The gametes produced by an organism define its sex: males produce small gametes (spermatozoa, or sperm, in animals; pollen in plants) while females produce large gametes (ova, or egg cells); individual organisms which produce both male and female gametes are termed hermaphroditic. Frequently, physical differences are associated with the different sexes of an organism; these sexual dimorphisms can reflect the different reproductive pressures the sexes experience. For instance, mate choice and sexual selection can accelerate the evolution of physical differences between the sexes.
Among humans and other mammals, males typically carry XY chromosomes, whereas females typically carry XX chromosomes, which are a part of the XY sex-determination system. Other animals have sex-determination systems as well, such as the ZW sex-determination system in birds, the X0 sex-determination system in insects, and various environmental sex determination systems .
One of the basic properties of life is reproduction, the capacity to generate new individuals, and sex is an aspect of this process. Life has evolved from simple stages to more complex ones, and so have the reproduction mechanisms. Initially the reproduction was a replicating process that consists in producing new individuals that contain the same genetic information as the original or parent individual. This mode of reproduction is called asexual, and it is still used by many species, particularly unicellular, but it is also very common in multicellular organisms. In sexual reproduction, the genetic material of the offspring comes from two different individuals. As sexual reproduction developed by way of a long process of evolution, intermediates exist. Bacteria, for instance, reproduce asexually, but undergo a process by which a part of the genetic material of an individual donor is transferred to another recipient.
Disregarding intermediates, the basic distinction between asexual and sexual reproduction is the way in which the genetic material is processed. Typically, prior to an asexual division, a cell duplicates its genetic information content, and then divides. This process of cell division is called mitosis. In sexual reproduction, there are special kinds of cells that divide without prior duplication of its genetic material, in a process named meiosis. The resulting cells are called gametes, and contain only half the genetic material of the parent cells. These gametes are the cells that are prepared for the sexual reproduction of the organism. Sex comprises the arrangements that enable sexual reproduction, and has evolved alongside the reproduction system, starting with similar gametes (isogamy) and progressing to systems that have different gamete types, such as those involving a large female gamete (ovum) and a small male gamete (sperm).
In complex organisms, the sex organs are the parts that are involved in the production and exchange of gametes in sexual reproduction. Many species, particularly animals, have sexual specialization, and their populations are divided into male and female individuals. Conversely, there are also species in which there is no sexual specialization, and the same individuals both contain masculine and feminine reproductive organs, and they are called hermaphrodites. This is very frequent in plants.