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Overview The male gamete sperm fertilizing the female gamete ovum 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, including many of those with sexual reproduction. 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. 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. Many species, both plants and 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. Evolution of sexual reproduction Different forms of anisogamy: A anisogamy of motile cells, B oogamy egg cell and sperm cell , C anisogamy of non-motile cells egg cell and spermatia. Different forms of isogamy: A isogamy of motile cells , B isogamy of non-motile cells, C conjugation. Sexual reproduction first probably evolved about a billion years ago within ancestral single-celled eukaryotes. Some of the many plausible theories include: 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 e. Some bacteria use conjugation to transfer genetic material between cells; while not the same as sexual reproduction, this also results in the mixture of genetic traits. The defining characteristic of sexual reproduction in eukaryotes 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 type is known in multicellular plants or animals. The ZW sex-determination system is shared by birds, some fish and some crustaceans. XY sex determination is used by most mammals,  but also some insects,  and plants Silene latifolia. A paper from compared the chicken Z chromosome with platypus X chromosomes and suggested that the two systems are related. Isogamy and Anisogamy The life cycle of sexually reproducing organisms cycles through haploid and diploid stages Sexual reproduction in eukaryotes is a process whereby organisms form offspring that combine genetic traits from both parents. Chromosomes are passed on from one generation to the next in this process. Each cell in the offspring has half the chromosomes of the mother and half of the father. 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 meiosis. 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, such as cryptogamic plants 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. An individual that produces exclusively large gametes is female, and one that produces exclusively small gametes is male. These gametes combine to form embryos which develop into a new organism. The male gamete, a spermatozoon produced in vertebrates within the testes , is a small cell containing a single long flagellum which propels it. 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 in vertebrates within the ovaries , large immobile cells that contain the nutrients and cellular components necessary for a developing embryo. 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. 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 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. 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. Plant reproduction Like animals, plants have developed specialized male and female gametes. The female gametes of seed 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. Female left and male right cones are the sex organs of pines and other conifers. 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 pistils , each unit consisting of a carpel , a style and a stigma. One or more of these reproductive units may be merged to form a single compound pistil. Within the carpels are ovules which develop into seeds after fertilization. The male parts of the flower are the stamens: 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. In pines and other conifers the sex organs are conifer 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 or larger animals such as humming birds and bats with nectar-containing flowers. These animals transport the pollen as they move to other flowers, which also contain female reproductive organs, resulting in pollination. Mating in fungi Mushrooms are produced as part of fungal sexual reproduction Most fungi reproduce sexually, having both a haploid and diploid stage in their life cycles. These fungi are typically isogamous , lacking male and female specialization: In some of these cases, the fusion is asymmetric, and the cell which donates only a nucleus and not accompanying cellular material could arguably be considered "male". Yeast with the same mating type will not fuse with each other to form diploid cells, only with yeast carrying the other mating type. 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. Sex-determination system Sex helps the spread of advantageous traits through recombination. The diagrams compare evolution of allele frequency in a sexual population top and an asexual population bottom. The vertical axis shows frequency and the horizontal axis shows time. The advantageous alleles A and B, arising independently, can be rapidly combined by sexual reproduction into the most advantageous combination AB. Asexual reproduction takes longer to achieve this combination, because it can only produce AB if A arises in an individual which already has B, or vice versa. The most basic sexual system is one in which all organisms are hermaphrodites , producing both male and female gametes—[ citation needed ] this is true of some animals e. The biological cause for an organism developing into one sex or the other is called sex determination. In the majority of species with sex specialization, organisms are either male producing only male gametes or female producing only female gametes. Exceptions are common—for example, the roundworm C. 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. Genetic Like humans and other mammals, the common fruit fly has an XY sex-determination system. In genetic sex-determination systems, an organism's sex is determined by the genome it inherits. Genetic sex-determination 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: Humans and other mammals have an XY sex-determination system: The "default sex," in the absence of a Y chromosome, is female-like. Thus, XX mammals are female and XY are male. In humans, biological sex is determined by five factors present at birth: In birds, which have a ZW sex-determination system , the opposite is true: 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. This is called X0 sex-determination —the 0 indicates the absence of the sex chromosome.
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