Arlington, Va. -- A genetic trait enabling some Tibetan women to achieve relatively high oxygen levels in their blood, despite living at oxygen-scarce altitudes, is associated with higher infant survival, according to research reported in the online edition Proceedings of the National Academy of Sciences. Tibetan mothers who have the oxygen-enriching gene(s) also give birth to infants who are more likely to survive their first year of life than are children of mothers without such genes.
Beall and an interdisciplinary team took data on mothers and their children in over 900 Tibetan households who lived at altitudes up to about 13,800 feet above sea level. For comparison, the peak of Hawaii’s volcanic mountain, Mauna Loa, is 13,680 feet above sea level.
The researchers measured the women’s blood oxygen and collected genealogical and fertility data, including the numbers of pregnancies and births, surviving children, and other health status factors.
Although the researchers did not examine the women’s DNA directly to identify a high-oxygen-conferring gene, they used an established statistical method to infer from blood samples and family history data the gene’s existence and path of inheritance from mother to child. According to their findings, one copy of the suspect gene, out of a possible two, gives Tibetan mothers the ability to infuse extra oxygen into their bloodstreams, an important survival asset because it decreases the high-altitude stress of oxygen deficiency.
The researchers then compared women’s blood-oxygen levels to the likelihood that they would become pregnant and give birth to living children, and to the likelihood that those children would survive to age 15 years.
They found the predisposition to achieve high oxygen levels did not affect a woman’s likelihood of becoming pregnant or delivering a live child, but it did affect that child’s chances of living past infancy. Children of moms with the genetic advantage were about five times more likely to live to the age of one year than were other children.
A higher survival rate for children of parents with certain genes is a defining feature of natural selection. “Questions about how humans adapt to their environments are important,” comments Mark Weiss, program officer at the National Science Foundation, which was a sponsor of the research. “This study links Darwinian fitness to a genetic adaptation – the ability to compensate for oxygen deprivation at high altitudes. As people migrated out of the tropics and into inhospitable environments they had to adapt both culturally and biologically. The work by Beall and colleagues is a really neat example of selection molding the genetic composition of a population to allow life in an extreme environment."
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