Baneat said:
starwarsgeek said:
When science backs ethics up, there's really no argument. Also, gross.
Expand on this?
Well, it's already been mentioned a little earlier in the thread, but the science of it is that if you have a child with someone who is closely related to you genetically, the risk of the child having two identical copies of a gene goes up substantially.
Everyone (except men, to a small extent, due to the y chromosome being essentially defective, and not properly matching it's partner the X chromosome), has 2 sets of pretty much all genes.
One from each parent. Thus, if one of those 2 is defective, the other can take over, and the person will still be healthy.
A surprising amount of problems result from recessive genes though; For instance, colour blindness & sickle cell anemia.
If the parents are unrelated, they won't have too many genes in common, so for any given gene, your parent will each have 2 copies, and you will have one copy from each parent.
(so, 2 out of 4)
The worst form of inbreeding, genetically, (aside from identical twins or a clone of yourself, which is currently impossible because you can't have a clone or identical twin of the opposite sex to yourself), is having a child with your own parent.
This is because, instead of there being 4 possible copies of a gene you can pass on, there are now at most 3.
Worse. Normally, for any gene, if all 4 variants are different, there are 4 possible end results.
With only 3 genes though, the end result is now that there are only 3 combinations. Worse, one of these involves the same gene twice. Contrary to popular belief, this isn't always a bad thing, because inbreeding can actually create massive improvements for the same reason that it can create serious problems:
Duplicating the same gene. - if this gene is a really good variant, the child will be better than the incestuous parents. But, if, as is much more likely, there is a bad gene in there somewhere, the child will be worse off.
(and if there is a long history of inbreeding, the problem gets a lot worse.)
So, let's say there's 4 variants of a gene, labelled A, b, C, d (with capital letters being dominant, small letters recessive.) - d is a faulty gene, and A is a really good version...
Starting with two parents:
Mother: Ab
Father: Cd
Possible children: AC, Ad, bC, bd
Since only d is faulty, none of the children will have problems, and 2 of 4 children will potentially be able to pass on the faulty gene.
Two out of 4 will have a better gene than the other two. (but this is statistical, so the real results will be kind of random amongst the 4 possibilities)
But, let's say we now have a few incestuous relationships.
First, two of the children.
You can see if you combine AC + bd, the possible end results are identical to the parents.
AC & Ad together produce: AA, Ad, CA, Cd. Of these two (Ad & CA) are exactly the same as one of the parents, but AA is a particularly good result, while the others are no better or worse than anyone else.
You can also see that Ad + bd has the potential to create dd, which would be incredibly bad.
But, there are worse examples, because it's possible both children are, say, bd. This can create only a handful of options: bb, bd, or dd. dd is bad, bb is alright since it eliminated d altogether, and bd is identical.
AC + AC can create AA or CC quite frequently, which is good because AA is very good, but bad in the sense that AA alone has no genetic diversity anymore. (But since it's such a good version of a gene, that isn't such a huge loss.)
The real problem happens when you get, say, bd + bd, creating multiple dd children... since dd has no alternate versions, and d is a bad gene, at this point, if inbreeding continues, all the children from that point on will only ever have d, the bad gene.
Notice that there's a chance of reducing the genetic diversity, alongside a risk of making bad (or good) genes the only ones left, but there's a lot of possible outcomes, many of which don't have a huge negative effect (but some of which clearly do)
OK, so that's brother and sister, so why is parent & child even worse?
Well, if you look closely, while brother & sister can produce bd + bd as a combination, (for instance), statistically, AC + AC is equally likely, and the overall number of possible combinations is quite large.
For parent and child, let's say Cd (the father) has a child with his own daughter.
We know the daughter could be any of the following: AC, Ad, bC, bd
But notice that while none of those is identical to the father, every single one contains either the C or d gene from the father.
So, with a parent, you are
guaranteed to have at least 50% of your genes in common. (with a brother or sister, it could be as high as 100%, but equally you could share no genes at all with them).
So, not knowing what specific combination the daughter inherited, if she has a child with her father, the possible outcomes are:
Cd + AC : CA, CC, dA, dC
Cd + Ad : CA, Cd, dA, dd
Cd + bC : Cb, CC, db, dC
Cd + bd : Cb, Cd, db, dd
You can see here that of 12 possible outcomes, 2 are really bad, 4 (1/3 of all options) identical to the father (older person's genes), 8 (2/3) of the results pass on the worst of the genes , only 1/3 pass on the best (again due to the older person's genes dominating), with none passing on two copies of it, and in general the father's genes dominate all possible results, while everything else has a much smaller chance of being passed on. This is good if the father/older person had good genes, but not so good if they didn't.
Also note that every single possible outcome here, (unlike the brother/sister example, or the initial one of unrelated people) results in a loss of possible genes.
There is some chance that the next generation will have all 3 gene variants, but a much larger chance they will lose at least one, (or both).
And if that keeps happening generation after generation, it won't take long to lose almost all genetic diversity, and suddenly you've only got one variant of a gene.
(thus everyone in the family will have it, for better or worse.)
Well, that was incredibly long-winded, but that, is the science behind why inbreeding is bad. (more or less.) of course, since this was only about the variations of a single gene, and for humans, the bare minimum is doing this with 46 chromosomes (each chromosome is part of a pair; you get one from each parent, but each chromosome has thousands of genes), the math for a real person is thus a lot more complicated, but the basics of this are still valid.
