omg what a small world
Anyway if you want the actual mathematical breakdown for the 98.75% figure, I also supplied that on /v/ as follows:
The reason inbreeding is risky is because there's a possibility you both carry a recessive, deleterious mutation from your ancestors, and therefore a possibility the child will receive both of these alleles and express the trait.
Let's assume the mother carries an allele that is not only recessive, but deleterious. Since it's recessive, you need two of them to actually express the trait, which is why inbreeding can cause these traits to appear (since the alleles are unique to the bloodline).
Anyway, the father is not related to the mother, so does not carry these harmful traits. We will label the retarded allele "R", and any non-retarded allele "N". The mother has one R and one N, and the father has two N's.
The possible combination of alleles from the mother and father will be RN, RN, NN, and NN.
Half of these combinations have an R, which is a 50% probability of a child carrying the allele—but since we are looking at TWO siblings, you times that by another 50% to get the probability of two children in a row carrying the retarded allele, which is 25%.
Of course, that's just carrying, since the mutation doesn't exist in the father's bloodline.
If Andrew and Ashley indeed both carry the recessive allele, then these are the possible outcomes for one child: RN, RR, NR, NN.
Only one out of four combinations has two retarded alleles, which is 25%. So, we times that 25% by the 25% likelihood that Andrew and Ashley even carry the retarded allele to get a 6.25% chance that their first child will be retarded.
And that's assuming the mutation is even harmful. Mutation is random, so the vast majority are neither harmful nor beneficial. So really, that 6.25% figure is the likelihood of the child inheriting a recessive allele regardless of whether it is harmful. The absolute highest estimate by science is that 20% of mutations are harmful, so we times that 6.25% by 20% to get the worst-case scenario of a mere 1.25%—the likelihood of imouto nakadashi resulting in retarded babies.