I know that Incest threads have been around before, but the search I did (poor hamsters) couldn’t answer this question so I thought I would pose it here. If it has indeed been answered in the past, I would be grateful for a link
I was at my Wife’s Uncles’ (I will now refer to him just as “Uncle”) ranch a few months ago, and he was showing me his horses. Some of them were gorgeous. One Filly in particular was the product (unintentionally) of the mare mating with her son… the son wasn’t supposed to be sexually mature enough for almost another year, but he was obviously a fast grower
Anyhow, I asked her Uncle (who also happens to be an MD, a very bright and educated man) if there was anything wrong with the filly since it was “inbred.” He said no. It appeared to be perfectly fine. He said it is usually more of a problem after a few generations of inbreeding. IE - This Filly mates another Stallion from the family, etc…
Now, there are cases of incest with humans where fathers have impregnated their daughters and the child was not “normal”, but I also assume there have been cases where the child was “normal” … like the Horse we were looking at. However, I think it is safe to say (in my uneducated opinion) that the more closely related the parents are, the greater chance of there being a genetic abnormality, correct?
So, I wonder how “distantly” related would parents have to be for there to be no real greater risk of the child being “abnormal” that the rest of the population having offspring? Lets not complicate the issue of further generations inbreeding with each other more, a one-off scenario.
I’m not sure that one can make a hard-and-fast line, where it doesn’t matter how closely related the parents are. For instance, if both parents are from certain parts of Africa, there is a much greater risk for sickle-cell anemia than if one parent was African and the other, say, Inuit. Likewise, there are probably some heriditary diseases to which the Inuit are subject which are unheard of in tropical climes, and a child born of mixed parents would again have almost no chance of expressing them.
Now, before you argue that this isn’t a matter of inbreeding, the parents of a child expressing sickle cell must have had a common ancestor, who passed down the sickle-cell gene to both of them. Admittedly, this common ancestor might have been a long time ago (sickle cell has been around for a while), but one can find individuals who are even more widely separated.
The risk in inbreeding is the reinforcement of harmful recessives. If there aren’t any, the danger is nil. If there’s something that requires several generations of reinforcement (a condition caused by several dozen copies of a normal gene, for example), then some risk exists. If there are recessives present, and they’re harmful, then inbreeding requires culling. If culling is not allowed, then inbreeding (and incest) carry greater risk.
The question, then, is “On average, what is the probability of reinforcing a harmful recessive when related pairs of unknown genotype are mated, and when is it too great?” The answer is “I don’t know, but it’s mathematically related to the presence of such alleles in the population” and “that’s not for me to say.”
I was hopefully looking for something akin to a genetic Rule of Thumb if there could be such a thing… such as
Siblings that have children together are 75% more likely to have a defect caused by a recessive gene. Uncles/nieces are 50%, First cousins are 10%, second cousins have no more of a chance than the “regular” population… or something along those lines, if even possible.
It appears (the second post wasn’t up when I was composing my first reply) that there must be just waaaay to many variables to give any answer with the slightest degree of accuracy.
I suppose in the case of Sickle-cell is a good example. Their common ancestors could he 100 generations (or more away) but the gene still effects them.
Thanks for the attempts to help answer my question.
I think that the answer to that last would depend on several factors, including whether any traits referrable to that gene show up in the individuals, how common the gene is in the general population, etc.
It’s important to realize that inbreeding simply reinforces characteristics carried on recessive genes (plus of course those on dominant genes). If coupled with a ruthless culling program (not quite so heartless as it sounds, being simply where any offspring with undesirable traits are not permitted to breed), inbreeding programs can stabilize a desirable new trait most effectively. For obvious reasons, this is rarely done in humans – and the reinforcement of negative characteristics from marrying within a relatively small gene pool has a lot to do with the decline in capability in European royal families. (As John Corrado and I have occasion to remember the earlier discussion of here, there was a tradition in Egypt of brother-sister marriages in the royal family lasting well over a thousand years.)
So mating a genius and his sister after a check of their ancestors and collaterals shows few if any undesirable recessive traits would be genetically positive, though morally dubious to say the least.
With humans, there’s a simple equation you can do. It tells you the chance of having a genetic defect in a specific gene caused specifically by inbreeding. Unfortunately, I can’t remember what it is.
Incidentally, inbreeding in and of itself isn’t harmful. It just increases the chance of expressing a recessive allele that’s already present. If two siblings mated, and neither one had any harmful recessive alleles (which is, of course, highly unlikely), there would be no problems. That family could interbreed forever, barring new mutations.
A couple of years ago, News of the Weird ran a story which I regrettably cannot find on their website.
It was about a not-so-dignified gent who sexually molested his daughter, who had a child–a daughter–by him. Then many years later the fellow molested the daughter/granddaughter, and she also had a child by him, making that offspring the old man’s simultaneous daughter/great-granddaughter, and perhaps other family titles which my feeble mind cannot now grasp.
