In biology, sex is a process of combining and mixing genetic traits, often resulting in the specialization of organisms into male and female types (or sexes). Sexual reproduction involves combining specialized cells (gametes) to form offspring that inherit traits from both parents. Gametes can be identical in form and function (known as isogametes), but in many cases an asymmetry has evolved such that two sex-specific types of gametes (heterogametes) exist: male gametes are small, motile, and optimized to transport their genetic information over a distance, while female gametes are large, non-motile and contain the nutrients necessary for the early development of the young organism.
Sexual reproduction is a process where organisms form offspring that combine genetic traits from both parents. Genetic traits are contained within the dioxyribonucleic acid (DNA) of chromosomes — by combining one of each type of chromosomes from each parent, an organism is formed containing a doubled set of chromosomes. This double-chromosome stage is called "diploid", while the single chromosome stage is "haploid". Diploid organisms can, in turn, form haploid cells (gametes) that randomly contain one of each of the chromosome pairs, via a process called meiosis. Meiosis also involves a stage of chromosomal crossover, in which regions of DNA are exchanged between matched types of chromosomes to form a new pair of mixed chromosomes. Crossing over and fertilization (the recombining of single sets of chromosomes to make a new diploid) result in the new organism containing a different set of genetic traits from either parent.
In many organisms the haploid stage has been reduced to just gametes specialized to recombine and form a new diploid organism; in others, the gametes are capable of undergoing cell division to produce multicellular haploid organisms. In either case, gametes may be externally similar, particularly in size (isogamy), or may have evolved an asymmetry such that the gametes are different in size and other aspects (anisogamy). By convention, the larger gamete (called an ovum, or egg cell) is considered female, while the smaller gamete (called a spermatozoon, or sperm cell) is considered male. An individual that produces exclusively large gametes is female, and one that produces exclusively small gametes is male. An individual that produces both types of gametes is a hermaphrodite; in some cases hermaphrodites are able to self-fertilize and produce offspring on their own, without a second organism.
The male gamete, a spermatozoan (produced within a testicle), is a small cell containing a single long flagellum which propels it. Spermatozoa are extremely reduced cells, lacking many cellular components that would be necessary for embryonic development. They are specialized for motility, seeking out an egg cell and fusing with it in a process called fertilization.
Female gametes are egg cells (produced within ovaries), large immobile cells that contain the nutrients and cellular components necessary for a developing embryo. Egg cells are often associated with other cells which support the development of the embryo, forming an egg. In mammals, the fertilized embryo instead develops within the female, receiving nutrition directly from its mother.
Animals are usually mobile and seek out a partner of the opposite sex for mating. Animals which live in the water can mate using external fertilization, where the eggs and sperm are released into and combine within the surrounding water. Most animals that live outside of water, however, must transfer sperm from male to female to achieve internal fertilization.
In most birds, both excretion and reproduction is done through a single posterior opening, called the cloaca—male and female birds touch cloaca to transfer sperm, a process called "cloacal kissing". In many other terrestrial animals, males use specialized sex organs to assist the transport of sperm—these male sex organs are called intromittent organs. In humans and other mammals this male organ is the penis, which enters the female reproductive tract (called the vagina) to achieve insemination—a process called sexual intercourse. The penis contains a tube through which semen (a fluid containing sperm) travels. In female mammals the vagina connects with the uterus, an organ which directly supports the development of a fertilized embryo within (a process called gestation).
Like animals, plants have developed specialized male and female gametes. Within most familiar plants, male gametes are contained within hard coats, forming pollen. The female gametes of plants are contained within ovules; once fertilized by pollen these form seeds which, like eggs, contain the nutrients necessary for the development of the embryonic plant.Many plants have flowers and these are the sexual organs of those plants. Flowers are usually hermaphroditic, producing both male and female gametes. The female parts, in the center of a flower, are the carpels—one or more of these may be merged to form a single pistil. Within carpels are ovules which develop into seeds after fertilization. The male parts of the flower are the stamens: these long filamentous organs are arranged between the pistil and the petals and produce pollen at their tips. When a pollen grain lands upon the top of a carpel, the tissues of the plant react to transport the grain down into the carpel to merge with an ovule, eventually forming seeds.
In pines and other conifers the sex organs are cones and have male and female forms. The more familiar female cones are typically more durable, containing ovules within them. Male cones are smaller and produce pollen which is transported by wind to land in female cones. As with flowers, seeds form within the female cone after pollination.
Because plants are immobile, they depend upon passive methods for transporting pollen grains to other plants. Many plants, including conifers and grasses, produce lightweight pollen which is carried by wind to neighboring plants. Other plants have heavier, sticky pollen that is specialized for transportation by insects. The plants attract these insects with nectar-containing flowers. Insects transport the pollen as they move to other flowers, which also contain female reproductive organs, resulting in pollination.
Some fungi, including baker's yeast, have mating types that create a duality similar to male and female roles. Yeast with the same mating type will not fuse with each other to form diploid cells, only with yeast carrying the other mating type.
