Natural selection for lactose tolerance

December 14, 2000

New data in the January issue of The American Journal of Human Genetics suggest that the habits of your ancestors may have determined whether you can eat an ice cream sundae without experiencing the unpleasant side-effects of lactose intolerance.

Before fresh animal milk was readily available, humans did not need to be able to digest milk after they were weaned. However, the domestication of mammals approximately 9,000 years ago presented humans with the first opportunity to drink fresh milk throughout life. This provided a potential selective force for lactase persistence, whereby high levels of intestinal lactase, the enzyme that digests lactose, persist into adulthood. Individuals without significant levels of lactase are lactose intolerant, and the frequency of lactose tolerance varies between populations. Dr. Dallas Swallow and colleagues have examined the lactase gene in several human populations to learn more about its evolutionary history. They were able to determine the forces that shaped the lactase gene during two evolutionary periods, one before and one after fresh milk was available.

A significant event in human evolution was the movement of early humans from their roots in Africa to the rest of the world. Scientists estimate that this migration occurred before fresh animal milk was available to humans. Dr. Swallow compared the gene sequences of the lactase gene region between modern Africans and non-Africans in order to study the selective forces on the lactase gene during this time period. While African populations have high levels of genetic diversity at this locus, non-African populations show lower, and generally similar, levels of diversity. These results suggest that, as small populations moved away from the larger African population during this period, genetic diversity was lost in the migrating groups due to random fluctuations in the gene frequencies, termed genetic drift. After early human populations separated to different corners of the world, some populations developed a taste for fresh milk as it became available through the domestication of animals. This event therefore provides a second, and more recent, stage at which to evaluate evolution at the lactase locus, and it allowed the researchers to find evidence that milk consumption might have affected the evolution of the lactase gene. Northern Europeans have a long history of fresh milk consumption, and they were the only population in this study with a high frequency of lactase persistence. Dr. Swallow reports that the northern Europeans also have an unusually high frequency of one particular set of DNA sequence variations, known as a haplotype, at the lactase locus. This led her and her colleagues to propose that this haplotype is common in the northern Europeans because it is associated with the ability to digest milk. They believe that this property has (in the past) provided a selective advantage to individuals carrying the haplotype. Over time, the advantageous haplotype became more prevalent in this milk-drinking population. Thus, culture can influence human genetics.

Although the selective advantage of milk consumption is not clear, previous studies have suggested that it may be due to such things as better nutrition or calcium absorption. The authors hope to perform additional comparisons between closely related populations in order to further study the relationship between culture and genetics at the lactase locus.
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For the full text of this article, please see "Lactase haplotype diversity in the Old World" by Edward J. Hollox et al. on the electronic edition of the January issue of The American Journal of Human Genetics at www.ajhg.org.

For further information, contact Dr. Swallow at the University College London. Phone: 44 207 679 5040. Fax: 44 207 387 3496. Email: dswallow@hgmp.mrc.ac.uk.

Contributed by Kate Beauregard, The American Journal of Human Genetics. Phone: 404-712-9985. Email: kbeaure@emory.edu.

The American Journal of Human Genetics

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