Lecture_04_hum_variation_II0-1 - Lecture 4 Other Examples...

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8/31/2006 Lecture 4 Other Examples of Human Adaptations Lactose tolerance : * * The enzyme lactase is needed to digest lactose (the sugar in milk) - All humans produce lactase from birth to 4-6 yrs . - After this, lactase secretion ceases; drinking milk (etc) will cause gastro- intestinal distress - BUT: People remain LACTOSE TOLERENT in a few populations ~ Vitamin D has three principal sources in humans 1) synthesized in the epidermis (skin) via a steroid plus UVB from sunlight. 2) dietary (fish and fish livers, mammal livers, shellfish, and egg yolks) 3) lactose in milk Lactose+lactase biochemically mimic vitamin D in the intestine * Infants depend on lactose in milk for Vitamin D Why are some human populations lactose tolerant? * lactose intolerance in late infancy = may encourage weaning * Hypothesis: Lactose tolerance selected for in the North (not much sunlight, lots of clothes) * Hypothesis : Pastoralists = consume milk, cheese, yogurt . - Pastoralists: good to able to digest milk as adults -- outweighs need to wean infants Lactose tolerance in adults evolved independently in several separate populations during the last 8000 years * lactose tolerance = most common in northern Europe, West Africa, East Africa, and Arabia. * Mutation in ONE gene (LAC*P) prevents the shut-down of lactase secretion. (Dominant) - Natural Selection can act rapidly on this trait. * Variation in the LAC*P gene show that mutations occurred in several unrelated populations, then spread Further aspects of Lactose Tolerance * Some populations in the Middle East and Central Asia that are lactose intolerant make milk edible by turning it into yoghurt, koumis, kefir - Bacteria digest the lactose first -- No problem for humans after this * In the 1960s, U.S. aid organizations sent large quantities of powdered milk to famine stricken people in Africa and Asia
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8/31/2006 - Although infants could benefit from this milk, most people over the age of 6 couldn’t drink the milk
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Lecture_04_hum_variation_II0-1 - Lecture 4 Other Examples...

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