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Sex-determination system
 
 
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A sex-determination system is a biological
Biology

Biology is a branch of the natural sciences concerned with the study of living organisms and their interaction with each other and their environment ....
 system that determines the development of sexual characteristics
Sex

In biology, sex is a process of combining and mixing genetics traits, often resulting in the specialization of organisms into male and female types ....
 in an organism
Organism

In biology, an organism is any life thing . In at least some form, all organisms are capable of response to stimulus , reproduction, growth and developmental biology, and maintenance of homeostasis as a stable whole....
. Most sexual organisms have two sex
Sex

In biology, sex is a process of combining and mixing genetics traits, often resulting in the specialization of organisms into male and female types ....
es. In many cases, sex determination is genetic: male
Malé

Mal? , population 104,403 , is the Capital , the largest city in terms of population, and the name of an island in the Maldives. It is located at the southern edge of North Male' Atoll Kaafu Atoll....
s and female
Female

Female is the sex of an organism, or a part of an organism, which produces mobile ovum . 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....
s have different allele
Allele

An allele is one member of a pair or series of different forms of a gene. Usually alleles are coding region, but sometimes the term is used to refer to a junk DNA....
s or even different gene
Gene

A gene is the basic unit of heredity in a living organism. All living things depend on genes. Genes hold the information to build and maintain their cell and pass genetic trait to offspring....
s that specify their sexual morphology
Comparative anatomy

Comparative anatomy is the study of similarities and differences in the anatomy of organisms. It is closely related to evolutionary biology and phylogeny ....
. In animal
Animal

Animals are a major group of multicellular, eukaryotic organisms of the Kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life....
s, this is often accompanied by chromosomal
Chromosome

A chromosome is an organized structure of DNA and protein that is found in Cell . A chromosome is a single piece of DNA that contains many genes, regulatory sequence and other genetic sequence....
 differences. In other cases, sex is determined by environmental
Ecosystem

An ecosystem is a natural unit consisting of all plants, animals and micro-organisms in an area functioning together with all of the non-living physical factors of the environment....
 variables (such as temperature
Temperature

In physics, temperature is a physical property of a Physical system that underlies the common notions of hot and cold; something that feels hotter generally has the greater temperature....
) or social variables (the size of an organism
Organism

In biology, an organism is any life thing . In at least some form, all organisms are capable of response to stimulus , reproduction, growth and developmental biology, and maintenance of homeostasis as a stable whole....
 relative to other members of its population
Population

File:Population density.pngIn biology, a population is the collection of inter-breeding organisms of a particular species; in sociology, a collection of human beings....
).






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A sex-determination system is a biological
Biology

Biology is a branch of the natural sciences concerned with the study of living organisms and their interaction with each other and their environment ....
 system that determines the development of sexual characteristics
Sex

In biology, sex is a process of combining and mixing genetics traits, often resulting in the specialization of organisms into male and female types ....
 in an organism
Organism

In biology, an organism is any life thing . In at least some form, all organisms are capable of response to stimulus , reproduction, growth and developmental biology, and maintenance of homeostasis as a stable whole....
. Most sexual organisms have two sex
Sex

In biology, sex is a process of combining and mixing genetics traits, often resulting in the specialization of organisms into male and female types ....
es. In many cases, sex determination is genetic: male
Malé

Mal? , population 104,403 , is the Capital , the largest city in terms of population, and the name of an island in the Maldives. It is located at the southern edge of North Male' Atoll Kaafu Atoll....
s and female
Female

Female is the sex of an organism, or a part of an organism, which produces mobile ovum . 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....
s have different allele
Allele

An allele is one member of a pair or series of different forms of a gene. Usually alleles are coding region, but sometimes the term is used to refer to a junk DNA....
s or even different gene
Gene

A gene is the basic unit of heredity in a living organism. All living things depend on genes. Genes hold the information to build and maintain their cell and pass genetic trait to offspring....
s that specify their sexual morphology
Comparative anatomy

Comparative anatomy is the study of similarities and differences in the anatomy of organisms. It is closely related to evolutionary biology and phylogeny ....
. In animal
Animal

Animals are a major group of multicellular, eukaryotic organisms of the Kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life....
s, this is often accompanied by chromosomal
Chromosome

A chromosome is an organized structure of DNA and protein that is found in Cell . A chromosome is a single piece of DNA that contains many genes, regulatory sequence and other genetic sequence....
 differences. In other cases, sex is determined by environmental
Ecosystem

An ecosystem is a natural unit consisting of all plants, animals and micro-organisms in an area functioning together with all of the non-living physical factors of the environment....
 variables (such as temperature
Temperature

In physics, temperature is a physical property of a Physical system that underlies the common notions of hot and cold; something that feels hotter generally has the greater temperature....
) or social variables (the size of an organism
Organism

In biology, an organism is any life thing . In at least some form, all organisms are capable of response to stimulus , reproduction, growth and developmental biology, and maintenance of homeostasis as a stable whole....
 relative to other members of its population
Population

File:Population density.pngIn biology, a population is the collection of inter-breeding organisms of a particular species; in sociology, a collection of human beings....
). The details of some sex-determination systems are not yet fully understood.

