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). Foundations of modern biology There are five unifying principles Genetics (from Ancient Greek grc-Latn genetikos, “genitive” and that from grc-Latn genesis, “origin” a discipline of Biology, is Male (♂ refers to the sex of an organism or part of an organism which produces small mobile Gametes called spermatozoa. Female (♀ is the Sex of an Organism, or a part of an organism which produces ova (egg cells Sexual reproduction involves combining specialized cells (gametes) to form offspring that inherit traits from both parents. The Evolution of sexual reproduction is a major puzzle The first Fossilized evidence of sexually reproducing Organisms is from Eukaryotes of the Stenian A gamete (from Ancient Greek γαμέτης; translated gamete = wife gametes = husband is a cell that fuses with another gamete 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. Male (♂ refers to the sex of an organism or part of an organism which produces small mobile Gametes called spermatozoa. Female (♀ is the Sex of an Organism, or a part of an organism which produces ova (egg cells
An organism's sex is defined by the gametes it produces: males produce male gametes (spermatozoa, or sperm) while females produce female gametes (ova, or egg cells); individual organisms which produce both male and female gametes are termed hermaphroditic. The term sperm is derived from the Greek word (σπέρμα sperma (meaning "seed" and refers to the male reproductive cells. An ovum (plural ova) is a Haploid Female reproductive cell or Gamete. A hermaphrodite is an organism having both male and female reproductive organs Frequently, physical differences are associated with the different sexes of an organism; these sexual dimorphisms can reflect the different reproductive pressures the sexes experience. Sexual dimorphism is the systematic difference in form between individuals of different Sex in the same Species. In some cases male or (more commonly) female organisms also have the role of caring for offspring through the first part of development. Developmental Biology is the official journal of the Society for Developmental Biology.
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Sexual reproduction
Sexual reproduction is a process where organisms form offspring that combine genetic traits from both parents. [1] 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. Deoxyribonucleic acid ( DNA) is a Nucleic acid that contains the genetic instructions used in the development and functioning of all known A chromosome is an organized structure of DNA and Protein that is found in cells. This double-chromosome stage is called "diploid", while the single chromosome stage is "haploid". "Haplo" redirects here For the fictional character see The Death Gate Cycle. "Haplo" redirects here For the fictional character see The Death Gate Cycle. Diploid organisms can, in turn, form haploid cells (gametes) that randomly contain one of each of the chromosome pairs, via a process called meiosis. A gamete (from Ancient Greek γαμέτης; translated gamete = wife gametes = husband is a cell that fuses with another gamete In Biology or life science meiosis (pronounced my-oh-sis is a process of reductional division in which the number of chromosomes per cell is cut in half [2] 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. For soil improvement see Fertilization (soil.
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. A gamete (from Ancient Greek γαμέτης; translated gamete = wife gametes = husband is a cell that fuses with another gamete Mitosis is the process in which a Eukaryotic cell separates the Chromosomes in its Cell nucleus, into two identical sets in two daughter nuclei Multicellular organisms are Organisms consisting of more than one cell, and having Differentiated cells that perform specialized functions 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). Isogamy refers to a form of Sexual reproduction involving Gametes of the same size eVolution is the third Album by eLDee, it was due to be released in 2008 Anisogamy (also called Heterogamy) refers to a form of Sexual reproduction involving Gametes of different sizes [3] 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 ovum (plural ova) is a Haploid Female reproductive cell or Gamete. The term sperm is derived from the Greek word (σπέρμα sperma (meaning "seed" and refers to the male reproductive cells. An individual that produces exclusively large gametes is female, and one that produces exclusively small gametes is male. Female (♀ is the Sex of an Organism, or a part of an organism which produces ova (egg cells Male (♂ refers to the sex of an organism or part of an organism which produces small mobile Gametes called spermatozoa. 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. A hermaphrodite is an organism having both male and female reproductive organs Reproduction is the Biological process by which new individual Organisms are produced [4]
Animals
Most sexually reproducing animals spend their lives as diploid organisms, with the haploid stage reduced to single cell gametes. Flies in the Diptera family Syrphidae are commonly known as hoverflies, flower flies, or Syrphid flies. Sexual intercourse, in its biological sense is the act in which the male reproductive organ (in humans and other higher animals enters the female reproductive tract [5] The gametes of animals have male and female forms—spermatozoa and egg cells. A spermatozoon or spermatozoan ( pl spermatozoa) from the Ancient Greek σπέρμα (seed and ζῷον (living being and more commonly known An ovum (plural ova) is a Haploid Female reproductive cell or Gamete. These gametes combine to form embryos which develop into a new organism.
