ON A HOT summer day around 1900, a young Walter Sutton left the varsity basketball courts at the University of Kansas and headed home to his family farm. While in the flat wheat fields of Russell County, so the story goes, he discovered a grasshopper with an endowment that would prove seminal to the later history of sex. After sending off several grasshopper specimens to his mentor in the biology department for analysis, this one turned out to be memorable. “For what you say of the ‘immensus,’” Sutton later wrote, “I infer that the gentleman’s cells are about the largest that have ever been discovered and if they are so our department may derive a little fame from the fact.” The discovery of one additional chromosome in the gentleman grasshopper — an “accessory chromosome,” as it was called — and which happened to appear in only half of the grasshopper’s sperm, was also thought to be a determiner of its sex. It was, Sutton’s mentor argued, “the bearer of those qualities which pertain to the male organism.”

Add a few decades and the X chromosome became associated with all things female and the Y with all things male. Or so we might be tempted to think.

In her fascinating book Sex Itself: The Search for Male and Female in the Human Genome, Sarah S. Richardson, a philosopher of science at Harvard University, attacks any such easy correlations among sex, gender, and chromosomes. (And for those of us who may not fully recall the birds and bees from high school biology class, she also provides beautifully illustrated and easily digestible diagrams.) While an earlier generation of scholars and scientists tended to think of sex as fundamentally biological and gender as fundamentally social, Richardson suggests that our very attempts to characterize the biological details of the genetics of sex are deeply gendered, no matter what newfangled technologies we are using. The quest to find the roots of “sex itself” in the components of bodies dates back at least to the invention of the microscope, and to 17th-century tales of copulating male and female sperm — but the discovery of chromosomes in the late 19th century were what gave geneticists a fresh locus and arena of action for fixing sexual difference.

Richardson describes how, in fact, Sutton and his mentor got the sex-determination story exactly backward: the accessory chromosome they had so happily discovered was associated with females, not with males. How was it possible to get it so wrong? In part, sex was messier at the turn of the century: chromosomes were small and hard to stain and see, and the early discovery that sex could be correlated with chromosomes in multiple ways — some species have W and Z rather than X and Y — also complicated matters. A narrow focus on examining sperm alone was another significant factor. Some early researchers were also, quite rightfully, concerned about the use of “terms with improper associations” (like “sex chromosome”) rather than other, more sex-neutral terms that might more easily roll off the tongue (like “idiochromosome,” allegedly!). One prominent geneticist for a time even called it a “travesty” to reduce our understanding of sex-determination to the chromosomes alone.

So how did the X and Y in humans become the “sex chromosomes” and then the roots of “sex itself”? In part, it was a matter of timing. According to Richardson, sex chromosomes came into focus at just the right time, serving as “conceptual innovations carved into the fibers and foundations of modern genetics.” In 1900 or thereabouts, a new theory of heredity, Mendelism, called for the association of visible traits with invisible hereditary “factors” that mixed and matched like cards reshuffled in every generation. Those “factors” funneled into our modern concept of the gene. The discovery that some traits, like white eyes in fruit flies, were sex-linked also suggested that sexual traits were associated with “sex chromosomes.” Interpreting such new discoveries through familiar gender frameworks, sex rapidly became a matter of “the X.” (Richardson even recounts a dirty ditty of the time about “Naughty Sadie,” a sexy X chromosome on the roam: “Till at last she met a man upon her ruin bent […] Sadie soon went slipping down that keen microtome, / Section after section of that sex chromosome […]”) Decades later, following the further development of sex hormone research, and with the dawn of a newly refined science of human genetics in the 1960s, the die was cast: sex chromosomes had become “potent symbols” and “molecular pillars of biological femaleness and maleness.”

Casting a critical eye on the idea that “at the level of the chromosomes, the gender rainbow […] falls away,” Richardson launches, in her later chapters, a full-on critique of the recent scientific gendering of accessory chromosomes and the larger implications of this gendering for science itself. After years of ignoring the tiny Y chromosome, notions of the “genetic essence of maleness” entered into human genetics through a notorious episode of problematic research into XYY “supermales” in the 1960s and 1970s. Such ideas persist in more recent debates over the discovery of Y-chromosome degeneration — replete with claims of a “wimpy Y” and strident calls to “save the males!”

