How is sex determined? In humans, it’s a matter of genetics, and it begins the moment a sperm cell fertilizes an egg, thereby producing a certain combination of sex chromosomes. Throughout the animal kingdom, though, sex determination varies widely and isn’t always genetically based, as biologist Aaron Reedy explains in the TED-Ed video below. For example, in some species—such as alligators and turtles—sex is actually determined by the environment. Specifically, the sex of these reptiles depends upon the temperature of their eggs at a critical point in development (cooler = male; warmer = female). Check out the video below to learn more about other fascinating variations in how sex is determined throughout nature.

A couple of disclaimers before you watch this video: Reedy describes what typically happens during development—and while XX chromosomes usually lead toward female development and XY chromosomes usually lead toward male development, this isn’t always the case due to other factors (e.g., hormonal exposure, sensitivity to hormones). The end result is that some people are intersex. Also, the focus of this video is on sex determination and it doesn't touch at all on gender identity. Keep in mind that biological sex and gender identity are different things and need not be consistent with one another.