Female (symbol: ♀) is the sex of an organism that produces non-mobile ova (egg cells), which is the gamete that fuses with the male gamete during sexual reproduction. Most female mammals, including female humans, have two X chromosomes. Female characteristics vary between different species with some species having pronounced female characteristics, such as the presence of pronounced mammary glands in mammals. There is no single genetic mechanism behind sex differences in different species and the existence of two sexes seems to have evolved multiple times independently in different evolutionary lineages.