Fungi produce mushrooms as part of their sexual reproduction. Within the mushroom diploid cells are formed, later dividing into haploid spores—the height of the mushroom aids the dispersal of these sexually produced offspring.
Sexual reproduction is a process specific to eukaryotes, organisms whose cells contain a nucleus and mitochondria. In addition to animals, plants, and fungi, other eukaryotes (eg. the malaria parasite) also engage in sexual reproduction. Some bacteria use conjugation to transfer genetic material between bacteria; while not the same as sexual reproduction, this also results in the mixture of genetic traits.
What is considered defining of sexual reproduction is the difference between the gametes and the binary nature of fertilization. Multiplicity of gamete types within a species would still be considered a form of sexual reproduction. However, no third gamete is known in multicellular animals.
In the majority of species with sex specialization organisms are either male (producing only male gametes) or female (producing only female gametes). A few exceptions exist—for example, in the roundworm C. elegans the two sexes are hermaphrodite and male (a system called androdioecy).
Sometimes an organism's development is intermediate between male and female, a condition called intersex. Sometimes intersex individuals are called "hermaphrodite" but, unlike biological hermaphrodites, intersex individuals are unusual cases and are not typically fertile in both male and female aspects.
In genetic sex determination systems, an organism's sex is determined by the genome it inherits. Genetic sex determines usually depends on asymmetrically inherited sex chromosomes which carry genetic features that influence development; sex may be determined either by the presence of a sex chromosome or by how many the organism has. Genetic sex determination, because it is determined by chromosome assortment, usually results in a 1:1 ratio of male and female offspring.
Humans and other mammals have an XY sex determination system: the Y chromosome carries factors responsible for triggering male development. The default sex, in the absence of a Y chromosome, is female. Thus, XX mammals are female and XY are male. XY sex determination is found in other organisms, including the common fruit fly and some plants. In some cases, including in the fruit fly, it is the number of X chromosomes that determines sex rather than the presence of a Y chromosome.
In birds, which have a ZW sex-determination system, the opposite is true: the W chromosome carries factors responsible for female development, and default development is male. In this case ZZ individuals are male and ZW are female. The majority of butterflies and moths also have a ZW sex-determination system. In both XY and ZW sex determination systems the sex chromosome carrying the critical factors is often significantly smaller, carrying little more than the genes necessary for triggering the development of a given sex.
Many insects use a sex determination system based on the number of sex chromosomes. This is called XX/XO sex determination—the O indicates the absence of the sex chromosome. All other chromosomes in these organisms are diploid, but organisms may inherit one or two X chromosomes. In field crickets, for example, insects with a single X chromosome develop as male, while those with two develop as female. In the nematode C. elegans most worms are self-fertilizing XX hermaphrodites, but occasionally abnormalities in chromosome inheritance regularly give rise to individuals with only one X chromosome—these XO individuals are fertile males (and half their offspring are male).
Other insects, including honey bees and ants, use a haplodiploid sex-determination system. In this case diploid individuals are generally female, and haploid individuals (which develop from unfertilized eggs) are male. This sex-determination system results in highly biased sex ratios, as the sex of offspring is determined by fertilization rather than the assortment of chromosomes during meiosis.
Many fish change sex over the course of their lifespan, a phenomenon called sequential hermaphroditism. In clownfish, smaller fish are male, and the dominant and largest fish in a group becomes female. In many wrasses the opposite is true—most fish are initially female and become male when they reach a certain size. Sequential hermaphrodites may produce both types of gametes over the course of their lifetime, but at any given point they are either female or male.
In some ferns the default sex is hermaphrodite, but ferns which grow in soil that has previously supported hermaphrodites are influenced by residual hormones to instead develop as male.
Many animals have differences between the male and female sexes in size and appearance, a phenomenon called sexual dimorphism. Sexual dimorphisms are often associated with sexual selection - the competition between individuals of one sex to mate with the opposite sex. Antlers in male deer, for example, are used in combat between males to win reproductive access to female deer. In many cases the male of a species is larger in size; in mammals species with high sexual size dimorphism tend to have highly polygynous mating systems—presumably due to selection for success in competition with other males.
Other animals, including most insects and many fish, have larger females. This may be associated with the cost of producing egg cells, which requires more nutrition than producing sperm—larger females are able to produce more eggs. Occasionally this dimorphism is extreme, with males reduced to living as parasites dependent on the female.
In birds, males often have a more colorful appearance and may have features (like the long tail of male peacocks) that would seem to put the organism at a disadvantage (e.g. bright colors would seem to make a bird more visible to predators). One proposed explanation for this is the handicap principle. This hypothesis says that, by demonstrating he can survive with such handicaps, the male is advertising his genetic fitness to females—traits that will benefit daughters as well, who will not be encumbered with such handicaps.
Sex differences in humans include, generally, a larger size and more body hair in men; women have breasts, wider hips, and a higher body fat percentage.