Public release date: 29-Aug-2013 [ | E-mail | Share ]
Contact: Clare Ryan clare.ryan@ucl.ac.uk 44-020-310-83846 University College London
A genetic phenomenon that allows for the selection of multiple genetic mutations that all lead to a similar outcome -- for instance the ability to digest milk -- has been characterised for the first time in humans.
The phenomenon, known as a 'soft selective sweep', was described in the population of Ethiopia and reveals that individuals from the Eastern African population have adapted to be able to digest milk, but via different mutations in their genetic material.
A team of geneticists from UCL, University of Addis Ababa and Roskilde University have shown that five different alleles are found in the Ethiopian population that cause adult lactase production, one of which is newly confirmed. Their study is published in The American Journal of Human Genetics.
Professor Dallas Swallow, from the Department of Genetics, Evolution and Environment, senior author of the paper said: "Our genetic make-up determines our ability to digest milk into adulthood. Just over a third of the global population have inherited genes that allow us to make lactase, the enzyme that digests milk, as adults.
"This study shows that several different genetic changes that allow our bodies to make lactase have emerged independently. Changes to our lifestyle over the past 10,000 years -- including diet, altitude acclimatisation and infectious disease resistance -- will likely have caused many genetic adaptations of this kind."
We need lactase when we are babies to digest our mother's milk, so in babies large amounts of lactase enzyme are expressed by our genes. When we are older we no longer rely on our mother's milk for essential nutrients, so in most humans manufacture of the lactase enzyme stops through de-activation of the corresponding gene.
However, subtle mutations in the regulatory region of the gene in some individuals cause lactase to carry on being expressed into adulthood. Different mutations are likely to affect lactase expression using slightly different mechanisms. This parallel selection of different gene mutations that have the same phenotypic effect - in this case lactase persistence - is known as a soft selective sweep.
Soft selective sweeps have not been so clearly described before in humans, one reason being that variations caused by soft selective sweeps are more likely to be caused by genetic mutations in regulatory sequences, rather than mutations found in coding regions of genes.
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Digesting milk in Ethiopia: A case of multiple genetic adaptations