Mammals are a subclass of animals with vertebrates who have features including breasts, hair, and a neocortex. They are distinguished from reptiles, birds, and other vertebrates.
Studies have shown there is no single determinant of sex, gender, or sexuality. Thus, a binary distinction of “male or female” fails to describe a wide range of humans in practice.
Nettie Stevens and Edmund Beecher Wilson independently discovered the chromosomal XY sex-determination system (that males have XY and females have XX sex chromosomes).