What’s supposed to be simple is, you got a sperm and you got an egg—each one carrying roughly half the genes of the person who made it. They fuse. You get an embryo, and it’s destined to be male or female.

So, not so simple.

Sex (broadly, the biology of reproduction) and gender (broadly, one’s sense of self as masculine, feminine, neither, or both) fit neatly into precisely no strict definitions—unless, of course, you are making policy for the Trump Administration, which has for two years been trying to define gender identity out of civil rights protections. The binary distinction between two sexes that are also two genders has held throughout human history, goes this philosophy—a hard and fast (if that’s what you’re into) split.

According to The New York Times this weekend, the US Department of Health and Human Services is planning on going even further. The agency is preparing a memo to be promulgated throughout the administration defining “sex” under Title IX, the civil rights law against gender discrimination in education, “on a biological basis that is clear, grounded in science, objective, and administratable.” Furthermore, the Times reported, that basis would be grounded in a person’s genitals at birth, with disputes resolved by genetic testing. “Sex means a person’s status as male or female based on immutable biological traits identifiable by or before birth,” the Times’ quote continued. “The sex listed on a person’s birth certificate, as originally issued, shall constitute definitive proof of a person’s sex unless rebutted by reliable genetic evidence.”

It seems important to be clear at this juncture: That is not going to work. At all.

Genes carried by a sperm and an egg are packaged into bundles called chromosomes, and depending on whether the sperm was ferrying one of a pair of chromosomes designated “X” or “Y,” the resulting embryo will develop with every cell carrying two Xs or an X and a Y. Generally—generally—XX means female, a woman, a vulva-and-ovaries-and-uterus haver. The XY means male, a penis-and-testes-and-prostate haver. It’s the circle of sex life.

But a lot can happen on the road from embryogenesis to personhood. Sometimes the fusion of egg and sperm goes differently. People can be XXX, XXY, or XYY with no physiological indications. People can have some XX cells and some XY cells. Sometimes a person can be XX but have “male” physiognomy, or the other way ’round. Sometimes, to the tune of one in a hundred, a baby is born with genitalia that people in the room can’t agree on. In some cases physicians perform surgery on those children to assign a sex, and that sex doesn’t always align with how that person sees themself as an adult. The X chromosome has genes for making sperm! A gene called SRY triggers a complex developmental pathway that usually leads to a person being male, but not always! All sorts of nominal sex differences—size of various brain regions, hormone levels, socioemotional development, personality, affinities—work on average and fall apart upon closer statistical investigation. Basically no scientist who knows anything about this stuff subscribes to the idea of the strict “gender binary” anymore.

Some adults—perhaps 2 million Americans, by one estimate—transition from one gender to another. Some adults feel altogether nonbinary, which is to say, they don’t identify as either fully masculine or feminine, completely apart from whatever their biology suggests.

Are most people mostly one way? Sure. External genitals, internal genitals, sex chromosomes, ability to make a baby, levels of certain hormones—they’re all useful physiological markers of sex and gender, but they don’t say anything about social influences, and they ain’t the end of the story. “In a number of cases, these parameters are not going to be congruent with each other. They’re not going to be all male or all female,” says Eric Vilain, a pediatrician and geneticist at George Washington University who studies sex differentiation. “That’s the complexity of biology. There’s no one biological marker.” Sex and gender, am I right? We humans are blessed to live in a world of infinite variety.