Mudkips said:
You only have problems if you have bad genes to begin with.
You can be a carrier for many diseases and not know it, and have carrier children. If those carrier children then fuck, the odds that the recessive trait will be expressed are increased vs the general population, but they're exactly the same odds as any two random carriers fucking.
There is zero practical increase in risk for anyone with a brain because:
If you or your parents had a disease, you'd know about it, and would avoid having kids with your siblings, just as you would with people with a similar family history. Or you'd screen for it when planning.
If you or your parents of unknown status (were carriers or were clean), you wouldn't know the situation and statistically you'd be in the same boat as fucking a random person.
The flawed reasoning that people use to claim an increased chance of having recessive traits be expressed relies on assuming carrier status for both parties and assuming normal odds with the general population. It's comparing the probably of X vs the probability of X given Y when there's no reason to assume Y.
Even if we ignore that most people WILL be too stupid to see the risks if their parents already had genetic disorders, that's not really how it works.
When your parents fuck, you will get randomly one chromosome of each chromosome pair from your mother and the other one from your father.
Any recessive mutations that affect the amino acid code / the function of the resulting protein(s) located on one chromosome can normally be compensated for by the other copy you received by your other parent.
There's a very low chance that you will ever have a child with someone that has the same gene mutated. While it's much, MUCH more likely that your brother/sister has one of the same chromosomes as you have:
A = chromosome with normal gene
a = chromosome with mutated gene (needs to be on both chromosomes to show an efect)
Father [AA] x Mother [Aa] ---(both are healthy)
Offspring: [AA] (50%) or [Aa] (50%) ---(all are healthy)
Let's say they had four kids and that 2 of them are [Aa], one male and one female
Brother [Aa] x Sister [Aa]
Offspring: [AA] (25%) or [Aa] (50%) or [aa] (25%) ---(the latter one will show the defect)
So statistically a quarter of the kids will get the mutation after just one inbreeding step (repeat to fuck up your gene pool).
Also keep it mind that ALL chromosomes in this scenario have only 4 different "versions" (starting with the parents, 2x2), so you will very likely not only get just one mutation but a lot of the recessive shit that was carried through generations.
So while it's true that basically all of us carry "bad" genes, you must be really unlucky to find another [Aa] (remember, the same gene) person if you are not related to them (however, yes, in this case the statistics for this gene would be the same as described above). But even in this case the chance of getting a non-relative that shares the mutations in several of your chromosomes is basically zero.
So no, it's really not the same as fucking the general population...