the X-linked glucose-6-phosphate dehydrogenase deficiency (G6PD, or favism) in those from areas like southern Italy. As our knowledge of genetic differences between one group and another improves, so does our ability to make predictions about individuals in those groups.
Population genetics gives us a quantitative perspective on variation, and new techniques are expanding the field. Analytical approaches like genomics, proteomics, and metabolomics are beginning to make their mark. In this chapter, we will explore some of the ways that knowing about genetic makeup of a population can provide valuable predictive tools, and we will see how these new approaches promise to change the future of genetic assessment. But first, history is also important. Let's take a moment to consider the implications of population thinking in an ethical context. Our example will be a case in which, because of poor understanding of population genetics and even poorer humanitarian concern, authorities caused terrible pain for innocent people.
The story is about a woman who was ordered to undergo sterilization because she was labeled "feebleminded." Carrie Buck (Figure 15-1) was born in 1906 and became pregnant when she was 17 after being raped by a presumed member of her foster family. Perhaps from embarrassment, her foster parents had her committed to the Virginia State Colony for Epileptics and Feebleminded, which took patients for being feebleminded or for displaying unmanageable behavior or promiscuity. Her daughter, Vivian, was born in March 1924. Carrie was the first person ordered to be sterilized under a new Virginia law as part of the state's eugenics program. The case eventually ended in the United States Supreme Court. ...