A: Actually, humans come in a variety of X and Y chromosome combinations (not just XX and XY) and body parts (i.e., anatomy).
TL; DR: Most people that have XX chromosomes develop what is considered “typical” female anatomy and most that have XY chromosomes develop “typical” male anatomy. But not always. Other sex chromosome combinations and anatomy are also possible.
Biologic sex doesn’t exist as a simple binary (i.e., boy vs. girl), with *all* people having *only* XX or XY sex chromosomes and body parts that are “typical” for those sex chromosomes. Some people have different combinations of sex chromosomes (for example, XXY or XO) and have slightly different anatomy from what is “typical” for their XY or XX counterparts. Even for people with sex chromosomes that are XX or XY, their body parts don’t always develop into strictly female or male anatomy. For example, some XX individuals are born with testes, penis, and scrotum, and some XY individuals are born with a vulva and vagina but no uterus. Other times, people’s external anatomy looks like something in between a penis and a vulva, which is often called “ambiguous genitalia.” For some people their external anatomy may not align with their internal reproductive organs.
These are just a few examples – there are lots of different variations that exist! These are often referred to as intersex conditions (i.e., when a person is born with a reproductive or sexual anatomy that doesn’t seem to fit the “typical” definitions of female or male) and they occur in about 1-2% of humans. That’s about as common as having red hair! Some variations are obvious at birth, such as ambiguous genitalia. But others can be subtle, so not everyone who has a difference may even be aware of it.
This variety in anatomy is possible because of how our body parts form in the first place. For the first 6 weeks after sperm meets egg, all embryos have the same anatomy, regardless of whether they will later develop male or female parts. We all start out exactly the same! Externally, we have what looks like a tiny penis with folds of skin on either side. As the embryo develops, the tiny “penis” may become either a clitoris or a penis. The folds of skin may become the vulva or the scrotum. Internally, we all start with two sets of tubes. One set of tubes can turn into a uterus and fallopian tubes, and the other set can turn into the vas deferens (tube that carries sperm) and other male-typical structures. How those parts develop depends on a complicated dance between many different genes and hormones. Most of the time, things go as expected, but not always! If a certain gene is altered, a receptor is missing, or a hormone level is too high or too low, you can end up with different combinations or slightly altered body parts.
So, even at the level of chromosomes and body parts, things are not always black and white (or pink and blue)! Humans exist on a spectrum of biological sex, not a binary. This is the reality of human bodies, even though it’s not commonly taught or recognized. Gender identity, which is one’s internal sense of self as being either a woman, a man, or non-binary, is an additional layer of complexity – look for a post on that soon!
Stay safe, stay well!
Those Nerdy Girls
For more information:
Video animation of typical fetal sexual differentiation
Video animation of differences in development
Video lecture on fetal sexual differentiation
BuzzFeed Video – What it’s like to be Intersex
TED The way we think about biological sex is wrong