Now that Jon and Dany seem likely to get together, I’ve seen a lot of people try to work out their exact relation. Well, I got bored and did out the math for you. or I tried to- i’m not 100% sure if it's right. please tell me if i’m wrong.
Usually, parents and full siblings share 50% of their DNA Aunts/uncles, half siblings, and grandparents share 25% Cousins share 12.5%. So Dany and Jon should share 25% of their DNA, right? well, no. Targaryen family trees are a special kind of special. They look more like ladders than trees. Dany’s father and mother, Aerys and Rhaella, were full siblings. So were her grandparents, Jaehaerys and Shaera. You have to go all the way to her great-grandparents, Aegon V (Egg) and Betha Blackwood to find a couple that wasn’t closely related.* Genetically, this makes Dany half Blackwood, a forth Dayne, and a forth Targaryen.
(they were still related, of course. This is Westeros. Just not *closely* related.)
So because of all this incest Rhaegar and Daenerys weren’t just siblings. They were super-siblings. Normal siblings share 50% of their DNA. Rhaegar and Daenerys shared 88%. That’s approaching identical twin level of incest.
This means Jon and Dany share 44% of their DNA. Genetically, they are closer to being full siblings than to being aunt/nephew.
For comparison:
Cersei and Jaime share 56.3% Jon and the Stark kids share 13.3%
Of course, Dany and Jon still are aunt and nephew. But they are also first cousins once removed. And second cousins once removed… and second cousins once removed. Again.
If you want to fully understand how crazy Targaryen incest is, Daenerys’s coefficient of inbreeding is 0.375 (The higher this number, the more inbred the person is)
Alfonso XII of Spain, who basically wins at being like, the most inbred person ever, had a coefficient of only 0.25.
Now think of the original plan: marry Viserys and Daenerys. Their children would have had a coefficient of 0.5. If Craster wanted to match that level of incest, he would have to become immortal and have kids with his daughter-wives an infinite number of times.
tldr: Targaryen incest > all other incest.
Jon and Dany are more related than you think.