If men have XY and women have XX, what would happen if scientists created a YY human?

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A person with two Y chromosomes and no X chromosome is biologically impossible, because the X chromosome is essential in order for your organs to form correctly. If a person were conceived with two Y chromosomes and no X chromosomes whatsoever, that person would probably die in the womb as a miscarriage. You can live without a Y chromosome. (I’ve been doing it for over twenty-two years!) You can’t live without an X chromosome.

The default sex for every human being is female. The only thing the Y chromosome is good for is turning people who would otherwise be totally female into men. That’s literally all it’s good for. The X chromosome, by contrast, is the one with all the vital organ-building information, including information on how to form organs unrelated to sexual reproduction. The X chromosome is a bit like your heart: everyone needs one and everyone who’s alive has at least one.

That’s also why the Y chromosome is absurdly tiny. It’s not even half the size of the X chromosome because it has to carry way less genetic information. The X chromosome carries 155 million base pairs and 804 known genes, but the Y chromosome only carries fifty-nine million base pairs and only sixty-three known genes:

Technically speaking, men and women actually have the same organs; the Y chromosome just causes those organs to form differently in males. You see, sex is genetically determined at conception, but, in the first few weeks after conception, the embryo has no anatomical or hormonal sex. You can’t tell a male zygote from a female zygote by looking at it. It’s what we call “undifferentiated.”

An undifferentiated male embryo basically develops as it would if it had no Y chromosome and is indistinguishable from a female embryo at the same stage of development. They both have a generic set of undifferentiated genitalia that can develop into either male sex organs or female sex organs (or the process can sometimes go differently from normal and the infant can end up with neither male nor female sex organs, but those cases are generally pretty rare).

Eventually, after the first few weeks, the Y chromosome kicks in in males and they start to develop male primary sex characteristics, such as a penis and testicles. Meanwhile, females continue to develop without the Y chromosome, causing them to develop a uterus, ovaries, a vagina, and a vulva. Here’s a diagram showing human sex differentiation:

As a result of this, male and female sex organs are homologous to each other. The penis is homologous to the clitoris. The foreskin is homologous to the clitoral hood. The scrotum is homologous to the labia majora. The scrotal raphe is homologous to the labia minora. The testes are homologous to the ovaries. The appendices of the testes are homologous to the fallopian tubes. The prostatic utricle is homologous to the vagina, cervix, and uterus. (Sorry men, I guess you all technically sort of have pussies.) The prostate is homologous to Skene’s glands. (You’ve probably all at least heard of female ejaculation, right? The fluid that’s ejaculated is probably produced by Skene’s glands.)

The fact that males and females start out development the same is probably one of the main reasons why adult males actually produce low levels of the female hormones estrogen and progesterone and adult females produce low levels of the male hormone testosterone. Even as adults, we’ve all still got a bit of the other gender left in us.

The only reason why men are men and not women is because of the Y chromosome. The thing is, the Y chromosome only works if you also have an X chromosome, because the Y chromosome essentially just modifies the genes carried by your other chromosomes, including the X chromosome. Without the X chromosome, the Y chromosome is basically useless. It’s sort of like how wearing glasses is useless if you don’t have eyes. That’s why scientists would never be able to create a person with two Y chromosomes and no X chromosome.

In any case, there would probably be no benefit to having a second Y chromosome anyway, because having two of the same chromosome generally doesn’t have much of an impact. Women usually have two X chromosomes, but very early in fetal development, one of those X chromosomes in every cell is inactivated. Again, the X chromosome is kind of like your heart: you need one to live, but you only really need one and having a second one is mostly just redundant. (Sorry Time Lords!)

There are some men who have one X chromosome and two Y chromosomes. Usually, all that happens with the second Y chromosome is the exact same thing that happens to women’s second X chromosome: it just shrivels up and goes dormant while the other acts as the dominant one.