The Crazy Ways Some Animals Determine Sex

Photo by Samuel Regan-Asante on Unsplash

Humans, like most mammals, determine sex chromosomally. Most of the time, men are XY and women are XX.

But this is far from the only way the natural world has come to distinguish males from females. Let’s take a look at some of the other fun ones.

Boys are XX, Girls are XO

In some species of rodent, most notably the unassuming creeping vole, which is found in the Pacific northwest, the Y chromosome has vanished and the SRY gene has moved to the X chromosome.

In this species, males have two X chromosomes and girls have one. The SRY genes on one chromosome appear to be silenced somehow, allowing for two sexes, no Y chromosome.

How did this happen? Most likely some kind of weird chromosomal fusion in the past.

Some moths are ZZ, Z0, indicating that the W chromosome, found in birds, some reptiles, and some insects has been lost in a similar way.

Boys are ZZ, Girls are ZW

Or, maybe we can just reverse it and have boys have two chromosomes the same. Weird, unless you’re a bird.

Birds are just flipped around! The Z chromosome is the big one with all of the genetic material. And not only that, but in humans the sperm determines the sex by either…