Dictionary of Human Evolution and Biology

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Disaccharide Intolerance Type III

Refers to any of a number of autosomal recessive mutations resulting in a deficient enzyme, adult lactase; results in an inability to metabolize milk sugar (lactose), causing symptomatic abdominal gas, cramps, and bloating after the ingestion of milk products. Type III is the adult form, in which the enzyme functions only in youth, but is then lost in some populations, producing a persistence/nonpersistence polymorphism. Adult heterozygotes usually experience gas pain after consuming dairy products. This nonpersistence condition is polymorphic and found in high frequency among many human populations that did not domesticate cows or other mammals, and that have not maintained through natural selection a functional allele of the enzyme. Sometimes called adult lactase deficiency, when described in terms of the enzyme itself. Because adult animals lose the ability to digest lactose, and because of the worldwide prevalence of the deficiency in humans, lactose malabsorption and intolerance is considered the normal state for human adults. Adult lactase persistence, found in fewer populations, is the rarer, derived form of the polymorphism, and generally characterizes pastoral populations in northern Europe, Arabia, and East Africa. Diversity in the African U-type haplotypes is consistent with the out of Africa II model of recent human dispersion.

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