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Digging deep into high altitude living

Emilia Huerta-Sánchez

 

Emilia is an accomplished scientist who was the first in her family to obtain an undergraduate degree (at Mills College, a small liberal arts college in Oakland, CA) and then went on to do her Ph.D. work at Cornell. "If people like what they are doing and are motivated, they can have a good career in science — whether they start at a highly competitive school or not," says Emilia. "Good mentorship is also really important in science. Throughout my education, I've had wonderful mentors. Without them, I wouldn't be where I am today."

Emilia's background in applied math provided her with the tools to find EPAS1's signature of selection, but it is biological curiosity that keeps her going. "It's so interesting that just one mutation or a few mutations could make all the difference in who can live in a particular place," she says. Judging by the complex response of Tibetans to high altitudes, there may also be other genes that were undergoing selection at the same time as EPAS1. She and her colleagues are currently designing studies that will help identify variants in these genes that complement EPAS1. In addition to trying to find these mutations, Emilia is currently working on improving her model of the population's history so that she can learn more about the timing and strength of selection on EPAS1. If her current model is right about the split between the Han and Tibetan lineages, the rate at which the Tibetan EPAS1 allele increased in frequency is the fastest of any allele yet studied in humans!

Emilia is also excited about a collaboration with scientists in Britain and Ethiopia to study the genetic basis of the Ethiopian highlanders' adaptations to life at high altitudes. The results will be particularly intriguing because Ethiopians seem to have different high-altitude adaptations than Tibetans do. Perhaps the targets of selection in Ethiopians will turn out to be entirely different genes — or the same genes but different mutations.

Emilia tackles all of these questions from her computer terminal in California, but she may be traveling to other places as well. Biology's lure seems to be pulling her further away, and Emilia says that she would like to do some fieldwork next: "I haven't been to Tibet. I would love to go, but I'm afraid that I'll get altitude sickness …"


Discussion and extension questions:

  1. In your own words, explain what an evolutionary adaptation is. Give one example of a human evolutionary adaptation not described in this article.
  2. In your own words, explain what phenotypic plasticity is. Give one example of phenotypic plasticity not described in this article.
  3. Most people experience increased infant mortalities at high altitudes. Explain how a mutation that allows normal levels of infant survival at high altitudes would spread through a human population that had just begun living high in the mountains. Make sure to include the concepts of variation, selection, and inheritance in your explanation.
  4. Examine the graph of allele frequencies among Tibetans and Han Chinese from this article. There are four or five outlying data points in the upper left hand corner of the graph. Explain what these points represent in terms of allele frequencies. What scenarios involving natural selection are consistent with the observation of these allele frequencies?
  5. Examine the table of genotypes, hemoglobin levels, and red blood cell counts from this article.

    1. What is the most common allele among Tibetans?
    2. What is the most common genotype among Tibetans?
    3. Which allele is most likely to be adaptive for Tibetans living at high altitudes? Which data from the table support this inference and how?
    4. Calculate the allele frequencies for C and G and the expected number of individuals of each genotype in the Tibetan sample under Hardy-Weinberg equilibrium.
    5. Perform a chi-squared test to determine whether this locus conforms to Hardy-Weinberg equilibrium in Tibetans.
    6. Does your answer for item e above have any implications for the evolution of the EPAS1 locus among Tibetans? Include random or assortative mating, mutation, migration, and natural selection in your response. Does this conclusion support or conflict with the idea that EPAS1 has been under selection in the Tibetan highland population? Explain your answer.


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Photo by Dave Strauss