I'm still sort of vaguely amazed that we have the ability to change someone's physical sex (or is it gender? Trans stuff confuses me) basically by taking pills. That you get from a freaking Walgreens.
Strange, yet amazing times we live in.
Think that's cool? Wanna know the crazy shit that's on the horizon?
In the lab, they've partially transdifferentiated adult mammal gonads between the two morphs; ovaries and testes. So they're close to being able to allow mammals like us do the thing that some amphibians and fish can do naturally, and change sex from the ground up, complete with regular fertility and self-produced hormones and etc.
Human self-engineering, ho!
Sex refers to the body - the state of the organs and tissues that can be differentiated between 2 human morphs (Male, Female, or points between called Intersex or Androgyne).
Gender is sort of arbitrary and Culture-Specific, but describes a social role, standard of behavior, or performance/act. This usually includes styles of clothing, behavior, and some lingering cliches of what's appropriate for a person to do, and what isn't okay.
In traditional cultures there were generally 2 Genders linked to Sex, with a few other Genders popping up for Androgyne or "Two-Spirit" or whatever folks. Modern Culture these days presents us with a lot more than 2 identifiable Genders; Tomboy or Feminine Lesbian are easy to see as distinct from a Traditional Woman role, you know? So, you can combine those two categories and try to describe people in a nutshell. And you can also use them in unexpected combinations to describe folks with more accuracy; like a Male (Sex) Traditional Woman (Gender) would lead you to describe some Trans-gals.