From Elizabeth Finn via Uproxx
If we consider just the bastard children that are mentioned in the books, [Ned Stark] didn’t have enough evidence to really make this determination. We know of four bastard children, all with dark hair. If Robert is homozygous BB, then this is by far the most likely outcome. However, if Robert is heterozygous Bb, he would still have dark hair, and it would still be fairly likely for four bastard children to have dark hair (think about flipping a coin and getting four heads in a row). When you add in the fact that some of his mistresses and whores likely had dark coloration as well, it becomes even more likely that all four illegitimate children have dark hair. So as far as Ned knows, Robert could well be Bb. And if he is, then the fact that he had three blonde children with the almost certainly bb Cersei Lannister is again reasonably likely. This is why Tyrion remarks that Joffrey’s – and hence Cersei’s – claim would be foolproof if she had had even one child with Robert: someone with dark hair married to someone with light hair having some children with light hair and some children with dark hair is utterly normal.
Banana Scoop 
Plus I don’t know about you guys, but I don’t think they have access to genetic research. But yeah Ned compared Robert to all of his ancestors not just his brothers. So it would be unlikely that Joffrey would be his.
But an interesting theory nonetheless.
the claim was that ALL of robert’s ancestors had dark hair, and did not have children with blonde hair.
Naaaaaaah… I doubt this person read the books.
Genetically, yes. Per Cersei’s lack of sex with Robert, no.