[yetanother4968]

The sex of an organism is defined by the type of gamete an organism produces (or will produce, or used to produce, or would produce if it was healthy, etc.) If an organism produces relatively large gametes that don’t move much, if at all (i.e eggs), it is female. If it produces small motile gametes (i.e. sperm) it is male. This holds true for all animals and plants that reproduce sexually, as it is the basic biological distinction between the sexes.

Chromosomes themselves aren’t sex, and only some animals’ sexes are “programmed” by their chromosomes. For example, alligator embryos’ (future) sex is determined by their incubation temperature.

People with Klinefelter syndrome are male because they have testicles, underdeveloped as they may be.

Edit: typos