In the case of Ari's family, what we see looks a lot like recessive inheritance of a disease gene (Figure 17.2), but it can sometimes be hard to tell if someone has lactose intolerance if they are not exposed to a lot of lactose. Her father's family, with western European ancestry and a standard American diet full of dairy products, appears to be full of lactose-persistent people. However, it is actually harder to tell about people on her mother's side of the family, who consume a more standard Middle Eastern diet. Although they also use dairy products, they eat recipes based on yogurt more often than milk, and drink tea or wine or water more often than milk. Lactose intolerance can be hard to diagnose even in someone who is exposed to a lot of milk, as happened with Ari; someone who is consuming a nearly lactose-free diet simply because of cultural context may not even know whether milk gives them problems. We know from many studies of other families and individuals that lactase persistence is dominant over lactose intolerance.
Telling just who is or isn't affected with a particular problem is a common problem in many genetic studies, but especially so with situations such as lactose intolerance that have an environmental component. A genetic disease called favism is only detected in individuals with the defect who eat fava beans. Normally, detection of malignant hyperthermia happens because someone undergoes general anaesthetic for surgery. Individuals susceptible to steroid glaucoma will only develop this potentially blinding eye disease if they are exposed to certain corticosteroid medications. So for many of these complex traits with both genetic and environmental elements, many of us have no idea what our phenotype or genotype might be because we have not encountered the conditions that would elicit the trait in someone with the predisposing genotype.
This process of bringing about the expression of a phenotype in response to exposure to something is called induction. Although many inherited phe-notypes are congenital (present from birth) or developmental (develop at a particular stage in the course of development and aging), there are many phenotypes that are inducible. Tanning is the process of inducing more skin melanin in response to sunlight (or tanning beds). Certain allergies might be
FIGURE 17.2 Recessive inheritance of lactose intolerance in Ari's family. The black symbol marks the one known case of lactose intolerance on both sides of the family. It is clear that her father's family has many individuals with lactase persistence, but the phenotype is unclear for most of her mother's side of the family because of their relatively low-lactose diet in which milk products tend to be yogurt based. We do know that Ari's mother and her father can both drink milk without problems, so they both appear to be carriers. This implies that at least one of her maternal grandparents should be lactase persistent.
said to be induced by exposure to the allergen, such as penicillin or a bee sting. In genetic terms, we usually use the term induction to refer to a process that brings about increased expression of a gene.
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