There is no indication of the sex of the doctor, and families that consist of two mothers do actually exist and probably doesn't even count as that unusual.
Speaking as a 50-something year old man whose mother finished her career in medicine and the very pointy end of politics, when I first heard this joke in the 1980s it stumped me and made me feel really stupid. But my 1970s kindergarten class mates who told me “your mum can’t be a doctor, she has to be a nurse” were clearly seriously misinformed then. I believe that things are somewhat better now but not as good as they should be …
Ah, but have you considered the fact that he's undergone a sex change operation, and was actually originally a female, the birth mother? Elementary, really...
I wonder if this interpretation is a result of attempts to make the model more inclusive than the corpus text, resulting in a guess that's unlikely, but not strictly impossible.
I think its more likely this is just an easy way to trick this model. It's seen lots of riddles, so when it's sees something that looks like a riddle but isn't one it gets confused.
So the riddle could have two answers: mother or father? Usually riddles have only one definitive answer. There's nothing in the wording of the riddle that excludes the doctor being the father.
