Victoria Woolaston, Daily Mail, October 19, 2015
It may not be a conscious decision, but a study has found we seek out partners who have a similar genetic ancestry to our own.
Researchers studying children in Mexico and Puerto Rico discovered their parents tended to share similar genes, even though they weren’t related.
In fact, the average mix was so alike in some cases, the couples were as genetically similar as third or fourth cousins. This is known as ‘assortative mating’.
The research was led by Dr Noah Zaitlen from the University of California, San Francisco.
His team specifically studied the parents of Mexican and Puerto Rican children with asthma.
In the case of Mexicans, the parents typically had a similar proportion of mostly European and Native American ancestry, with some genomic heritage from Africa.
For Puerto Ricans, this ‘assortative mating’ meant they had similar amounts of European and African ancestry, with some Native American.
The average mix was similar enough to make the couples equivalent to between third and fourth cousins, said the researchers.
This degree of closeness may have implications for genetic diseases.
In particular, because Puerto Ricans have mutations from both the Spaniards who colonised the island and the Native American women who bore their children, the researchers estimated assortative mating could increase the chances of recessive diseases by two to 14 per cent after 10 generations of mixing.
This may help explain the high prevalence of certain diseases such as asthma among Puerto Ricans.
However, assortative mating could also have health benefits.
A separate study carried out in Iceland, for example, found that the most fertile couples were about as closely related as fourth cousins.
Among the Puerto Rican, but not the Mexican couples, the researchers discovered that parents had similar genes associated with facial characteristics.
The strength of the ancestry in both groups was stronger than ‘education assortment’, in which people choose a mate based on similar education levels.
Dr Zaitlen said: ‘To avoid mathematical complexities, population and medical geneticists typically assume that people choose their mates randomly when modelling everything from demographic history to the diseases in a population.
‘We now have evidence that these choices may not be random at all, and we should incorporate this new understanding to more accurately model human history and improve our understanding of the genetic basis of disease.’
The researchers stressed that they only found associations between factors such as ancestry and partner selection, not evidence that one was influencing the other.
They also warned that some of the factors they measured, such as the genes known to be involved in facial development and genomic ancestry, could be entangled with one another, or related to other factors they didn’t measure, such as culture and religion.
Given their subjects’ mixed European, Native American and African ancestry, the researchers said the participants’ high genomic diversity is likely to have contributed to a wider array of facial features and this may have magnified the effect of their tendency to choose similar-looking partners.