Androgen insensitivity, where you end up with XY "females." This can be a good topic because it give you a chance to look at some very interesting questions about development, about gender, and of course sports sociology. For example, should such a person compete as a woman?
Or, you could look at how in the sixties no one was sure whether femaleness resulted from two copies of X, and maleness from a single copy, with Y being irrelevant. Or, was the number of X chromosomes unimportant (so long as there was at least 1), and long as there was a Y you got a male and if no Y, a female? We all know the answer now, but this was a difficult question to answer in those days ... look up Klinefelter's and Turner's syndromes, because they (and the development of karytyping) gave the answer. This one can be good because you get to talk about two equally reasonable hypotheses in a time when no one could gather evidence either way ... enter karotyping ... find XO and XXY individuals...it all comes together.
When I was a college professor I always favored papers in which the student showed a sense of history, talked about unsolved problems, the evidence that had been needed to solve them or at least get part way there, how the evidence finally turned up, and what was done with it.