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Genetic Traits

Genetics Behind Lactose Intolerance

All mammalian babies are born with the ability to digest lactose, a sugar found in mother’s milk.  Lactose is present abundantly in milk for the nourishment of new born babies until weaning.  Lactose itself cannot be absorbed by intestine; it has to be broken down into absorbable sugar glucose and galactose by lactase anchored on the surface of small intestine epithelial cells.  The lactase (also called lactase-phlorizin hydrolase (LPH) in human) is encoded by the lactase gene (LCT) on human chromosome 2 (2q21).

The expression of lactase in small intestine usually cease gradually after weaning.  The result is that adult mammals eventually lose the ability to digest lactose.  In Humans, however, certain populations will have normal lactase expression (called Lactase Persistence, LP), and therefore can continue to enjoy milk and other dairy foods into adulthood,  while the other populations will have an insufficient level of lactase to digest dairy foods properly. For those people, dairy food will cause indigestion (so called lactose intolerance) because undigested lactose stimulates fast growth of gas-producing gut bacteria, causing symptom such as bloating, gassing and diarrhea.

Genetic studies revealed that lactase persistence (LP), a new phenotype unique to humans, is associated with various autosomal dominant alleles maintaining the expression of lactase beyond infancy; those LP alleles are located far upstream of lactase gene (LCT).  Those genetic changes emerged relatively recently in the last 10,000 years among some populations.  This coincides with the transition from nomadic to agricultural life style, cattle domestication and milk/diary food becoming daily food in pastoralstic populations.  In the northern Europe region, allele T in SNP C/T-13910 and allele A in SNP G/A-22018 (13 or 22 thousand base upstream LCT gene) were identified as the causal alleles for LP.  In the North Africa and the Middle East, a different set of alleles (G/C-14010, T/G-13915 and C/G-13907) was identified for LP. 

The observation of different SNPs responsible for the same phenotype is a great example of how human have evolved with nature and how culture can influence our genes.  The availability of milk as a rich source of nutrients for adult put a selection pressure on the population to take that advantage.  In two separate locations, different SNP were emerged in the same chromosomal region where those changes can maintain persistent lactase level into  adulthood.  In the evolution term, this is called convergent evolution or convergent adaptation.  On the other side of coin, it is also interesting that in the non-pastoralistic regions where milk/diary food is not a part the adult daily diet, LP alleles are very low (~1% in Chinese,~5%-20% in West African agriculturalists).  Of course, one would expect there are other genetic changes that offer survival advantages based on agriculturalists dietary habits.

Further readings:

Sarah A Tishkoff  et al. Convergent adaptation of human lactase persistence in Africa and Europe.  Nat Genet. 2007; 39(1): 31–40

Rejane Mattar et al. Lactose intolerance: diagnosis, genetic, and clinical factors.  Clinical and Experimental Gastroenterology.  2012 (5):113 - 121

Itan et al. A worldwide correlation of lactase persistence phenotype and genotypes.BMC Evolutionary Biology 2010, 10:36

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