| Popis: |
Variation in the ability of adult humans to digest the lactose in milk is a genetically determined trait that has excited the interest of many for the past 50 years. The trait seems to have risen to high frequencies by one of the strongest selection pressures ever described. This section describes the phenotypic polymorphism and the evidence that the genetic trait involves regulation of expression of the lactase gene and is caused by multiple independent mutations that have reached high frequencies in different populations, because of the benefits of drinking milk. The evolutionary history in Europe and Africa are rather different; there being a single high-frequency allele in Europe, in contrast to multiple alleles in Africa. Also the selective forces, which are likely to include both the nutritional and water content of milk, may have been different in these regions. Key Concepts: The enzyme, lactase, is restricted to the small intestine where it digests lactose in the milk of suckling mammals. Lactase persists into adult life in some, but not all, people. Lactase persistence is a genetic trait, which varies in frequency in different populations of the world. Mutations in an enhancer sequence, at a distance from the lactase gene, appear to be responsible for lactase persistence. These alleles arose in the last few thousand years and have been under strong positive selection. The selection may be attributable to one of the nutrients or water in fresh milk and/or its calorific value. Keywords: lactose; milk; lactase; selection; persistence; tolerance |
