Just ask directly: are you a man or a woman? (casually: are you a guy or a girl?) "What is your sex?" makes me feel like a farm animal or house pet, examined by a veterinarian. "What is your gender?" makes me think you're looking at me but can't decide if I'm male or female. (I'll grow a beard, OK?) If this is for online chat, just "asl?" is fine (age, sex, location?).

you should ask :" what type of chromo combo (i.e. chromosome combination) are you--XX, or XY, or undetermined?"

LOL Your question is funny. I hope it means that you are asking someone online that you cannot see. If you have to ask this question often when you can see them that would be rather disturbing. You can ask it both ways: "What is your sex?" or "What is your gender?"

Haha I agree with Dark Panther. Even when ur asking it online, it could be a little awkward depending on the person. I suggest asking other questions and figuring it out for urself. If all else fails u could try the old 'ASL' thing. Simple and quick.