What Makes an Individual a Male or a Female

In mammals, including humans, males and females differ not only in physical appearance but in every cell of their body: male cells have a tiny Y-chromosome which females lack. Females instead have two X-chromosomes while males have only one. It is not universally true though, as majority of fishes, frogs, lizards and turtles have no sex chromosomes. Their sex is generally determined based on the environment (e.g. temperature) in which the eggs grow. Though, the Y-chromosome gene, Sry, triggers male development, it alone is not enough to differentiate the two sexes; orderly expression of a number of genes, generally present on the autosomes, is required to ensure differentiation of a specific gonad - testis or ovary. Individuals bearing testes become male while those having ovaries become female. Excepting the Y-chromosomal Sry, almost all other genes in this cascade are evolutionarily conserved throughout vertebrates. Mutually antagonistic interactions of the male and female pathway genes lead to the formation of the gonads that eventually determine the sex of the individual. Most disorders of sexual development occur due to mutations in any of these genes.