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In biology, sex refers to the categorization of organisms (or their parts) with regard to the role they play in reproduction. When a species reproduces sexually, it usually does so by producing two types of gametes that have to come together to form a new member of the species. One type of gamete is large and non-mobile (ova), whereas the other type is small and mobile (spermia). When an organism or an organ produces large and non-mobile gametes, it's called female. If it produces small and mobile gametes, it's called male.
In most mammals, in particular humans, female members of the species can be identified by having two X chromosomes, ovaries, a uterus and a vagina. The ovaries produce ova (also called egg cells), which may be fertilized into a zygote, which travels to the uterus to develop into an embryo, a fetus, and ultimately a baby which is brought to the world through the vaginal canal. Males can be identified by having an X and a Y chromosome, testes (testicles) and a penis. The testes produce sperm, which are ejaculated into the vagina during sexual intercourse, from where they might travel towards the ova to fertilize them.
Some disorders may cause a person not to develop typical sex organs. The umbrella term intersex covers persons who show ambiguity in their sex due to such a disorder.
Some legal systems also classify people according to sex. Under such systems, a person's legal sex may not correspond to their biological sex, because of errors in documentation, or because of legal sex-change procedures that may be allowed for transgender or transsexual people. Some systems allow a third sex category for those who don't want to be legally identified as female or male, either because they have an intersex condition, or because they claim to have a gender identity that is non-binary.