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Information about Sex Determination System

A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most sexual organisms have two sexes. In many cases, sex determination is genetic: males and females have different alleles or even different genes that specify their sexual morphology. In animals, this is often accompanied by chromosomal differences. In other cases, sex is determined by environmental variables (such as temperature) or social variables (the size of an organism relative to other members of its population). The details of some sex-determination systems are not yet fully understood.

Chromosomal determination

XX/XY sex chromosomes

Main article: XY sex-determination system
The XX/XY sex-determination system is one of the most familiar sex-determination systems and is found in human beings and most other mammals. In the XY sex-determination system, females have two of the same kind of sex chromosome (XX), while males have two distinct sex chromosomes (XY). Some species (including humans) have a gene SRY on the Y chromosome that determines maleness; others (such as the fruit fly) use the presence of two X chromosomes to determine femaleness.

XX/X0 sex determination

Main article: X0 sex-determination system
In this variant of the XY system, females have two copies of the sex chromosome (XX) but males have only one (X0). The 0 denotes the absence of a second sex chromosome. This system is observed in a number of insects, including the grasshoppers and crickets of order Orthoptera and in cockroaches (order Blattodea).

The nematode C. elegans is male with one sex chromosome (X0); with a pair of chromosomes (XX) it is a hermaphrodite.

ZW sex chromosomes

Main article: ZW sex-determination system
The ZW sex-determination system is found in birds and some insects and other organisms. The ZW sex-determination system is reversed compared to the XY system: females have two different kinds of chromosomes (ZW), and males have two of the same kind of chromosomes (ZZ).

It is unknown whether the presence of the W chromosome induces female features or the duplication of the Z chromosome induces male ones; unlike mammals, no birds with a double W chromosome (ZWW) or a single Z (Z0) have been discovered. It is possible that either condition causes embryonic death, and both chromosomes are responsible for gender selection; or it may just be that ZWW is just as rare in birds as Turner's syndrome (XO) or Kleinfelter's syndrome (XXY) is in humans, and without having any reason to check individual animals for this, it has never been found.

In Lepidoptera (moths and butterflies), examples of Z0, ZZW and ZZWW females can be found. This suggests that the W chromosome is essential in female determination in some species (ZZW), but not in others (Z0). In Bombyx mori (the commercial silkworm), the W chromosome carries the female-determining genes.

Chromosomes in the ZW region in birds are autosomal in mammals, and vice-versa; therefore, it is theorized that the ZW and XY couples come from different chromosomes of the common ancestor. A paper published in 2004 (Frank Grützner et al, Nature; doi:10.1038/nature03021) suggests that the two systems may be related. According to the paper, platypuses have a ten-chromosome–based system, where the chromosomes form a multivalent chain in male meiosis, segregating into XXXXX-sperm and YYYYY-sperm, with XY-equivalent chromosomes at one end of this chain and the ZW-equivalent chromosomes at the other end.

Haplodiploidy

Main articles: Ploidy and Haplodiploid sex-determination system
Haplodiploidy is found in insects belonging to Hymenoptera, such as ants and bees. Unfertilized eggs develop into haploid individuals, which are the males. Diploid individuals are generally female but may be sterile males. Thus, if a queen bee mates with one drone, her daughters share ¾ of their genes with each other, not ½ as in the XY and ZW systems. This is believed to be significant for the development of eusociality, as it increases the significance of kin selection.

Non-genetic sex-determination systems

Main article: Temperature-dependent sex determination
Many other exotic sex-determination systems exist. In some species of reptiles, including alligators, some turtles, and the tuatara, sex is determined by the temperature at which the egg is incubated. Other species, such as some snails, practice sex change: adults start out male, then become female. In tropical clown fish, the dominant individual in a group becomes female while the other ones are male.

Some species have no sex-determination system. Earthworms and some snails are hermaphrodites; a few species of lizard, fish, and insect are all female and reproduce by parthenogenesis.

In some arthropods, sex is determined by infection. Bacteria of the genus Wolbachia alter their sexuality; some species consist entirely of ZZ individuals, with sex determined by the presence of Wolbachia.

Other unusual systems [this section still being researched]:

See also

External links

References

Evolution of chromosomes
 •  • [ e]
Basic topics: Chromosome | Karyotype | Ploidy | Meiosis
Classification: Autosome | Sex chromosome
Evolution: Chromosomal inversion | Chromosomal translocation | Polyploidy
Biology (from Greek: βίος, bio, "life"; and λόγος, logos, "knowledge"), also referred to as the biological sciences, is the scientific study of life.
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Sex refers to the male and female duality of biology and reproduction. Unlike organisms that only have the ability to reproduce asexually, sexed male and female pairs have the ability to produce offspring through meiosis and fertilization.
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Plantae
  • Chromalveolata
  • Heterokontophyta
  • Haptophyta
  • Cryptophyta
  • Alveolata

