Milk :: Lightning strikes twice for milk drinkers

The ability of Africans and Europeans to digest milk as adults is a textbook example of a trait arising independently in two populations in response to the same strong selective pressure. This ability — known as lactase persistence — is analyzed in the January issue of Nature Genetics.

Lactase persistence is frequent in Northern European populations, but much lower elsewhere, and may have become established as a consequence of cattle domestication and a pastoralist lifestyle. Previous work had shown that genetically, it could be attributed to variants that control the expression of the gene encoding the enzyme lactase-phlorizin hydrolase (LPH). This enzyme breaks down lactose into more easily absorbed sugars such as glucose and galactose.

Sarah Tishkoff and colleagues asked whether the genetic basis of lactase persistence in certain pastoralist populations in East Africa might be attributed to the same variants. They studied 470 lactase-persistent and lactase non-persistent individuals from Kenya, Tanzania, and Sudan and found a significant association between lactase persistence and one variant that is very close in location to the LPH variant previously associated with the trait in Europeans. Because other genetic markers in the region differed between the two populations, the authors conclude that the lactase persistent-associated LPH variants arose independently.

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