If you have been following Game of Thrones you have been noticing that there is a brewing romance between Jon Snow, King in the North, and Daenerys Targaryen, the aspiring claimant to her father’s Iron Throne.
Of course there is a twist to all of this: unbenknownst to either, Jon Snow’s biological father is Daenerys’ dead brother, Rhaegar. This means that Daenerys is Jon Snow’s aunt.
Long-time followers of the world of Game of Thrones are aware that incest between near relations is neither unknown nor shocking. But there is a non-trivial detail which it is important to note. Jon and Daenerys are far more closely related than typical aunts and nephews.
The reason is simple, Daenerys and her brother were the products of two generations of sibling incest. Incest results in inbreeding, and inbreeding as you know results in loss of genetic diversity. By Daenerys’ generation the coefficient of relationship between herself and her brothers was much higher than normal.
To be concrete, the coefficient of relationship of full-siblings is 0.50. That of half-siblings 0.25. Identical twins? Obviously 1.0. Another way to think about this is how much of the genome do any two pairs of individuals share in terms of long tracts of inheritance from recent ancestors. On the whole siblings share about half of their genomes in such a fashion. After two generations of inbreeding Daenerys and Rhaegar have a coefficient of relationship of 0.727 (using Wright’s method). They’re not identical twins, obviously, but their genetic relationship is far closer than full-siblings!
Dividing this in half gives 0.36 as the coefficient of relationship between Jon and Daenerys, as opposed to 0.50 for full-siblings and 0.25 for a conventional aunt-nephew. Jon and Daenerys have almost the same genetic relationship as 3/4 siblings; two individuals who share a common parent, like half-siblings, but whose unshared parents are first order relatives (full-siblings or parent-child).
Not Jaime & Cersei creepy, but still creepy.
Addendum: Though Daenerys is quite inbred, Jon is not at all. One generation of outbreeding can eliminate all inbreeding.