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  1. Where do the male and female sex cells meet
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  79. It contains the female nucleus anda store of food. Gametes are necessary for DNA to be passed from one organism down to the next generation. Pregnancy lasts an average of 280 days — about 9 months.
  81. Sex: Either of the two main categories male and female into which humans and most other living things are divided on the basis of their reproductive functions. Toward the end of puberty, girls begin to release eggs as part of a monthly period called the menstrual cycle. The ovaries of a female release eggs female sex cells and a male's testes produce sperm male sex cells.
  83. Reproductive system - At each stage in the cycle, there can be variations, and up to half of the species, will depart from the conventional reproductive activity and do something completely different.
  85. This article is about sex in sexually reproducing organisms. For the act, see. For other uses, see. The gametes produced by an organism define its sex: males produce small gametes e. Individual organisms which produce both male and female gametes are termed. Gametes can be identical in form and function known as , but, in many cases, an asymmetry has evolved such that two different types of gametes heterogametes exist known as. Physical differences are often associated with the different sexes of an organism; these can reflect the different reproductive pressures the sexes experience. For instance, and can accelerate the evolution of physical differences between the sexes. Among humans and other , males typically carry an X and a XY , whereas females typically carry two XX , which are a part of the. Other animals have different , such as the in birds, the in insects, and various , for example in crustaceans. The male gamete fertilizing the female gamete 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. 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. In sexual reproduction, there are special kinds of cells that divide without prior duplication of its genetic material, in a process named. The resulting cells are called , 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 are the parts that are involved in the production and exchange of gametes in sexual reproduction. 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. This is very frequent in plants. Sexual reproduction first probably evolved about a billion years ago within ancestral single-celled. The reason for the evolution of sex, and the reason s it has survived to the present, are still matters of debate. Some of the many plausible theories include: that sex creates variation among offspring, sex helps in the spread of advantageous traits, that sex helps in the removal of disadvantageous traits, and that sex facilitates repair of germ-line DNA. Sexual reproduction is a process specific to , organisms whose cells contain a nucleus and mitochondria. In addition to animals, plants, and fungi, e. Some bacteria use 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. While the evolution of sex dates to the prokaryote or early eukaryote stage, the origin of may have been fairly early in eukaryotes see. The is shared by birds, some fish and some. XY sex determination is used by most mammals, but also some insects, and plants Silene latifolia. The is found in most , insects such as , and , and some nematodes, crustaceans, and gastropods. No genes are shared between the avian ZW and mammal XY chromosomes, and from a comparison between chicken and human, the Z chromosome appeared similar to the chromosome 9 in human, rather than X or Y, suggesting that the ZW and XY sex-determination systems do not share an origin, but that the sex chromosomes are derived from autosomal chromosomes of the of birds and mammals. A paper from 2004 compared the chicken Z chromosome with X chromosomes and suggested that the two systems are related. 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. Genetic traits are contained within the DNA of —by combining one of each type of chromosomes from each parent, an organism is formed containing a doubled set of chromosomes. Diploid organisms can, in turn, form haploid cells that randomly contain one of each of the chromosome pairs, via. Meiosis also involves a stage of , in which regions of DNA are exchanged between matched types of chromosomes, to form a new pair of mixed chromosomes. In many organisms, the haploid stage has been reduced to just specialized to recombine and form a new diploid organism; in others, such as the gametes are capable of undergoing to produce haploid organisms. In either case, gametes may be externally similar, particularly in size , or may have an asymmetry such that the gametes are different in size and other aspects. By convention, the larger gamete called an , 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 ; in some cases hermaphrodites are able to and produce offspring on their own, without a second organism. Animals mating 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— and egg cells. These gametes combine to form embryos which develop into a new organism. The male gamete, a produced in vertebrates within the , is a small cell containing a single long 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. Female gametes are egg cells produced in vertebrates within the , 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. 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. Animals which live in the water can mate using , where the eggs and sperm are released into and combine within the surrounding water. Most animals that live outside of water, however, use , transferring sperm directly from to female to prevent the gametes from drying up. In many other terrestrial animals, males use specialized sex organs to assist the transport of sperm—these are called. In humans and other mammals this male organ is the , which enters the female reproductive tract called the to achieve —a process called. The penis contains a tube through which a fluid containing sperm travels. In female mammals the vagina connects with the , an organ which directly supports the development of a fertilized embryo within a process called. Because of their motility, can involve coercive sex. Plants Main article: Like animals, plants have developed specialized male and female gametes. Within seed plants, male gametes are contained within hard coats, forming. The female gametes of seed plants are contained within ; once fertilized by pollen these form 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 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 , each unit consisting of a , a and a. One or more of these reproductive units may be merged to form a single compound. Within the carpels are which develop into seeds after fertilization. The male parts of the flower are the : these consist of long filaments arranged between the pistil and the petals that produce pollen in at their tips. When a pollen grain lands upon the stigma on top of a carpel's style, it germinates to produce a 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 and other the sex organs are 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 and with nectar-containing flowers. These animals transport the pollen as they move to other flowers, which also contain female reproductive organs, resulting in. Fungi Mushrooms are produced as part of fungal sexual reproduction Most reproduce sexually, having both a haploid and diploid stage in their life cycles. These fungi are typically , lacking male and female specialization: haploid fungi grow into contact with each other and then fuse their cells. Fungi may also have more complex allelic mating systems, with other sexes not accurately described as male, female, or hermaphroditic. Some fungi, including , have 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. Many species of produce as part of their. Within the mushroom diploid cells are formed, later dividing into haploid. The height of the mushroom aids the of these sexually produced offspring. 