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    • Sex refers to the male and female duality of biology and reproduction. Unlike organisms that only have the ability to reproduce asexually, sexed male and female pairs have the ability to produce offspring through meiosis and fertilization.
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    Male (♂) refers to the sex of an organism, or part of an organism, which produces small mobile gametes, called spermatozoa. Each spermatozoon can fuse with a larger female gamete or ovum, in the process of fertilisation.
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    Female (♀) is the sex of an organism, or a part of an organism, which produces ova (egg cells). The ova are defined as the larger gametes in a heterogamous reproduction system, while the smaller, usually motile gamete, the spermatozoon is produced by the male.
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    For the hard rock band, see .
    An allele (Pronounced: /əˈlil/) is a viable DNA (deoxyribonucleic acid) coding that occupies a given locus (position) on a chromosome.
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    For a non-technical introduction to the topic, see .
    A gene is a locatable region of genomic sequence, corresponding to a unit of inheritance, which is associated with regulatory regions, transcribed regions and/or other functional sequence regions.
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    Comparative anatomy is the study of similarities and differences in the anatomy of organisms. It is closely related to evolutionary biology and phylogeny (the evolution of species).
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    Editing of this page by unregistered or newly registered users is currently disabled until (UTC) due to vandalism.
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    Figure 1: A representation of a condensed eukaryotic chromosome, as seen during cell division.]] A chromosome is a single large macromolecule of DNA, and constitutes a physically organized form of DNA in a cell.
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    ecosystem is a natural unit consisting of all plants, animals and micro-organisms in an area functioning together with all the non-living physical factors of the environment.
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    trillion fold).]]

    Temperature is a physical property of a system that underlies the common notions of hot and cold; something that is hotter generally has the greater temperature. Temperature is one of the principal parameters of thermodynamics.
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    Plantae
  • Chromalveolata
  • Heterokontophyta
  • Haptophyta
  • Cryptophyta
  • Alveolata

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    • population is the collection of people or organisms of a particular species living in a given geographic area or mortality, and migration, though the field encompasses many dimensions of population change including the family (marriage and divorce), public health, work and the
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    The XY sex-determination system is the sex-determination system found in humans, most other mammals, some insects (Drosophila) and some plants (Ginkgo). In the XY sex-determination system, females have two of the same kind of sex chromosome (XX), and are called
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    Mammalia
    Linnaeus, 1758

    Subclasses & Infraclasses
    • Subclass †Allotheria*
    • Subclass Prototheria
    • Subclass Theria

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    Figure 1: A representation of a condensed eukaryotic chromosome, as seen during cell division.]] A chromosome is a single large macromolecule of DNA, and constitutes a physically organized form of DNA in a cell.
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    SRY (Sex-determining Region Y) is a sex-determining gene on the Y chromosome in humans and other primates. It is not the only, or even most common sex-determining gene in mammals. Most non-primate mammals use the Y chromosome gene UBE1 for the same purpose.
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    D. melanogaster

    Binomial name
    Drosophila melanogaster
    Meigen, 1830[1]

    Drosophila melanogaster (from the Greek for black-bellied dew-lover
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    The X0 sex-determination system is a system that grasshoppers, crickets, roaches, and some other insects use to determine the sex of their offspring. In this system, there is only one sex chromosome, referred to as X.
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    Insecta
    Linnaeus, 1758

    Orders
    Subclass Apterygota
    * Archaeognatha (bristletails)
    * Thysanura (silverfish)
    Subclass Pterygota
    * Infraclass Paleoptera (Probably paraphyletic)

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    Orthoptera
    Latreille, 1793

    Extant suborders and superfamilies

    Suborder Ensifera
    • Grylloidea
    • Hagloidea
    • Rhaphidophoroidea
    • Schizodactyloidea
    • Stenopelmatoidea
    • Tettigonioidea
    Suborder Caelifera
    • Acridoidea

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    Blattodea

    Families

    Blaberidae
    Blattellidae
    Blattidae
    Cryptocercidae
    Polyphagidae
    Nocticolidae

    Cockroaches (or simply "roaches") are insects of the order Blattodea.
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    Nematoda
    Rudolphi, 1808

    Classes

    Adenophorea
       Subclass Enoplia
       Subclass Chromadoria
    Secernentea
       Subclass Rhabditia
       Subclass Spiruria
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    elegans

    Binomial name
    Caenorhabditis elegans
    Maupas, 1900

    Caenorhabditis elegans (IPA:
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    hermaphrodite is an organism that posses both male and female genetalia.[1] In many species, hermaphroditism is a common part of the life-cycle, particularly in some asexual animals and some plants.
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    The ZW sex-determination system is a system that birds, some fishes, and some insects (including butterflies and moths) use to determine the sex of their offspring. The ovum determines the sex of the offspring in this system, in contrast to the XY sex-determination system and the
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    Aves
    Linnaeus, 1758

    Orders

    About two dozen - see section below

    Birds (class Aves) are bipedal, warm-blooded, egg-laying vertebrate animals.
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