Archonsod said:
IIRC the bad thing about incest is rather than receiving two versions of each gene, the child only receives one.
Mostly correct.
Basically, each person has 2 copies of each gene: one from mommy and one from daddy. A copy of a gene in a given cell can change due to mutation, but that is not very likely for one given gene (although a few certain mutations in a single cell will cause cancer).
A gene is basically a recipe for a protein. A mutation in a gene can be:
Silent - has no effect on protein
Can change a single amino acid - can have little effect if it is a not an important part of a protein and amino acid is changed to a similar one. Say, Alanine to Glycine. Or, it can disrupt function of a protein by changing an amino acid in an important region of a protein and/or changing an aminoacid to a very different one. Say, a non-polar one to a charged one. Or, it can introduce a stop codon, cutting off the rest of the protein sequence.
Or, it can be a frameshift mutation that totally ****s up the protein.
It is rare that a mutation will enhance effect of the protein. Most likely, the protein will become useless piece of junk. In some cases, genes are inactivated in most cells (telomerase gene for example) and a mutation will not be in gene itself, but will activate the gene (in case of telomerase gene, it is a requirement for cancer).
So, here is the fact that we have 2 copies of each gene help: if one copy is inactive, the other copy will come to rescue. In most cases, one copy of gene will produce sufficient amount of protein. Therefore, phenotype will be the same as for a person with both copies intact.
Now, since we have a large number of genes, chances are a large number of them has 1 copy "damaged". Since normal people have sex with people who are not closely related to us (although all of us are relatives a large number of generations back), chances are our mate will have different genes having a copy "damaged".
A kid gets one copy of gene from mother and one copy from father. Which copy ends up there can be assumed to be random (some genes go together, but in case of this discussion random assumption is OK). Since one parent will have both "healthy" copies, no matter what gene comes from other parent, the child still ends up with a healthy copy of a gene.
A brother and a sister, however, will have many genes where they each have a "damaged" copy. When they have a child, the child will have a 25% chance of getting both damaged copies... for
EACH gene matching. Therefore, chance of kid being healthy is 0.75^(# of genes where both parents have a bad copy). Since intelligence is a quite complex characteristic, it is likely to be the first thing affected through those genetic disabilities. That is why incest results in retarded babies. Provided those babies do not die before they are born because they will have a number of disabilities.
This is also the reason why small closed communities have higher prevalence of genetic disabilities - parents are too closely related. If you were thinking marrying someone from a community where people are only allowed to marry people from that community (we are talking a relatively small communities, not a whole country), don't. Chances are, they are retarded.
So, since consequences of brother and sister are pretty dire, nature had no choice but to put factors that prevent incest. Brother and sister must find each other sexually repulsive. That must overcome otherwise unavoidable convenience of incest - brother and sister are usually geographically close to each other, so all other things equal they would have sex with each other more often that with others. evolution introduced that repulsiveness. Since people did not know why exactly brother and sister usually find each other repulsive, they just concluded that it is just bad. Therefore, taboos on incest came around.
Also, this is likely to be the same reason why generally same sex people do not want to have sex - which is why homosexuals are a minority - their sex will not produce kids and, therefore, their genes will die without being passes on. Most likely that is why taboo on gay sex came around. From evolutionary standpoint, homosexualism is the same as abstention. Nowadays, however, that is no problem for humanity as a whole - humanity does not need to have its population increased since we own the planet anyway and can keep owing it even with 1% of our number. The only reason why gay people are acted against is reactionary clinging to a taboo that is obsolete.
Taboo on incest, however, is and will always be necessary: we do not need retards around. It would be bad for them to live like that and bad for us for having to support them...