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. In many cases, however, specialization of sex has evolved such that some organisms produce only male or only female gametes. 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 has an hermaphrodite and a male sex a system called. Sometimes an organism's development is intermediate between male and female, a condition called. Genetic Like humans and other mammals, the common fruit fly has an. 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 ; 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 have an : the carries factors responsible for triggering male development. Thus, XX mammals are female and XY are male. In humans, biological sex is determined by five factors present at birth: the presence or absence of a Y chromosome which alone determines the individual's genetic sex , the type of , the , the internal reproductive anatomy such as the in females , and the external genitalia. XY sex determination is found in other organisms, including the 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 see below. In birds, which have a , 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 —the 0 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 , for example, insects with a single X chromosome develop as male, while those with two develop as female. In the nematode most worms are self-fertilizing XX hermaphrodites, but occasionally abnormalities in chromosome inheritance regularly give rise to individuals with only one X chromosome—these X0 individuals are fertile males and half their offspring are male. Other insects, including and , use a. 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 , as the sex of offspring is determined by fertilization rather than the assortment of chromosomes during meiosis. Nongenetic Main article: For many species, sex is not determined by inherited traits, but instead by environmental factors experienced during development or later in life. Many reptiles have : the temperature embryos experience during their development determines the sex of the organism. In some turtles, for example, males are produced at lower incubation temperatures than females; this difference in critical temperatures can be as little as 1—2 °C. Many fish change sex over the course of their lifespan, a phenomenon called. In , smaller fish are male, and the dominant and largest fish in a group becomes female. In many 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 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 and some plants have differences between the male and female sexes in size and appearance, a phenomenon called. In other species, the differences may be more extreme, such as differences in coloration or bodyweight. Sexual dimorphisms in animals are often associated with — 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 than the female. Mammal species with extreme sexual size dimorphism tend to have highly mating systems—presumably due to selection for success in with other males—such as the. Other examples demonstrate that it is the preference of females that drive sexual dimorphism, such as in the case of the. 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. For example, female spiders are typically twice as long as the males. Occasionally this dimorphism is extreme, with males reduced to living as dependent on the female, such as in the. Some plant species also exhibit dimorphism in which the females are significantly larger than the males, such as in the moss and the liverwort. There is some evidence that, in these genera, the dimorphism may be tied to a sex chromosome, or to chemical signalling from females. In birds, males often have a more appearance and may have features like the long tail of male peacocks that would seem to put the organism at a disadvantage e. One proposed explanation for this is the. This hypothesis says that, by demonstrating he can survive with such handicaps, the male is advertising his to females—traits that will benefit daughters as well, who will not be encumbered with such handicaps. Retrieved March 23, 2018. Sex: Either of the two main categories male and female into which humans and most other living things are divided on the basis of their reproductive functions. The fact of belonging to one of these categories. The group of all members of either sex. Retrieved March 23, 2018. A single body can function as both male and female. Sexual reproduction requires both male and female haploid gametes. In most species, these gametes are produced by individuals that are either male or female. Species that have male and female members are called dioecious from the Greek for 'two houses'. In some species, a single individual may possess both female and male reproductive systems. Retrieved Feb 18, 2018. Biology of Plants 7 th ed. NY: Freeman and Company Publishers. Genetics: Conjugation 4 th ed. Biological Science 3rd ed. Living at Micro Scale. Cambridge, Massachusetts: Harvard University Press. The Evolution of Sex Determination. Retrieved 7 April 2008. Seminars in Cell and Developmental Biology. Evolutionary Origin and Adaptive Function of Meiosis. In Meiosis: Bernstein C and Bernstein H, editors. Retrieved 4 June 2018. Caenorhabditis Elegans: Development as Indiv. Retrieved 3 April 2008. Power, Sex, Suicide: Mitochondria and the Meaning of Life. Scott; Paul Matsudaira; Harvey Lodish; James Darnell; Lawrence Zipursky; Chris A. Kaiser; Arnold Berk; Monty Krieger 2000. Molecular Cell Biology Fourth ed. WH Freeman and Co. American Society of Plant Biologists. Retrieved 1 April 2008. Cold Spring Harbor Laboratory Press. Proceedings of the Royal Society B. Archived from on 31 August 2003. Retrieved 8 August 2012. Cambridge: Cambridge University Press. New Manual of Bryology. Nichinan, Miyazaki, Japan: The Hattori botanical Laboratory. Mosses of Eastern North America. New York: Columbia University Press. The handicap principle: a missing piece of Darwin's puzzle. Molecular Biology of the Cell 4th ed. New York: Garland Science. Developmental Biology 6th ed. The Evolution of Sex.
  86. In humans, biological sex is civil by five factors present at birth: the presence or absence of a Y chromosome which alone determines the individual's genetic sexthe type ofthethe internal reproductive anatomy such as the in femalesand the external genitalia. For instance, and can accelerate the evolution of social differences between the sexes. These tubes connect each ovary to the uterus. The vagina connects with the uterus, or womb, at the cervix which means neck. The testes also secrete the male hormone testosterone, which stimulates development of the reproductive structures and secondary sexual characteristics such as met voice at puberty. If a sperm cell containing an X chromosome fertilizes an egg, the resulting zygote will be XX, or female. An example of an X-linked gene is red-green colorblindness in humans. When a baby girl is born, her ovaries contain hundreds of thousands of eggs, which piece inactive until puberty begins. With a few exceptions, this pattern holds true for all sexually reproducing organisms. Hormone: A chemical produced in living cells that is carried by the blood to organs and tissues in distant parts of the body, where it regulates cellular activity.