Before 500 years ago people rarely went far to find a mate, choosing a husband or wife from the locally available pool of men and women. But with the dawn of European colonialism people from different parts of the world were suddenly living side by side, and had a whole new set of people to choose from when picking a mate. This trend continues especially in today’s world, where education and work opportunities have flung young people across the globe, allowing them to interact – and potentially marry – people from opposite sides of the globe.
There is ample historical evidence for an increase in exogamy (that is, marrying someone outside your village or town) over the past few hundred years, but could there be evidence stored in our genes as well? In this week’s PLoS Genetics, an international team of scientists claims to have found genetic clues that allow them to see decreases in inbreeding across the 20th century.
Every person has two copies of each gene – one from our mother and the other from our father. The more closely related a person’s parents are, the more similar those two copies are to one another.
The authors of the PLoS Genetics study examined the DNA of more than 800 Americans of European ancestry who ranged in age from 19 to 99. Then they looked at how closely each person’s two DNA strands matched one another, a measure called autozygosity that roughly corresponds to the amount of inbreeding in an individual’s pedigree.
The results overwhelmingly confirmed the notion that levels of autozygosity have significantly decreased over time. When the authors examined the DNA of the youngest subjects, they found significantly lower levels of autozygosity when compared to the oldest subjects. In fact, they found a steady decrease in levels as they went from oldest to youngest.
What does this mean? In the last several generations, there’s been a steady decrease in marrying someone from the same town, village, or region (who could potentially be a distant relative). Instead, the percentage of people who’ve chosen to marry someone from a different town, village, region, or even continent, has been on the rise.
According to the authors there are benefits to this increase in genetic diversity, primarily a reduction in the risk of recessive genetic diseases.