Barbas said:
As for what genetic issues would be produced, I think I need filling in on that.
Well the reason behind genetic issues in inbred children is from the stacking of similar genetic defects.
If two perfect (And i mean PERFECT, all alleles perfectly functional and working, no errors, no mutations, no defunct genes) individuals inbred and were related they could hypothetically do so forever with no inborn errors. Healthy as far as the eye can see.
However humans are extremely genetically messy. Our genome is disgusting and wasteful and FULL of crap that does nothing or is broken. Each individual has a lot of stuff that doesnt work from their mother and their father, fortunately in the VAST majority of situations you will have one functional chromosome from one parent to make up for the broken one from the other parent. Or perhaps only one of the 40 genes that codes for a specific thing is broken and the other 39 pick up the slack. The point is in unrelated individuals both have messy genomes with some errors, but two strangers will likely compensate for eachothers minor errors.
Now the reason genetic defects occur when incest is practiced (particularly multiple times) is because youre extremely likely to have a similar "genetic mess" as your siblings and cousins. Whatever faults lie in your genome are likely to lie in theirs as well. So when you breed with them those faults combine and theres no copy of a healthy gene in the mix to mask the defective one.
With cousins the odds of having a defective child increase by 2% (flat value, from 2% in normal couples to 4% in cousin cousin). With siblings the odds are 25% flat out. However if you are fortunate enough to have a very stable or "neat" genome the odds will be far less likely. But thats something you just cant know. A large amount of your genes could be defunct but as long as you had one copy youd appear healthy and normal. You could be a genetic Adonis/Aphrodite and have the chance very low.
The reason this chance increases as you repeat generations is the errors stack together. Lets say about 34 out of 40 of your genes work for a certain feature (Like cornea shape) and only 32 of your siblings work. Your kids might end up with 28 working. If you do so again they might end up with 20 working. This is when it might start to show and the child will be deformed. Cousin incest has INCREDIBLY low chance of causing a genetic defect. Hell due to my TERRIBLE genes for eyesight im basically 100% likely to create a child with an abnormal cornea and they will need glasses like a lot of people now-a-days do. Its an interesting line between "acceptable genetic error" and "unacceptable genetic error".
I always try and dump the biology into these threads. A lot of misconceptions about how it works.
OT: I figure if two Haemophilia carriers can do the do with a 25% chance of creating a haemophilic baby and be legal I see no reason to forbid incest. Sure ill discourage it if theres chance a child could suffer down the line but if its unethical to forbid someone who is 100% likely to create children with a genetic disease (Like Y linked diseases) the same logic might apply.