Biological sex is what determines whether an individual is male, female, or intersex. It is determined by the genes, hormones, and sexual anatomy of an individual. Biological sex is often simply referred to as sex.
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The biological sex of an individual is determined based on sexual anatomy, genes, and hormones. A person born with XX chromosomes will typically have the sexual anatomy of a female, and will develop female secondary sex characteristics during puberty due to the production of female-specific hormones. A person born with XY chromosomes will typically have the sexual anatomy of a male, and during puberty will develop the secondary sex characteristics of a male due to the production of male-specific hormones. Sometimes, a person is born with either XX or XY chromosomes, but has ambiguous sexual anatomy. In these cases, the individual is considered intersex.