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Peter McKnight is an adjunct professor in the School of Criminology at Simon Fraser University.

So which washroom is Caitlyn Jenner supposed to use? If you haven't been keeping up with the Kardashians, you probably haven't heard that Caitlyn is the name former Olympian and Kardashian patriarch Bruce Jenner has given herself.

And if you haven't been keeping up with transgender issues, you probably haven't heard that these things always come down to washrooms. Those who oppose all matters transgender inevitably get their knickers in a knot about who belongs in which washroom.

Witness the Senate's recent efforts to gut NDP MP Randall Garrison's transgender rights bill by exempting washrooms from its purview. And witness Conservative Senator Donald Plett's comments that "vulnerable women" need to be protected from "biological males" who might enter the women's washroom.

For many people, biology defines sex, and sex is always a binary affair. Sure, postmodernists have been playing with the concept of gender for decades, but sex, well, sex is sacred, which means you're either biologically male or female. But never both. Or neither.

But biology doesn't work that way. Biological phenomena don't necessarily fit into human-ordained binary categories. So while humans insist that you're either male or female – that you have either XY or XX sex chromosomes – biology begs to differ.

For example, genetic men with Klinefelter syndrome possess an extra X chromosome (XXY) or more rarely, two or three extra Xs (XXXY, XXXXY); they typically produce low levels of testosterone, leading to less-developed masculine sexual characteristics and more-developed feminine characteristics than other men. In contrast, some men receive an extra Y chromosome (XYY) in the genetic lottery, and while they have been referred to as "supermales" that is more sensationalism than science.

Genetic women with Turner syndrome have only one X chromosome; they often display less-developed female sexual characteristics than other women. And people with a genetic mosaic possess XX chromosomes in some cells and XY in others. So how do we determine if they're male or female? Hint: Don't say that it depends on the chromosomal makeup of the majority of their cells, since women with more than 90 per cent XY genetic material have given birth.

Even if you get the "right" combination of sex chromosomes, it's no guarantee that you'll fit into the carefully circumscribed human definitions of male and female.

For example, genetic women (XX) with congenital adrenal hyperplasia produced unusually high levels of virilizing hormones in utero and develop stereotypically masculine sexual characteristics, including masculinized genitals.

Similarly, genetic men (XY) with complete androgen insensitivity syndrome don't respond to male hormones and fail to develop masculine sexual characteristics. Most live their lives as women. Some historians suggest that Joan of Arc, Elizabeth I and Wallis Simpson all suffered from this syndrome.

So what's the answer? There isn't one, at least if we're looking for the answer in biology. We must not fall back on biology. Rather, we must always remember that it is we, not biology, who decide who counts as male or female. And it is we who must take responsibility for our decisions.

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