I'm writing a story, in which the detective will recognize a prescription for the victim is for relief of some genetic condition. He then claims that another character was the murderer because she was in fact his daughter (shocking everyone). He proves it by revealing some physical trait she had hidden.
I'm looking for some genetic condition that both mother and father could have that would express itself differently in the child.
I've done a lot of research, but search engines weren't made for this sort of question, and deadlines approach. I've discovered codominant genetic expressions, which seem to fit the bill, but the only examples I can find (in humans) are AB blood type and Sickle Cell Anemia, neither of which will work in the narrative. Perhaps I'm barking up the wrong tree.
My question is: Is there a genetic condition that a father could be receiving treatment for, that would express itself very differently in his daughter? I bring codominance into it because I want it to be 100%, and if both parents were carriers, that ought to do it.
Note: Successful answers should present physical symptoms that can be hidden and then revealed.
Thanks for any suggestions.