There’s a New Explanation for “Genetic” Trait Pairs: Your Parents

Border and his colleagues are not the first to raise the possibility of false genetic correlations. When designing studies, geneticists can control for the effects of factors such as parental traits and childhood environment by comparing people who have these things in common, i.e. siblings and sisters. Earlier this year, statistical geneticist Laurence Howe and a team of researchers did just that. When Howe compared the siblings to each other, he observed no genetic correlation between BMI and years of education. Somehow it was the parents, not the genes themselves, that made weight and upbringing seem genetically linked.

But Howe’s study did not explain exactly How? ‘Or’ What parents played a role. There were promising possibilities. Parents don’t just pass on genes to their children, they also pass on their socio-economic status, which has implications for both schooling and diet. And, of course, parents usually choose who they breed with. Loic Yengo, group leader of the University of Queensland’s Statistical Genomics Laboratory, says geneticists had realized that mating with cross-traits could, in theory, inflate genetic correlations. But no one had yet produced concrete evidence that this was the case.

Border and his colleagues found this evidence. Studying cross-character matched mating in detail requires knowing how much of it actually happens in the real world. It seems reasonable that depressed people might end up with anxious people because of their shared experience of living with mental illness, or that educated people tend to marry people who have high IQ test scores, but Border had to quantify these trends. . The team was able to find the information they needed in the UK Biobank, a massive dataset that includes genetic, medical and demographic data on hundreds of thousands of UK residents. They found that the more people who had a particular pair of traits tended to mate, the more those traits appeared to be genetically correlated. It was therefore reasonable to suspect that assorted mating actually caused certain genetic correlations to appear stronger than they would otherwise.

Yet this observation did not prove that assorted mating could create the illusion of a genetic link where none existed. So Border and his team turned to a computational approach: Following the marital trends they had observed in real Biobank data, they simulated a population of people who had partnered. These imaginary couples reproduced, and their children found partners, and their children’s children, and so on. Scientists tracked the genes and traits of all of these simulated individuals and, using this information, they were able to calculate genetic correlations across each generation. What they found confirmed their suspicions – even though two traits were completely unrelated genetically in the first generation, if people who had those traits tended to mate, the genes eventually started to seem correlated. Based on the simulations, they estimated that assorted mating alone could explain up to half of the genetic correlation between BMI and education.

But assorted mating didn’t go that far to explain some of the other apparent correlations they simulated. It appears to play a lesser role in genetic correlations between certain pairs of psychiatric disorders, such as bipolar disorder and schizophrenia, or major depression and anxiety. Since each pair of conditions shares so many genetic similarities, some scientists have questioned whether they should even be considered separate conditions. Even allowing for assorted mating, this argument would still seem valid.

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