A question which I have been pondering.
Say X gives birth to a son and a daughter – let call down A and B respectively. Now A marries someone else and has a daughter, while B marries someone else and has a son. Is it safe for the grand-daughter and grand-son to get married? Would their genetic materials be too silmiar?
“Now a study by the National Society of Genetic Counselors says that having a child with your first cousin raises the risk of a significant birth defect from about 3-to-4 percent to about 4-to-7 percent. According to the authors, that difference isn’t big enough to justify genetic testing of cousin couples, much less bans on cousin marriage.”
From this Slate article, which has a bunch of interesting things to say about cousin marriage.
Once isn’t too much of a problem, but it can have adverse effects in cultures that constantly practise cousin marriage. For example, 55% of British Pakistanis are married to their first cousin. This has led to more serious birth defects - this group has just 3% of births in the UK, but account for roughly 1/3 of the serious birth defects.
I’d tend to challenge the premise of this. ‘Constantly’ practicing cousin marriage doesn’t seem to me to have any direct effect of leading to higher birth defect risk in a culture, other than the risk to the direct progeny of the marriage. Overall, the chance of the surviving children to carry dangerous genes is actually slightly lower, allowing for the fact that there’s a risk that the children might get a dangerous double-gene combo and thus die young. Even if someone gets a fairly minor double-gene combo and grows to adulthood and marries, there’s no chance that they’ll ‘pass that combo on’ because they’ll only give one half of it to their kids… and the chances that their spouse also has a copy of that particular gene, even if they’re from the same family, is probably not that high.
I think what’s going on with the british pakistanis in that story is something a little subtle and different. If you have a relatively small population continually breeding within their own limits, then you’re going to start to lose some bits of healthy genetic diversity by pure chance. This can happen without first cousins or second cousins marrying, though of course no finite population can continue to interbreed without having n-cousin marriage, (the population of the earth gets us up to around the 30th cousin limit, which is probably pretty safe.)
Does any of that make sense? Am I totally out to lunch??
Cecil says so !
Odd, I read in The Economist last year that cousin-marriage was not as dangerous as we once thought. If Pakistani-Brits have a hug number of birth defects perhaps we should factor in other issues such as nutrition, exposure to chemicals and the lack of abortion in that community.
(Of course the results could still come out as the previous poster said. I must say, I rarely hear of birth defects here.)
Just a quick google check shows that Saudi Arabia has the second highest rate of birth defects in the world, after Sudan. Cite.
Yoinks! I had no idea! Thank you so much for the cite!
The Kingdom has just introduced mandatory DNA testing before marriage. (Oh yeah, that will be enforced.)