Meanwhile, X chromosomes came to be associated with “female biology, femaleness, and femininity.” Stereotypes of women as “complex, contradictory, and changeable” were even applied to the phenomenon of “X mosaicism,” where the cells of a woman (XX) can express either the maternal or paternal X chromosome. Describing the X chromosome as feminine in these ways (and as sex-determining) not only misses the obvious fact that men (XY) also have X chromosomes, but also fundamentally distorts the process of finding out just how chromosomes actually relate to sex. This makes for a complicated picture, indeed. By perpetuating subtle and not-so-subtle gender stereotypes, gendered discourse surrounding “sex” chromosomes has, argues Richardson, generally led to overstating the differences between the sexes and understating the diversity present within each sex.

And what about another claim — are men and women really more different from each other than humans are from chimpanzees? And why does so much seem to rest on finding the basis of “sex itself” in the chromosomes anyway? In an age when we are putatively comfortable with the idea of complex genetic interactions, why are some scientists still so tempted by the idea of a single master sex-determining gene? In case after case, and chapter after chapter, Richardson adeptly teases out how traditional gendered assumptions have fundamentally affected chromosome research. Even as we have moved away from thinking of sex as something inherent in “sex chromosomes” to more complex understandings of the interactions of different genes on different chromosomes in complex networks, gendered habits of thought persist even in contemporary research. Sometimes one just needs a “ball-breaking feminist Y chromosome knocker” (another story Richardson tells with particular aplomb).

While we’ve been zealous in searching out sex differences and assigning them to the chromosomes, Richardson argues that we remain far less motivated to question why we need to make such connections in the first place. This said, thanks to decades of feminist critique, we are more reflective than we used to be; and this has improved our scientific understandings even if individual scientists remain “painfully unaware of the contributions of feminism to their work.” Gender criticism, as Richardson puts it, is “part of specialist discourse in the field of sex determination research in a way that it was not in the 1980s and 1990s — and before.”

But even if we know that X isn’t female and Y isn’t male, or at least not in the ways that we might have readily assumed, how can we resist making a good X or Y joke? Indeed, some reviewers of Richardson’s work to date have scarcely resisted the opportunity to make “Mars and Venus”-type distinctions, or to wrap their reviews in pun after pun (“The X and Y Files”). A review in the journal Science was even entitled “YseX Is a Matter of Concern Rather Than a Matter of Fact.” But when even Richardson herself pens a piece on Slate.com entitled “Y All the Hype?” and when the very cover of Sex Itself has an array of pink and blue chromosomes, it seems there’s something more than mere punning going on.

In tracking our impulse to gender chromosomes across decades and contexts, Richardson shows how gendered cognitive schemas have always been present in the history of our search for “sex itself.” Problematic though these metaphors may sometimes be, the fact that she and her reviewers (guilty!) continue to deploy them suggests we need not be overly puritanical in stripping our language, our writing, and our thought of gendered associations. We should, however, be ever-conscious of what such gendering enables and obscures.

Indeed, rather than expect or hope that gender conceptions and values might disappear from science, leaving us high and dry in some sort of cognitive asexual utopia, Richardson suggests that such gendering in science should simply be open to debate and scrutiny as the truth is worked out. It’s less a matter of simply removing gender bias — indeed, “the ideal of an unbiased value-neutral science is misconceived,” she says — than acknowledging the ways in which gender always already valences our “normal” practice of science. Sometimes, she says, gender conceptions can do “constructive, even clarifying, work in the evaluation of competing scientific models.” Such a “gender-critical” perspective about the double rainbows of sex and gender seems essential not only to avoiding the embarrassing mistakes of the past, but to figuring out what it means to do better science in the future. Whatever new scientific insights may emerge as we continue to careen toward our post-genomic future (the subject of Richardson’s next project), I’ve a feeling we’re not in Kansas anymore!

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Luis Campos is associate professor in the Department of History at the University of New Mexico.