The male gamete, a spermatozoan (produced within a testicle), is a small cell containing a single long flagellum which propels it. A spermatozoon or spermatozoan ( pl spermatozoa) from the Ancient Greek σπέρμα (seed and ζῷον (living being and more commonly known The testicle (from Latin testiculus, diminutive of testis, meaning "witness" virility plural testes) is the male A flagellum ( plural flagella) is a tail-like structure that projects from the Cell body of certain Prokaryotic and Eukaryotic cells and it [6] 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. For soil improvement see Fertilization (soil.
Female gametes are egg cells (produced within ovaries), large immobile cells that contain the nutrients and cellular components necessary for a developing embryo. An ovum (plural ova) is a Haploid Female reproductive cell or Gamete. "Ovaria" redirects here This is also a proposed section and a Synonym of Solanum. [7] Egg cells are often associated with other cells which support the development of the embryo, forming an egg. In most Birds and Reptiles an egg ( Latin ovum) is the Zygote, resulting from Fertilization of the Ovum. 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. In Biology, mating is the pairing of opposite- Sex or hermaphroditic Organisms for copulation and in Social animals also to raise their 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. External fertilization is a form of Fertilization in which a Zilo cell is united with an Egg cell external to the body of the female [8] Most animals that live outside of water, however, must use transfer sperm from male to female to achieve internal fertilization. Internal fertilization is a form of Animal fertilization of an Ovum by Spermatozoon within the body of an inseminated animal whether female or hermaphroditic
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 zoological anatomy a cloaca is the posterior opening that serves as the only such opening for the intestinal and urinary tracts of certain animal species [9] In many other terrestrial animals, males use specialized sex organs to assist the transport of sperm—these male sex organs are called intromittent organs. An intromittent organ is a general term for an external organ of a Male organism that is specialized to deliver Sperm during copulation. 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 (plural penises, penes The vagina (from Latin, literally " Sheath " or " Scabbard " is a fibromuscular tubular tract leading from the Uterus Insemination is the introduction of sperm into the female Uterus of a Mammal or the Oviduct of an Oviparous (egg-laying animal during Sexual intercourse, in its biological sense is the act in which the male reproductive organ (in humans and other higher animals enters the female reproductive tract The penis contains a tube through which semen (a fluid containing sperm) travels. Physiological aspects Internal and external fertilization Depending on the Species, spermatozoa can fertilize 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). The uterus (from the Latin word for womb) is the major Female reproductive organ of most Mammals including Humans One end the Gestation is the carrying of an Embryo or Fetus inside a Female Viviparous Animal.
Plants
Like animals, plants have developed specialized male and female gametes. Plant sexuality covers the wide variety of Sexual reproduction systems found across the Plant kingdom [10] Within most familiar plants, male gametes are contained within hard coats, forming pollen. Pollen is a fine to coarse powder consisting of microgametophytes ( pollen grains) which produce the male Gametes (sperm cells of 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. Ovule literally means "small egg." In Seed plants the ovule is the structure that gives rise to and contains the female reproductive cells A seed (in some plants referred to as a kernel) is a small embryonic Plant enclosed in a covering called the seed coat usually with some stored
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Many plants have flowers and these are the sexual organs of those plants. A flower, also known as a bloom or Blossom, is the reproductive structure found in Flowering plants (plants of the division Magnoliophyta, also 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. A gynoecium (from Ancient Greek gyne, "woman" is the Female reproductive part of a Flower. A gynoecium (from Ancient Greek gyne, "woman" is the Female reproductive part of a Flower. 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. The stamen ( Plural stamina or stamens, from Latin stamen meaning "thread of the warp " is the male 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. This article is about the tree For other uses of the term "pine" see Pine (disambiguation. 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. Insects ( Class Insecta) are a major group of Arthropods and the most diverse group of Animals on the Earth with over a million described 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. Pollination in angiosperms and Gymnosperms is the process that transfers pollen grains, which contain the male Gametes (sperm to where the female
Fungi
Most fungi reproduce sexually, having both a haploid and diploid stage in their life cycles. Mating in fungi is like a fungal Mycelium, a complex tangle There are several sources of confusion These fungi are typically isogamous, lacking male and female specialization: haploid fungi grow into contact with each other and then fuse their cells. Isogamy refers to a form of Sexual reproduction involving Gametes of the same size 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". [11]
Some fungi, including baker's yeast, have mating types that create a duality similar to male and female roles. Baker's yeast is the common name for the strains of Yeast commonly used as a Leavening agent in Baking Bread and related products where it converts Mating types occur in Eukaryotes that undergo Sexual reproduction via Isogamy. Yeast with the same mating type will not fuse with each other to form diploid cells, only with yeast carrying the other mating type. [12]
Fungi produce mushrooms as part of their sexual reproduction. A mushroom is the fleshy Spore -bearing Fruiting body of a Fungus, typically produced above ground on soil or on its food source 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. In Biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions
Evolution
Sexual reproduction first appeared about a billion years ago, evolved within ancestral single-celled eukaryotes. [13] The reason for the initial evolution of sex, and the reason 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, and that sex helps in the removal of disadvantageous traits.
