Originally Posted by TeamOrange
Predisposition to genetic illness, ie tremendous increase a chance of being hemophiliac, bipolar, and various other illnesses.
Doing no research whatsoever, I feel like it can't be that bad. Any disease caused by a dominant gene is not an issue because it has the same chance of being passed on regardless of partner. The only case I can see is a very rare recessive gene for a disease. Lets say 1% of people are carriers of the gene. The chance that both your parents are carriers is 0.01%. Making your chances of getting the double recessive and expressing the gene 0.0025%.
However, if a carrier has 2 children the chance that they both are carriers is 25%. If they are both carriers the chance that their kid expresses the gene is 25%. So if 2 children of a carrier have a child it has a 6.25% of expressing. Therefore, the child of a pair of siblings has a 0.0625% chance of expressing.
So there is some truth to this, if 1% of people are carriers, a child's chances of expressing the gene (some horrible disease) are 25 times greater if his/her parents are siblings. However, that ratio is only that significant for rare genes. If 50% were carriers, 6.25% would express, but only 3.125% of sibling parents's children would express! Therefore, the child's chances of getting a disease are only greater for very rare diseases, in which case the chances go from very small, to 20 times that, but still very small.