The model reveals a surprising disconnect between physical characteristics and genetic ancestry in certain populations


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A new study by biologists at Stanford University finds an explanation for the idea that physical characteristics, such as skin pigmentation, are “only superficial.” Using genetic modeling, the team discovered that when two populations with different traits are combined over generations, the traits of the individuals within the resulting “mixed” population reveal very little about the ancestry of the individuals. Their findings were published on March 27 in a special issue of the American Journal of Physical Anthropology about race and racism.

“When two founding groups come together for the first time, a visible physical trait that differed between those founders initially carries information about the genetic ancestry of mixed individuals,” says Jaehee Kim, a postdoctoral researcher in biology at Stanford and first author of the study. “But this study shows that after enough time has passed, that is no longer true and you can no longer identify a person’s genetic ancestry based solely on those traits.”

A decreasing correlation

Working with Stanford biology professor Noah Rosenberg and others, Kim built a mathematical model to better understand genetic mixing, the process by which two populations that have been apart for a long time come together and create a third mixed population with ancestral roots. in both sources. They specifically studied how the relationship between physical traits and the level of genetic mixing changes over time.

The researchers considered several scenarios. In one, individuals within the mixed population mated randomly. In others, they were more likely to look for pairs with similar levels of genetic mixing or who had similar levels of a trait, in a process known as selective mating.

The study found that, over time, traits that might initially have been indicative of an individual’s genetic ancestry eventually no longer contain that information. While this ancestry and trait decoupling occurred more slowly if the mating was selective rather than random, the decoupling still occurred in all scenarios.

“In the model, if selective mating depends on a genetically inherited trait, a correlation between the trait and genetic ancestry would last longer than if the mating had occurred randomly, but the correlation would still eventually dissociate,” said Rosenberg, lead author. from the article. , who holds the Stanford Chair in Population Genetics and Society at the College of Humanities and Sciences.

The team’s research was inspired in part by a study conducted by a different team in Brazil, a country with a lot of genetic mixing in its history. After taking samples of individuals and studying their genomes, the biologists in the 2003 study hypothesized that there was a decoupling between physical traits and genetic mixing and claimed that, over time, traits such as skin pigmentation they revealed little about the fraction of a person’s ancestors originating from Europe, Africa, or Native American Origins. The Stanford team found that their model largely supported this hypothesis.

Only on the skin

To understand uncoupling, the researchers say, consider a trait like skin pigmentation that is due in part to variations between a number of genes. If a person receives most of their genetic ancestry from one population, but the key genetic variants that determine their skin pigmentation from another, their skin pigmentation may appear to be a “mismatch” with their genetic ancestry. The rearrangement of genetic variants that occurs in each generation increases the likelihood of such mismatches.

The researchers acknowledge that there are limitations to their modeling approach. The model did not consider environmental conditions that also play a role in the development of traits. The height of a person, for example, has some genetic basis, but it also depends on factors such as nutrition. The model also focused only on scenarios where the initial mixing occurred all at once and did not explore the role of new members of the source populations entering the mixed population over time. In the future, Rosenberg plans to add some of these features to the initial model.

According to Kim, the new findings have important implications for understanding the social meaning of physical traits.

“When societies attribute a social meaning to a trait such as skin pigmentation, the model suggests that after mixing has been going on long enough, that trait is not going to tell us much about genetic ancestry, or about other traits that are based on genetics, “she said.

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More information:
Jaehee Kim et al, Skin deep: Decoupling levels of genetic mixing of phenotypes that differ between source populations American Journal of Physical Anthropology (2021). DOI: 10.1002 / ajpa.24261

Provided by Stanford University

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