People with XY are dudes. People with XX are chicks. DNA tests can determine someone's gender, a fact I learned from CSI, CSI: Miami, Criminal Minds, Law and Order, and biology class. Ergo, gender is specified in the DNA. I see no reason why it couldn't apply to intersexual individuals (by which I exclusively mean people with mismatched parts, not people who are actively mutilating^[1] their parts in an attempt to mimic the opposite gender) either -- even with mismatched parts, most of their characteristics (even if only the secondary sex characteristics) tend to be in line with their genetic gender.

(Note that I am using "gender" to refer to a person's biological sex. "Gender identity" is the better IMO term for what a person considers themselves to be. A dude with an inside-out dick has a male gender and a female gender identity.)

^[1] I have nothing against transsexuality or against people who get sex changes. They're merely people attempting to rectify a comparatively-minor psychological discrepancy. I just look at it from a biological perspective. A dude with an inside-out dick, for example, isn't a chick. He's a dude with an inside-out dick. Call him what he is: a "he" who willingly endured genital mutilation. (Or perhaps a "they"/"it", if you feel like compromising yet want to maintain some semblance of scientific/biological accuracy.) When society learns how to create real fake parts, then it's an actual sex change. Otherwise, it's purely cosmetic and should be referred to as such.