Prevelence of Lactase Persistance

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Certain human populations have developed the ability to digest the nutrients found in milk throughout their life spans. As newborns, mammals rely solely on milk for their nutritional needs, and, thus, have the necessary enzymes to digest milk and its nutrients. However, all mammals, besides select populations of humans, lose the ability to digest the main carbohydrate in milk, lactose, after weaning (Ingram et. al, 2008). Lactase is the small intestinal enzyme responsible for breaking down lacrosse into the absorbable monosaccharides glucose and galactose. The presence or absence of lactase is a genetic polymorphism; individual are either lactase persistent or lactase nonpersistent. Individuals who are lactase nonpersistent do not absorb lactase in the intestines, so it travels to the colon, where it is metabolized by bacteria. This results in fermentation and excess gas, which causes abdominal symptoms such as bloating, cramps, nausea, diarrhea, and vomiting. On the other hand, those who display lactase persistence are able to breakdown lactase throughout the course of their lives. Lactase persistence started with the domestication of animals, which lead to dairying. Dairying and human adult milk consumption provided a selective pressure that lead certain human populations to evolve lactase persistence, allowing them to digest the beneficial nutrients of milk products throughout their lifetime.
Lactase Persistence Frequencies and Origin in Human Populations
It was thought for a long time that most humans had high lactase activities as adults, most likely due to that fact that early research was conducted in countries where lactase persistence was the norm (Swallow, 2003). However, lactase nonpersistence is actually more ...

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...ons have high frequencies of lactase persistence and low frequencies of the T-13910 allele. Two SNP have been lined to lactase persistence in Saudi population, T/C-3712 and T/G-13915 (Enattah et. al, 2008). Both have been show to increase transcription of lactase in vitro (Enattah et. al, 2008).
Conclusion
Lactase persistence in humans has been one of the most prominent examples of natural selection in humans. It is an example of convergent evolution, as the phenotype lactase persistence developed independently in different human populations. Although the single nucleotide polymorphism, -T 13910,is highly associated with lactase persistence in Europeans populations, there is more variants in SNPs that control lactase persistence in Middle Eastern and African populations. More research is needed to fully understand the evolution of lactase persistence worldwide.

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