Tired of hearing of these urban legends and popular opinions, I ask this questio

Question

Tired of hearing of these urban legends and popular opinions, I ask this question here to see if there really is scientific merit to this belief. My questions is, is it really "harmful" for a person to reproduce with his/her own first cousin? Everyone, at least in America, seems to think that this is incest which I completely dismiss because incest is purely a religious concept. The next argument I get is that we want more genetic diversity and having offspring with a first cousin is detrimental to our survival as a species. If you have any genetic weaknesses then your cousin is likely to be carrying them as well and then the chances of your offspring carrying them increase and so on.

I don't really believe any of this. It is rather well known (or may be not so well known) that only a very small percentage of a human individual is different from another human individual's genome. Differences due to skin color, race, ethnicity, and such are literally skin deep. So one reproducing with her cousin or someone half way across the globe is only negligibly different.

Forgetting about religion, has this been scientifically systematically tested and/or proven? Has any research been done on this? Is it true one way or the other? I don't mean to spark any debates here. Opinions are of course welcome. But if anyone can point me to any reputable sources or published results/references, that would be ideal.

Explanation / Answer

Inbreeding depression, "the reduced survival and fertility of offspring of related individuals" (quoting the linked article), is a well-known and well-understood biological effect. It does, indeed, affect humans.

The problem is that recessive mutations become more likely to affect the survival of the offspring of relatives. Imagine that you have a mutation affecting, say, some gene that's critical to your metabolism, which is entirely possible. But, since you have two copies of the gene and only one of them is faulty, you are not affected by this. Nevertheless, you, by chance, pass the non-functional one on to a couple of your kids, who are also not affected due to only having one copy as well. They, in turn, pass it on to one or more of their kids. Now, two of them, cousins, have a kid. The kid gets both copies and can no longer properly metabolize something due to having two faulty copies of the gene. This can prove to be fatal or at least severely debilitating.

Now, lest you think this is improbable, there are on average 70-something mutations per generation. Some of those will be neutral in effect, others will be detrimental. Once an offspring gets two copies of those detrimental alleles, then there could be serious health effects. The likelihood of an offspring getting two copies decreases as the relatedness of its parents decreases. So, sibling or parent/child pairings would be the most likely to produce offspring with the problems. Pairing between 3rd generation (First-cousins) or any pairings between them and their siblings, parents, aunts/uncles or grand-parents would be the next most likely.

I missed the "as a species" part of your question. Inbreeding will only likely have an effect within small, closed populations, though it will continue to have a lasting effect even as those populations grow and open up. Two textbook examples are French Canadians and Ashkenazi Jews. Even now, there is continued elevated risk of certain rare genetic diseases in these populations. As for the species as a whole, it is likely to be really detrimental only if the effective population size of the species becomes really low: a general rule of thumb used by conservation biologists is that the effective population size should be at least 50 to avoid the effects of inbreeding (and 500 to avoid the effects of genetic drift) (50/500 rule).

Related Questions

Navigate

• Browse (All)
• Browse T
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.