My question is about unusual hereditary traits in humans. I'm new here and this is my first question - apologies in advance if I've accidentally messed this up or covered existing ground.
I'm writing a post-apocalyptic story in which the protagonist realises partway through that a child he's rescued is biologically his own. This is a post-apocalyptic earth, relatively realistic, so genetics have to adhere to our current real-world rules. There is no genetic testing in this future, though the characters in question are educated and understand genetics and hereditary principles. My question is: in what way/s could I show that (child) is the daughter of (Male B) rather than (Male A)? In this scenario those two men are the only possible fathers.
I looked into birthmarks, but from what I can find, they're not actually hereditary, and this was just one of those little literary cheats - one I'd rather not use.
I've also considered having it become obvious that the little girl is colourblind, and so is (Male B), but not (Male A). A girl can only be colourblind if her father is (AND her mother carries the gene, but that's besides the point). This seems like a fiddly, talky way to handle this reveal though, so I'm wondering if there's anything like a birthmark - a medical condition? A distinct but subtle physical feature? - that can only be inherited from a father, can be identified visually, and provides incontrovertible proof of parentage.