My question is this: is the daughter/great-granddaughter more likely to display unwanted recessive traits than the daughter/granddaughter, or would the chances be roughly the same?
(Well, okay, to tell the truth, I just saw an opportunity to tell a really disturbing story–but it’s a real story and a real question, I promise.)
One factor is how relatively inbred the population the parents come from is. For example, a cousin marriage among the Amish, who are already quite inbred, having descended from a small founding population, is much more likely to produce detrimental results than one where one grandparent was from Singapore and the other from Brazil.
I am not an expert in genetics (so I can’t explain as well as someone who is an expert would) but inbreeding is very common in horses - and the result is often a superior animal - that’s why they do it. Not accidental breedings, as was the case of your uncle, but they breed closely related animals on purpose. And with good results. And they don’t like to call it “inbreeding”, breeders prefer calling it “linebreeding”. (Breeding horses with the same bloodlines to each other).
If these closely related horses were coming out defective, breeders would not continue doing it - there is a lot of $$$ tied up in breeding horses.
Aside from the risk of re-inforcing undesirable/unhealthy physiological traits, what is wrong with incest? Mind you I don’t endorse the practice or anything, I just want to know we think it is such a bad thing. I don’t think our abhorrence is instinctive, otherwise I doubt Berse’s great Uncle’s mare wouldn’t have allowed her son to mate with her.
I like to point out that everything depends on how inbred the population was originally. Populations that are severely inbred for many generations (ususally under laboratory conditions, but it does occur in the wild) go through a population crash, with many defective offspring being born as harmful recessives are reinforced. However, those few that get through such a population crash have been cleansed of harmful recessives and are free to continue inbreeding. (They often aren’t the healthiest, because the population crash removes a lot of good alleles, too, but ongoing inbreeding won’t make them any worse.) Horses that are linebred have ususally gone through this. The amish, unfourtunately, appear to be in the middle of this process.
Also, like Chronos said, if the incidence of a harmful allele is artificially high in a population, you may get reinforcement of it regardless of inbreeding.
As to Barking Dog’s question - actually, most wild animals DO have an instinctive avoidance of close relatives (although this is not absolute). Most domesticated animals do not, presumably because humans have bred it out of them. Humans who do breeding LIKE to mate siblings (see linebreeding, above) and so do not favor animals who will not cooperate.
Beyond that, I think incest in humans is insanely unhealthy for moral and mental health reasons. In the case of mother/child or father/child relationships, I think it is impossible for the child, however old, to truly give consent. A child grows up with his/her parents representing authority, and trying to please them, and (I think) cannot fully untangle this conditioning from his/her own wishes on such a sensitive topic. For sibling matings, it seems to me to be driven by profound insularity and unhealthy fear of strangers.
Note: I am a geneticist, so I know what I’m talking about regarding the biological effects of inbreeding. My opinion psychological effects of inbreeding, however, are pretty much that of a layman.
Ahem, and the corallary to what I was saying inbred population being purged of harmful alleles: outbred populations (like virutally all humans) have loads 'o them. Which is why close relative mating often produce problems.
What, you think humans have the same instincts as horses?
The incest taboo is universal among human cultures and societies. Studies have shown that unrelated people brought together will behave towards each other as brother and sister - that is to say, in a non-sexual fashion.
A google search for “Kissing Cousins” will probably yield more viewpoints/sources on the article. The basic gist however is, if you have a 1st degree cousin that you absolutely must have, genetically speaking, you’re in the clear.
Well, Im under the impression still that we are all related to one another if you go back far enough. You know if you take the bible into account, then we come from one couple, & if you take the story of an European King & Queen had kids & those kids more kids, then hey, we are related
As has been said, there are too many variables to make a single rule. IN order to say, definitively, “This couple does not produce undesirable duplication of recessive traits,” you need to look on a single case basis. I imagine that, given the number of genes, there are few couples which result in a flawless child.
Mind you, I mean flawless in that every single trait is dominant.
You COULD do it by number-of-“blood”-relations. That’s how the State determines it, I believe. It’s messed up by the fact that genes are rearranged in the creation of germ cells - every sperm is slightly different. You are related to your sibling by a full blood (actually, anywhere between zero and 100% blood, averaging 50%) and each parent by a half blood. Each grandparent shares a quarter blood with you.
A blood uncle or aunt shares an eighth blood. Their child, your first cousin, shares a sixteenth blood.
The State ( of CT at least) doesn’t allow marriages or intercourse between those who share 1/16 blood or more.
bernse, in post #4, I thought first cousins were 25% (1 in 4 odds)?
Most states apparently don’t care after 1st cousins, many allow 1st cuz marraiges.
Personally, I don’t want my mother and my mother-in-law to be sisters.
But here’s a useless trivia item: Franklin and Eleanor Roosevelt (said to be a president during some war or something) were first cousins.
