Only maybe one live birth in 100 has some non-standard sex anatomy, and genetic anomalies are slightly rarer than that. However, let's talk about those 1-in-100 or 1-in-1000 cases — because those could be you, or your aunt, or your best friend — and because our system of law has to deal with 1-in-1000 situations, too.
An illustrative example: Let's say you're an adult woman. You were always female; you never had any doubts about this. You have all the right parts and none of the wrong ones; you think of yourself as a gal. You get married. (Yes, I do mean to a man.) Somewhere down the road, your genes get tested: To your astonishment, it says you're not only not XX (double-X being the classic female chromosome pairing), but in fact XY (classic male chromosomes). Testing shows your blood to be loaded with testosterone and other classic androgens (male hormones). Ultrasound locates an undescended set of testes in your abdomen. What's going on? (Note that all women do produce androgens, normally, but at much lower levels than in men.)
What's going on is called Androgen Insensitivity Syndrome (AIS, found in both "Partial" = PAIS and "Complete" = CAIS sub-variants), where, at a critical early stage of development, because of a structural variation on your X chromosome, your cells' androgen receptors became unable to respond to those hormones. The hormones were there, but had no effect. Your entire subsequent development was thus into a woman. You're a gal — one with no ovaries / )womb and thus infertile, and with vestigial testes somewhere inside, but every other inch a woman.
Laws like Proposition 8 might very well cause such a woman's marriage to be retroactively annulled — or prohibited in the first place. Furthermore, if the law decides that this particular citizen is male on account of the Y chromosome, testosterone, and testes, then she would be permitted to marry... guess who?... another woman. So, courtesy of Prop. 8, we are likely to suddenly have court-mandated same-sex marriages.
Let's say you're an adult male. You've always been male; you seem to have exactly the right parts, and so on. Your genes get checked one day, and they come up "XX" — the classic chromosome pair of a genetic female. Yet, you're a 230-lb. guy with facial hair who was a high school running back. What's going on?
When your mother and father's genes fused, the key part of Dad's Y chromosome (the "SRY" = Sex-determining Region Y bloc) got translocated to somewhere other than chromosome 46. Are you an XY, a genetic male? Well, no, you're not — even though you think you're a guy, you look like a guy, and so on. Your marriage might end up getting annulled, too — and you might end up being permitted to marry only men.
It turns out, those two cases — AIS and transported SRY blocs — are only the beginning of the troubles we're facing. Here's a litany of other ways people can end up biologically difficult to classify, through no fault of their own. I'll keep the jargon factor low, but you can certainly skip this list if not interested:
You're a girl with female sex organs — but, at puberty, your voice deepens and some testes descend out of your lower abdomen. (I'm steering clear of some of the more graphic details, here.) It turns out you have XY chromosomes (male), but you look like a young woman. You might or might not be fertile, but it would (if so) be as a male. This is a recessive genetic condition called "5-alpha-steroid reductase deficiency", that causes testosterone to be chemically transformed in peripheral tissues before it can have its usual effect.
You're a man with all of the apparent parts, facial hair and all. One day, during unrelated abdominal surgery, doctors are surprised to find inside a full set of normal female parts (ovaries, Fallopian tubes, womb, etc.). Genetic typing shows you to be XX, but something called "Congenital Adrenal Hyperplasia", starting early in life, caused your adrenal glands (near your kidneys) to induce production of an unusual mix of sex hormones, changing your entire course of post-natal development, leading you to appear, and believe yourself to be, male.
You're a gal: All the gal parts. None of the others. Yet, you're XY. It turns out that, during development, you ended up short of a hormone called "Müllerian Inhibiting Factor". That factor would have let your Müllerian ducts develop into testes, which would have given you more androgens. Its absence means you end up looking, seeming, and believing yourself female. (This pattern of early development is called "gonadal dysgenesis".)
You're a girl — but you don't seem to be entering puberty. Genetic testing shows you're XY, although in all other respects apparently completely female. What's happened is that the sex-determining "SRY" region of the Y chromosome didn't trigger the development of testes, with resulting female development. This is called "Swyer syndrome", or "XY gonadal dysgenesis".
You're a man, albeit somewhat on the tall and thin side. You and your wife are having fertility problems. You're both checked: It turns out you, the husband, have "XXY" or "XXXY" or "XXYY" chromosomes. (There's also been one reported case of "XYYY".) You might be borderline fertile (as a man), or you might be out of luck. This is called "Klinefelter's syndrome". There's also a variant "mosaic" form of Klinefelter's where some of your body's cells have XY chromosomes and others are XXY.
Speaking of mosaicism: You're a male with all the fixings, but also female equivalents — and raised as a boy. You then get the worst parts of both types of puberty, with both menstruation and your voice cracking, and so on. This is an incredibly rare but documented "cellular mosaic" condition where you have some XY cells and some XX ones, the only known way of generating true (bi-fertile) hermaphrodism, which is otherwise impossible and a medical myth. (There can be diverse combinations of mosaicism: XY with XXY, XX, XXXY, and so on.)
You're a man with all the normal parts. You and your wife are having fertility problems. Genetic testing shows you to be "XX", but with the sex-determining "SRY" genetic bloc also present on some non-standard chromosome, and thus producing male development anyway.
You're an apparently normal man, with fertility problems. Genetic testing reveals "XX" genes, and no SRY block can be found anywhere. This "SRY-negative XX male syndrome" is observed but so far not fully understood.
You're a gal. You're late entering puberty. The doctors find no womb or a partial one. Otherwise, you're an (infertile, except via surrogacy) XX woman with absolutely nothing wrong with you. The causes are not really understood, but it's called Mayer Rokitansky Küster Hauser (MRKH) syndrome.
You're a gal, but your secondary sex characteristics at puberty seem underdeveloped, relatively speaking. You turn out not to be XX, but rather have a single, unpaired X chromosome (also called either "XO" or "45,X" as opposed to "46,XX") — or a second X is present but abnormal, or some single-X cells are mosaiced among XX ones. In any of those cases, you might be fertile, or maybe not, and it's called Turner (or Ullrich-Turner) syndrome.
You may be unsurprised to hear that babies also sometimes end up with mixed sexual characteristics caused by fetal conditions (e.g., Mom was prescribed a progestin-based medicine such as Danazol, a testosterone-related hormone once prescribed to treat endometriosis), or for no identifiable reason. Traditionally, obstetricians tend to pick a sex and use neonatal surgery to converge the baby towards it — something more common than people realize, as nobody wants to talk about it — yet another contentious, opinion-soaked issue I'm carefully avoiding. (But, anyway, the point is: Are you sure you're not a man, or a woman, primarily because a scalpel made you that way?)
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