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Sex is a system of categories, a way of putting kinds of bodies into categories. Living things of many species evolved to be specialized into their own male, female, and intersex kinds, each known as a sex. A sex is generally determined by reproductive body parts. In humans, these imply-- but do not prove-- a correlation with chromosomes. In gender studies, the sex and gender of a person are thought of as two distinct things.
When people speak of a person's "sex", usually what they really mean is their assigned gender at birth. This is because a person's sex is much more difficult to determine than most people believe. For example, chromosomes are part of defining someone's sex, but most people never get their chromosomes tested. A baby's assigned gender at birth is based on only one thing: the presence or absence of what a doctor thinks is probably a penis. This will be the only basis of that child's legal gender/sex. As the person grows up, the doctor's guess about their sex can turn out to be wrong, because some intersex conditions only become clear once a person has gone through puberty. Even then, the person might have unusual chromosomes or internal reproductive organs without ever knowing about it.