Sexual reproduction is a process specific to eukaryotes, organisms whose cells contain a nucleus and mitochondria. Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex In addition to animals, plants, and fungi, other eukaryotes (eg. Protists (ˈproʊtɨst are a diverse group of eukaryotic Microorganisms Historically protists were treated as the kingdom Protista but this the malaria parasite) also engage in sexual reproduction. Malaria is a vector -borne Infectious disease caused by Protozoan Parasites It is widespread in tropical and subtropical regions including 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, as far as we know no third gamete has appeared in multicellular animals. [14][15][16]
Sex determination
The most basic sexual system is one in which all organisms are hermaphrodites, producing both male and female gametes—this is true of some animals (eg. A sex-determination system is a biological system that determines the development of sexual characteristics in an Organism. A hermaphrodite is an organism having both male and female reproductive organs snails) and the majority of flowering plants. [17] 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. A sex-determination system is a biological system that determines the development of sexual characteristics in an Organism.
In the majority of species with sex specialization organisms are either male (producing only male gametes) or female (producing only female gametes). Male (♂ refers to the sex of an organism or part of an organism which produces small mobile Gametes called spermatozoa. Female (♀ is the Sex of an Organism, or a part of an organism which produces ova (egg cells A few exceptions exist—for example, in the roundworm C. elegans the two sexes are hermaphrodite and male (a system called androdioecy). Caenorhabditis elegans (ˌsiːnoʊræbˈdaɪtɪs ˈɛlɪgænz is a free-living Nematode (roundworm about 1 mm in length which Androdioecy is a reproductive system found in Species composed of a Male Population and a distinct Hermaphrodite population
Sometimes an organism's development is intermediate between male and female, a condition called intersex. Intersexuality is the state of a living thing of a gonochoristic species whose Sex chromosomes, Genitalia, and/or Secondary sex characteristics 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
In genetic sex determination systems, an organism's sex is determined by the genome it inherits. A sex-determination system is a biological system that determines the development of sexual characteristics in an Organism. 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. Developmental Biology is the official journal of the Society for Developmental Biology. 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. Human beings, humans or man (Origin 1590–1600 L homō man OL hemō the earthly one (see Humus Mammals ( class Mammalia) are a class of Vertebrate Animals characterized by the presence of Sweat glands, including sweat glands The XY sex-determination system is the Sex-determination system found in Humans most other Mammals some insects ( Drosophila) and some 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. Drosophila melanogaster (from the Greek for black-bellied dew-lover) is a two-winged insect that belongs to the Diptera, the order [17] 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. Birds ( class Aves) are bipedal endothermic ( Warm-blooded) Vertebrate animals that lay eggs. The ZW sex-determination system is a system that determines the sex of offspring in Birds, some Fish, and some Insects (including Butterflies [18] 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. [19]
Many insects use a sex determination system based on the number of sex chromosomes. Insects ( Class Insecta) are a major group of Arthropods and the most diverse group of Animals on the Earth with over a million described This is called XX/XO sex determination—the O indicates the absence of the sex chromosome. The X0 sex-determination system is a system that Grasshoppers, Crickets, Cockroaches, and some other Insects use to determine the sex of their 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. Field crickets are Insects of order Orthoptera. These crickets are in Subfamily Gryllinae of family Gryllidae. [20] 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). Caenorhabditis elegans (ˌsiːnoʊræbˈdaɪtɪs ˈɛlɪgænz is a free-living Nematode (roundworm about 1 mm in length which [21]
