Model reveals surprising disconnect between physical characteristics and genetic ancestry in some populations

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 found that when two populations with distinct traits combine over generations, the traits of individuals within the resulting “mixed” population reveal very little about the ancestry of the individuals. Their results were published on March 27 in a special edition of the American Journal of Physical Anthropology on race and racism.

“When two founding groups first come together, a visible physical trait that differed between these founders initially carries information about the genetic ancestry of the mixed individuals,” says Jaehee Kim, postdoctoral researcher in biology at Stanford and first author of the ‘study. “But this study shows that after enough time this is no longer true, and you can no longer identify a person’s genetic ancestry based solely on these 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 long been separated come together and create a third population mixed 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 case, individuals from the mixed population mated at random. In others, they were more likely to seek out mates with similar levels of genetic mixing or who had similar levels of a trait, in a process known as assortative mating.

The study found that over time, traits that might initially have indicated an individual’s genetic ancestry ultimately no longer carried this information. Although this decoupling of ancestry and trait would occur more slowly if the mating was assortative rather than random, the decoupling still occurred in all scenarios.

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

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

Only deep skin

To understand decoupling, say the researchers, consider a trait such as skin pigmentation which is in part due to variations among a series 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 shuffling of genetic variants that occurs with each generation increases the likelihood of such mismatches.

The researchers recognize that there are limits to their modeling approach. The model did not take into account environmental conditions which also play a role in the development of traits. A person’s height, for example, has a genetic basis, but also depends on factors like 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 significance of physical traits.

“When societies attach social significance to a trait like skin pigmentation, the model suggests that after mixing has been going on for enough time, that trait won’t tell us much about genetic ancestry – or on other traits based on genetics, “she says.