Marriage between close relatives increases the chance of certain genetic concerns. Specifically, it increases the chances of having a child with a recessive condition. You may remember from biology class that DNA is the instructional manual that tells our body how to grow, develop, and work properly. Genes are the individual units that make up our DNA. We all have two copies of every gene: one from mom and one from dad.
Some variants are responsible for physical differences, such as hair or eye color. And some DNA variants cause diseases. Sometimes, having a variant in just one copy of a gene is enough to cause a disease. But for recessive conditions, a person will only develop a disease when they have mutation in both copies of gene. For individuals to have recessive condition, they must have two non-working copies of a gene.
We each carry about 12 genetic variants associated with recessive conditions. But if you marry someone who has the same recessive variant as you, there is a chance your child may have the condition. The more DNA you share with someone, the more likely it is that you have the same disease-causing variants. However, Bittles notes that most research has not controlled for factors such as maternal age and education, birth spacing and socioeconomic status — those may correlate with consanguinity and may also affect health outcomes.
And the consensus about health risks appears to be shifting. In , the National Society of Genetic Counselors NSGC published the findings of a task force set up to look specifically at the risks for offspring of first cousins.
They estimated that health risks for those children were about 1. Have a question you would like answered here?
Send it to MonaChalabi or dearmona fivethirtyeight. She was previously a lead news writer for FiveThirtyEight. Ultimately, marrying your first cousin carries some risk. But the odds of healthy offspring dramatically improve with each new distance of relation. Second cousins share only 6. Seventh cousins—the average distance between modern American spouses—have no meaningful genetic relation at all.
The genetic data, branching off this way and that, reveals just how closely related we all already are. Small innovations can go a long way. Sign up to receive Popular Science's emails and get the highlights. Albert Einstein and his second wife and first cousin , Elsa Einstein, took family marriage to a new level. They were related through both maternal and paternal lineages: Their mothers were sisters and their fathers were first cousins.
Wikimedia Commons. Around one-quarter of the variable material is shared between uncles, aunties, nieces and nephews.
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