The model reveals surprising break between physical traits and genetic ancestry in certain populations

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A new study by Stanford University biologists finds an explanation for the idea that physical properties such as pigmentation of the skin ‘are only deep skin’. Using genetic modeling, the team found that when two populations with different traits come together over generations, characteristics of individuals within the resulting ‘mixed’ population reveal very little about the descent of individuals. Their findings were released on March 27 in a special issue of the American Journal of Physical Anthropology about race and racism.

“When two founding groups meet for the first time, a visible physical trait that differs between the founders initially contains information about the genetic ancestry of mixed individuals,” says Jaehee Kim, a postdoctoral fellow in biology at Stanford and the first author of the study. “But this study shows that, after enough time, it is no longer true, and that you can no longer identify a person’s genetic ancestry based on such traits.”

A decreasing correlation

In collaboration with Stanford biology professor Noah Rosenberg and others, Kim has built a mathematical model to better understand the genetic mix – the process by which two long-separated populations come together and create a third mixed population with ancestral roots in both sources . They specifically studied how the relationship between physical traits and genetic mixing level changes over time.

The researchers considered several scenarios. In one, individuals within the mixed population were randomly paired. In others, they were more likely to seek out partners with similar levels of genetic mixing or similar levels of a trait, in a process known as assortative mating.

The study found that traits that could initially be an indication of an individual’s genetic lineage, over time, no longer contained the information. Although this decoupling of origin and traits occurred more slowly than mating rather than randomization, decoupling still occurred in all scenarios.

“If the assortative mating in the model depends on a genetic hereditary trait, a correlation between the trait and genetic ancestry would last longer than if mating were to occur randomly, but the correlation would eventually still come loose,” said Rosenberg, senior author of the article, said. , who earned the Stanford Professorship in Population Genetics and Society from the School of Humanities.

The team’s research was inspired in part by a study conducted by another team in Brazil, a country with many genetic blends in its history. After sampling individuals and studying their genomes, biologists from the 2003 study hypothesized that there was a link between physical traits and genetic mix, claiming that traits such as pigmentation of the skin did not reveal much over time. fraction of the ancestors of a person of European, African or Native American descent. The Stanford team found that their model largely supported this hypothesis.

Only skin deep

To understand the linkage, the researchers say, consider a trait such as skin pigmentation that is due in part to variations between a range of genes. If someone happens to receive most of their genetic ancestry from one population but the major genetic variants that determine their skin pigmentation from another, it may seem that their skin pigmentation is a “mismatch” to their genetic ancestry. The reshuffling of genetic variants that occur in each generation increases the likelihood of such mismatches.

The researchers realize that their model approach is limited. The model did not take into account the environmental conditions that also played a role in the development of characteristics. The length of someone, for example, has a genetic basis, but also depends on factors such as nutrition. The model also focused only on scenarios in which the initial mix occurred simultaneously, and did not examine the role of new members of the source populations who eventually entered the mixed population. 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 significance of physical characteristics.

“When societies attach social significance to a trait such as skin pigmentation, the model suggests that after the mixture has been running for a long time, we will not tell much about genetic origins – or other traits based on genetics,” she says. said.


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

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Quotation: Model reveals surprising disconnect between physical traits and genetic ancestry in certain populations (2021, April 5) on April 6, 2021 detected from https://phys.org/news/2021-04-reveals-disconnect-physical-characteristics-genetic. html

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