Other insects, including honey bees and ants, use a haplodiploid sex-determination system. This article refers collectively to all true honey bees for the "common" domesticated honey bee see European honey bee Honey bees Ants are social Insects of the family Formicidae and along with the related families of Wasps and Bees belong to the order The haplodiploid sex-determination system determines the sex of the offspring of many Hymenopterans ( Bees Ants and Wasps, and coleopterans ( [22] 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. Sex ratio is the Ratio of Males to Females in a Population. The primary sex ratio is the ratio at the time of conception secondary sex ratio is
Nongenetic
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 temperature-dependent sex determination: the temperature embryos experience during their development determines the sex of the organism. Reptiles, or members of the class Reptilia are air-breathing Cold-blooded Vertebrates that have skin covered in scales as opposed to hair or feathers Temperature-dependent sex determination (TSD also called environmental sex determination, is where the surrounding temperature determines the sex of an 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. Turtles are Reptiles of the Order Testudines (all living turtles belong to the Crown group Chelonia) most of
Many fish change sex over the course of their lifespan, a phenomenon called sequential hermaphroditism. Fish are aquatic Vertebrate animals that are typically ectothermic (previously Cold-blooded) covered with scales, and equipped with two A hermaphrodite is an organism having both male and female reproductive organs In clownfish, smaller fish are male, and the dominant and largest fish in a group becomes female. Clownfish and anemonefish are Fishes from the subfamily Amphiprioninae in the family Pomacentridae. In many wrasses the opposite is true—most fish are initially female and become male when they reach a certain size. The wrasses are a family, Labridae, of marine Fish, many of which are brightly colored 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. A fern is any one of a group of about 20000 Species of Plants classified in the phylum or division Pteridophyta, also known as Filicophyta [23]
Sexual dimorphism
Many animals have differences between the male and female sexes in size and appearance, a phenomenon called sexual dimorphism. Sexual dimorphism is the systematic difference in form between individuals of different Sex in the same Species. Sexual dimorphism is the systematic difference in form between individuals of different Sex in the same Species. Sexual dimorphisms are often associated with sexual selection - the competition between individuals of one sex to mate with the opposite sex. Sexual selection is the Theory proposed by Charles Darwin that states that certain evolutionary traits can be explained by Intraspecific competition [24] 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. Polygyny (which comes from neo- Greek: πολύ poly "many" + γυνή gyny "woman" is a specific form of Polygamy,
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. [25] 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 (eg. bright colors would seem to make a bird more visible to predators). One proposed explanation for this is the handicap principle. The handicap principle is a Hypothesis originally proposed in 1975 by biologist Amotz Zahavi to explain how Evolution may lead to [26] 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 a larger size and more body hair in men; women have breasts, wider hips, and a higher body fat percentage. A sex difference is a distinction of biological and/or physiological characteristics typically associated with either males or females of a species in general
See also
References
- Alberts B, Johnson A, Lewis J, Raff M, Roberts K, and Walter P (2002). Sexual conflict occurs when the two Sexes have conflicting optimal fitness strategies concerning Reproduction, leading to Evolutionary arms race Molecular Biology of the Cell, 4th edition. ISBN 0-8153-3218-1.
- Gilbert SF (2000). Developmental Biology, 6th edition, Sinauer Associates, Inc. . ISBN 0-87893-243-7.
Notes
- ^ Alberts et al. (2002), V. 20. The Benefits of Sex
- ^ Alberts et al. (2002), V. 20. Meiosis
- ^ Gilbert (2000), 1.2. Multicellularity: The Evolution of Differentiation
- ^ Alberts et al. (2002), V. 21. Caenorhabditis Elegans: Development from the Perspective of the Individual Cell
- ^ Alberts et al. (2002), 3. Mendelian genetics in eukaryotic life cycles
- ^ Alberts et al. (2002), V.20. Sperm
- ^ Alberts et al. (2002), V.20. Eggs
- ^ Alberts et al. (2002), V.20. Fertilization
- ^ Ritchison G. Avian Reproduction. Eastern Kentucky University. Retrieved on 2008-04-03. 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Events 1043 - Edward the Confessor is crowned King of England.
- ^ Gilbert (2000), 4.20. Gamete Production in Angiosperms
- ^ Nick Lane (2005). Power, Sex, Suicide: Mitochondria and the Meaning of Life. Oxford University Press, 236-237. ISBN 0192804812.
