sex identity is quite binary: just one gene governs sex identity, resulting in the striking anatomical and physiological dimorphism that we observe between males and females. But gender and gender identity are far from binary. Imagine a gene—call it TGY—that determines how the brain responds to SRY. One child might inherit a TGY gene variant that is highly resistant to the action of SRY on the brain, resulting in a body that is anatomically male, but a brain that does not read or interpret that male signal. Such a brain might recognize itself as psychologically female; it might consider itself neither male or female, or imagine itself belonging to a third gender altogether.
These men (or women) have something akin to a Swyer syndrome of identity: their chromosomal and anatomical gender is male (or female), but their chromosomal/anatomical state does not generate a synonymous signal in their brains. In rats, notably, such a syndrome can be caused by changing a single gene in the brains of female embryos or exposing embryos to a drug that blocks the signaling of “femaleness” to the brain. Female mice engineered with this altered gene or treated with this drug have all the anatomical and physiological features of femaleness, but perform the activities associated with male mice, including mounting females: these animals might be anatomically female, but they are behaviorally male.
The hierarchical organization of this genetic cascade illustrates a crucial principle about the link between genes and environments in general. The perennial debate rages on: nature or nurture, genes or environment? The battle has gone on for so long, and with such animosity, that both sides have capitulated. Identity, we are now told, is determined by nature and nurture, genes and environment, intrinsic and extrinsic inputs. But this too is nonsense—an armistice between fools. If genes that govern gender identity are hierarchically organized—starting with SRY on top and then fanning out into thousands of rivulets of information below—then whether nature predominates or nurture is not absolute, but depends quite acutely on the level of organization one chooses to examine.
At the top of the cascade, nature works forcefully and unilaterally. Up top, gender is quite simple—just one master gene flicking on and off. If we learned to toggle that switch—by genetic means or with a drug—we could control the production of men or women, and they would emerge with male versus female identity (and even large parts of anatomy) quite intact. At the bottom of the network, in contrast, a purely genetic view fails to perform; it does not provide a particularly sophisticated understanding of gender or its identity. Here, in the estuarine plains of crisscrossing information, history, society, and culture collide and intersect with genetics, like tides. Some waves cancel each other, while others reinforce each other. No force is particularly strong—but their combined effect produces the unique and rippled landscape that we call an individual’s identity.
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I. With such steep liabilities, it is a genuine wonder that the XY system of gender determination exists in the first place. Why did mammals evolve a mechanism of sex determination burdened with such obvious pitfalls? Why carry the sex-determination gene in, of all places, an unpaired, hostile chromosome, where it’s most likely to be assailed by mutations?
To answer the question, we need to step back and ask a more fundamental question: Why was sexual reproduction invented? Why, as Darwin wondered, should new beings “be produced by the union of two sexual elements, instead of by a process of parthenogenesis”?
Most evolutionary biologists agree that sex was created to enable rapid genetic reassortment. No quicker way exists, perhaps, to mix genes from two organisms than by mixing their eggs and sperm. And even the genesis of spermatozoa and egg cells causes genes to be shuffled through the gene recombination. The powerful reassortment of genes during sexual reproduction increases variation. Variation, in turn, increases an organism’s fitness and survival in the face of a constantly changing environment. The phrase sexual reproduction, then, is a perfect misnomer. The evolutionary purpose of sex is not “reproduction”: organisms can make superior facsimiles—re-productions—of themselves in the absence of sex. Sex was invented for quite the opposite reason: to enable recombination.
But “sexual reproduction” and “sex determination” are not the same. Even if we recognize the many advantages of sexual reproduction, we might still ask why most mammals use the XY system for gender determination. Why, in short, the Y? We do not know. The XY system for gender determination was clearly invented in evolution several