Sex
Sex is the biological trait of a reproducing organism in producing gametes of one of two different sizes or shapes—male or female gametes. Thus, the typical classification for such organisms, their sex, is also known as either male or female. During sexual reproduction, a male and a female gamete fuse to form a zygote, which develops into an offspring that inherits traits from each parent. By convention, organisms that produce smaller gametes are called male, while organisms that produce larger gametes are called female. An organism that produces both types of gamete is a hermaphrodite.
In non-hermaphroditic species, the sex of an individual is determined through one of several biological sex-determination systems. Most mammalian species have the XY sex-determination system, where the male usually carries an X and a Y chromosome, and the female usually carries two X chromosomes. Other chromosomal sex-determination systems in animals include the ZW system in birds, and the XO system in some insects. Various environmental systems include temperature-dependent sex determination in reptiles and crustaceans.
The male and female of a species may be physically alike or have physical differences. In sexually dimorphic species, including most birds and mammals, the sex of an individual is usually identified through observation of that individual's sexual characteristics. Sexual selection or mate choice can accelerate the evolution of differences between the sexes.
Some kinds of functional differences between individuals, such as in fungi, may be referred to as mating types.
Sexual reproduction
Sexual reproduction, in which two individuals produce an offspring that possesses a selection of the genetic traits of each parent, is exclusive to eukaryotes. Genetic traits are encoded in the deoxyribonucleic acid of chromosomes. The eukaryote cell has a set of paired homologous chromosomes, one from each parent, and this double-chromosome stage is called "diploid". During sexual reproduction, a diploid organism produces specialized haploid sex cells called gametes via meiosis, each of which has a single set of chromosomes. Meiosis involves a stage of genetic recombination via chromosomal crossover, in which regions of DNA are exchanged between matched pairs of chromosomes, to form new chromosomes, each with a new combination of the genes of the parents. Then the chromosomes are separated into single sets in the gametes. When gametes fuse during fertilization, the resulting zygote has half of the genetic material of the mother and half of the father. The combination of chromosomal crossover and fertilization, bringing the two single sets of chromosomes together to make a new diploid zygote, results in a new organism that contains a different set of the genetic traits of each parent.In animals, the haploid stage only occurs in the gametes, the sex cells that fuse to form a zygote that develops directly into a new diploid organism. In a plant species, the diploid organism produces a type of haploid spore by meiosis that is capable of undergoing repeated cell division to produce a multicellular haploid organism. In either case, the gametes may be externally similar as in the green alga Ulva or may be different in size and other aspects. The size difference is greatest in oogamy, a type of anisogamy in which a small, motile gamete combines with a much larger, non-motile gamete.
In anisogamic organisms, by convention, the larger gamete is considered female, while the smaller gamete is considered male. An individual that produces large gametes is female, and one that produces small gametes is male. An individual that produces both types of gamete is a hermaphrodite. In some species, a hermaphrodite can self-fertilize and produce an offspring on its own.
Animals
Most sexually reproducing animals spend their lives as diploid, with the haploid stage reduced to single-cell gametes. The gametes of animals have male and female forms—spermatozoa and egg cells, respectively. These gametes combine to form embryos, which develop into new organisms.The male gamete, a spermatozoon, 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. In vertebrates, they are produced within the ovaries. They are 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 that 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, use internal fertilization, transferring sperm directly into the female to prevent the gametes from drying up.
In most birds, both excretion and reproduction are 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 known as the penis, which enters the female reproductive tract to achieve insemination—a process called sexual intercourse. The penis contains a tube through which semen travels. In female mammals, the vagina connects with the uterus, an organ which directly supports the development of a fertilized embryo within.
Because of their motility, animal sexual behavior can involve coercive sex. Traumatic insemination, for example, is used by some insect species to inseminate females through a wound in the abdominal cavity—a process detrimental to the female's health.
Plants
Like animals, land plants have specialized male and female gametes. In seed plants, male gametes are produced by reduced male gametophytes that are contained within pollen which have hard coats that protect the male gamete forming cells during transport from the anthers to the stigma. The female gametes of seed plants are contained within ovules. Once fertilized, these form seeds which, like eggs, contain the nutrients necessary for the initial development of the embryonic plant.The flowers of flowering plants contain their sexual organs. Most flowering plants are hermaphroditic, with both male and female parts in the same flower or on the same plant in single sex flowers. About 5% of plant species have individual plants that are one sex or the other. The female parts, in the center of a hermaphroditic or female flower, are the pistils, each unit consisting of a carpel, a style and a stigma. Two or more of these reproductive units may be merged to form a single compound pistil, the fused carpels forming an ovary. Within the carpels are ovules which develop into seeds after fertilization. The male parts of the flower are the stamens: these consist of long filaments arranged between the pistil and the petals that produce pollen in anthers at their tips. When a pollen grain lands upon the stigma on top of a carpel's style, it germinates to produce a pollen tube that grows down through the tissues of the style into the carpel, where it delivers male gamete nuclei to fertilize an ovule that eventually develops into a seed.
Some hermaphroditic plants are self-fertile, but plants have evolved multiple different self-incompatibility mechanisms to avoid self-fertilization, involving sequential hermaphroditism, molecular recognition systems and morphological mechanisms such as heterostyly.
In pines and other conifers, the sex organs are produced within cones that have male and female forms. Male cones are smaller than female ones and produce pollen, which is transported by wind to land in female cones. The larger and longer-lived female cones are typically more durable and contain ovules within them that develop into seeds after fertilization.
Because seed plants are immobile, they depend upon passive methods for transporting pollen grains to other plants. Many, including conifers and grasses, produce lightweight pollen which is carried by wind to neighboring plants. Some flowering plants have heavier, sticky pollen that is specialized for transportation by insects or larger animals such as hummingbirds and bats, which may be attracted to flowers containing rewards of nectar and pollen. These animals transport the pollen as they move to other flowers, which also contain female reproductive organs, resulting in pollination.
Fungi
Most species of fungus can reproduce sexually and have life cycles with both haploid and diploid phases. These species of fungus are typically isogamous, i.e. lacking male and female specialization. One haploid fungus grows into contact with another, and then they fuse their cells. In some cases, the fusion is asymmetric, and the cell that donates only a nucleus could arguably be considered male. Fungi may also have more complex allelic mating systems, with other sexes not accurately described as male, female, or hermaphroditic.Some fungi, including baker's yeast, have mating types that determine compatibility. Yeasts with the same mating types will not fuse with each other to form diploid cells, only with yeast carrying another mating type.
Many species of higher fungi produce mushrooms as part of their sexual reproduction. Within the mushroom, diploid cells are formed, later dividing into haploid spores.