- ^ Matthew P Scott, Paul Matsudaira, Harvey Lodish, James Darnell, Lawrence Zipursky, Chris A Kaiser, Arnold Berk, Monty Krieger (2000). Molecular Cell Biology, Fourth Edition, WH Freeman and Co. 14.1. Cell-Type Specification and Mating-Type Conversion in Yeast
- ^ Book Review for Life: A Natural History of the First Four Billion Years of Life on Earth. Jupiter Scientific. Retrieved on 2008-04-07. 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Events 529 - First draft of Corpus Juris Civilis (a fundamental work in Jurisprudence) is issued by Eastern Roman Emperor
- ^ Amanda Schaffer, Pas de Deux: Why Are There Only Two Sexes?, Slate updated 27 September, 2007. Slate is an English-language online current affairs and culture Magazine created in 1996 by former New Republic editor Michael
- ^ Laurence D. Hurst, 'Why are There Only Two Sexes?', Proceedings: Biological Sciences 263 (1996): 415-422
- ^ ES Haag, 'Why two sexes? Sex determination in multicellular organisms and protistan mating types', Seminars in Cell and Developmental Biology 18 (2007): 348-9.
- ^ a b Dellaporta SL, Calderon-Urrea A (1993). "Sex Determination in Flowering Plants". The Plant Cell 5: 1241–1251. doi:. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
- ^ Smith CA, Katza M, Sinclair AH (2003). "DMRT1 Is Upregulated in the Gonads During Female-to-Male Sex Reversal in ZW Chicken Embryos". Biology of Reproduction 68: 560–570. doi:. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 12533420.
- ^ Evolution of the Y Chromosome. Annenberg Media. Retrieved on 2008-04-01. 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Events 527 - Byzantine Emperor Justin I names his nephew Justinian I as co-ruler and successor to the throne
- ^ Yoshimura A (2005). "Karyotypes of two American field crickets: Gryllus rubens and Gryllus sp. (Orthoptera: Gryllidae)". Entomological Science 8 (3): 219–222. doi:. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
- ^ Riddle DL, Blumenthal T, Meyer BJ, Priess JR (1997). C. Elegans II. Cold Spring Harbor Laboratory Press. ISBN 0-87969-532-3. 9.II. Sexual Dimorphism
- ^ Charlesworth B (2003). "Sex Determination in the Honeybee". Cell 114 (4): 397–398. doi:. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
- ^ Tanurdzic M and Banks JA (2004). "Sex-Determining Mechanisms in Land Plants". The Plant Cell 16: S61–S71. doi:. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 15084718.
- ^ Darwin C (1871). The Descent of Man and Selection in Relation to Sex. The Descent of Man and Selection in Relation to Sex is a book on Evolutionary theory by English naturalist Charles Darwin, first Murray, London.
- ^ Stuart-Smith J, Swain R, Stuart-Smith R, Wapstra E (2007). "Is fecundity the ultimate cause of female-biased size dimorphism in a dragon lizard?". Journal of Zoology 273 (3): 266–272. doi:. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
- ^ Zahavi, A. and Zahavi, A. Amotz Zahavi ( Hebrew: אמוץ זהבי) (born 1928 is an Israeli evolutionary biologist, a Professor Emeritus at the Zoology (1997) The handicap principle: a missing piece of Darwin's puzzle. Oxford University Press. Oxford. ISBN 0-19-510035-2
External links and further reading
- Human Sexual Differentiation by P. C. Sizonenko
- New Scientist article on Sex chromosomes in the platypus
- Maynard-Smith, J. The Evolution of Sex. Cambridge University Press, 1978.
Dictionary
sex
-noun
- (countable) Either of two main divisions (either male or female) into which many organisms can be placed, according to reproductive function or organs; gender.
- (uncountable) The distinguishing property, quality, or assemblage of properties by which organisms are classified as female or male on the basis of their reproductive organs and functions; the set of properties by which male is distinguished from female.
- (uncountable) Sexual intercourse; the act of sexual intercourse.
- (euphemism) Genitalia; a penis or vagina.
- (dated, with the definite article) the sex. Women; womankind. Also, the fair sex, the fairer sex, the whimsical sex.
-verb
- (zoology) To determine the sex of; often in reference to newly-hatched chicks.
- (colloquial) To have sex with.
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