The children of genetically-unrelated parents are more likely than those with similar genes to be taller and more intelligent according to the biggest study yet of human genetic diversity in an age when more people than ever are marrying people from different parts of the world.
Scientists found that height and general intelligence were two traits that appear to be increasing as a result of the mixing of DNA between genetically-diverse parents who on average share fewer genes than more closely related individuals.
However, the researchers also found that the increasing genetic diversity of the human population appeared to have little or no effect on a range of other medical traits, such as blood pressure, which could affect peoples’ health over their lifetimes.
Ever since Charles Darwin, scientists have argued over the effects of “inbreeding” between close relatives such as first cousins, but there have been few studies to look into the positive benefits of “outbreeding” between more distantly-related parents with widely different genetic backgrounds.
The latest research, published in the journal Nature, analysed more than 100 separate studies carried out around the world involving some 350,000 people living in both rural and urban environments.
The research compared the genetic diversity of genome data with 16 biomedical traits, from height and cholesterol levels to cognitive ability and academic achievements.
The researchers found that four traits – height, lung capacity, general cognitive ability and educational attainment – had increased significantly in line with the level of outbreeding resulting in increased genetic diversity within the genomes of individuals living as far apart as Finland and East Asia.
Peter Joshi of the University of Edinburgh, the first author of the study, said that the researchers attempted to compensate for environmental effects – such as differences in upbringing due to socio-economic status – that could have influenced the results.
However, even after taking these non-genetic factors into account, they still found that the degree of genetic-diversity was a significant factor affecting the four traits.
“We’ve found that the genetics are associated quite robustly across populations and although we tried to compensate for environmental factors, we think the genetic effects are real,” Dr Joshi said.
“There has been speculation ever since Charles Darwin that genetic diversity would be beneficial in terms of evolutionary fitness. We think genetic diversity decreases the chances of inheriting defecting copies of the same gene from both father and mother,” he said.
“Our research answers questions first posed by Darwin as to the benefits of genetic diversity. Our next step will be to hone in on the specific parts of the genome that most benefits from diversity,” he added.
Although the effects are relatively small, the scientists calculated that they would be equivalent to the children of first cousin marriages being an average 1.2cm shorter than the children of genetically-diverse parents, and having a typical academic attainment which is equivalent to 10 months less in full-time education.
The findings support the idea that increased height and cognitive ability have been positively selected for by evolution over many generations in different parts of the world such as Europe, Africa and Asia, the researchers said.
Co-author of the research Jim Wilson of the University of Edinburgh said: “This study highlights the power of large-scale analyses to uncover fundamental information about our evolutionary history.”
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