I have to point you to this post on royal inbreeding in A Song of Ice and Fire. They reference my post on the Habsburgs. Well done! In any case, one possibility is that the Targaryen lineage may have purged their genetic load through inbreeding. The basic logic is that all the recessive traits are going to be “exposed” every generation, resulting in a far stronger selection coefficient against those alleles than would be the case in a outbreed population (where most deleterious variants with recessive expression are masked by being present heterozygote genotypes).
A friend mentioned last night that he was watching a bit of A Game of Thrones, the new HBO series based on George R. R. Martin’s A Song of Ice and Fire series. I’ll probably wait until after the DVD version comes out, if I watch it at all. I’m not generally impressed by visual media adaptations of science fiction and fantasy literature, and have even less use for film & TV only science fiction & fantasy. I’m not a snob, I’m just easily bored. In any case, George R. R. Martin has gotten The New Yorker treatment. I had to laugh at this sentence from near the end of the piece: ‘Martin hopes that, after he surmounts the particular thorny problems of “A Dance with Dragons,” the final two books will come much faster.’ Of course he he said this after the last book. To be fair, there wasn’t that much of a gap between book two and book three.