Chromosomal determination


XX/XY sex chromosomes

The XX/XY sex-determination system is one of the most familiar sex-determination systems and is found in human
Human

A human being, also human or man, is a member of a species of bipedalism primates in the family Hominidae . Mitochondrial DNA evidence indicates that modern humans originated in east Africa about 200,000 years ago....
 beings and most other mammal
Mammal

Mammals are a class of vertebrate animals whose name is derived from their distinctive feature, mammary glands, with which they feed their young....
s, although at least one monotreme
Monotreme

Monotremes are mammals that lay eggs instead of giving birth to live young like Marsupialias and Placentalia .They are conventionally treated as comprising a single order Monotremata, though a recent classification proposes to divide them into the orders Platypoda and Tachyglossa ....
, the platypus
Platypus

The Platypus is a semi-aquatic mammal Endemic to Eastern states of Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay Egg instead of giving birth to live young....
, presents a particular sex determination scheme that in some ways resembles that of the ZW sex chromosomes of birds, and it also lacks the SRY
SRY

SRY is a sex-determining gene on the Y chromosome in the therians .This intronless gene encodes a transcription factor that is a member of the high mobility group -box family of DNA-binding proteins....
 gene. Several Arvicolinae
Arvicolinae

Arvicolinae is a subfamily of rodents that includes the voles, lemmings, and muskrats. Its closest relatives are members of the other subfamilies in the Cricetidae, the hamsters and New World rats and mice....
 (voles and lemmings) and some other rodent
Rodent

Rodentia is an Order of mammals also known as rodents, characterised by two continuously growing Incisors#The_Rodent_incisor in the upper and lower jaws which must be kept short by gnawing....
s are also noted for their unusual sex determination systems.

In the XY sex-determination system, females have two of the same kind of sex chromosome
Chromosome

A chromosome is an organized structure of DNA and protein that is found in Cell . A chromosome is a single piece of DNA that contains many genes, regulatory sequence and other genetic sequence....
 (XX), while males have two distinct sex chromosomes (XY). Some species (including humans) have a gene SRY
SRY

SRY is a sex-determining gene on the Y chromosome in the therians .This intronless gene encodes a transcription factor that is a member of the high mobility group -box family of DNA-binding proteins....
 on the Y chromosome that determines maleness; others (such as the fruit fly
Drosophila melanogaster

Drosophila melanogaster is a two-winged insect that belongs to the Diptera, the Order of the Fly. The species is commonly known as the Drosophilidae or vinegar fly, and is one of the most commonly used model organisms in biology, including studies in genetics, physiology and Life history theory....
) use the presence of two X chromosomes to determine femaleness. The XY sex chromosomes are different in shape and size from each other unlike the autosomes, and are termed allosome
Allosome

An allosome is a sex chromosome that differs from an ordinary autosome in form, size, or behavior. The human sex chromosomes are a typical pair of allosomes....
s.

XX/X0 sex determination

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 insect
Insect

Insects are the biggest class of arthropods and the only ones with wings. They are the most diverse group of animals on the planet. They are most diverse at the equator and their diversity declines toward the poles....
s, including the grasshoppers and crickets of order Orthoptera
Orthoptera

The Orthoptera are an order of insects with paurometabolous or incomplete metamorphosis, including the grasshoppers, cricket s and locusts. Many insects in this order produce sound by rubbing their wings against each other or their legs, the wings or legs containing rows of corrugated bumps....
 and in cockroaches (order Blattodea
Cockroach

Cockroaches are insects of the order Blattaria. This name derives from the Latin word for "cockroach", blatta.There are about 4,000 species of cockroach, of which 30 species are associated with human habitations and about four species are well known as pest s....
).

The nematode
Nematode

The "roundworms" or "nematodes" are the most diverse phylum of body cavity, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 80,000 have been described, of which over 15,000 are parasite....
 C. elegans
Caenorhabditis elegans

'Caenorhabditis elegans' is a free-living, transparent nematode , about 1 mm in length, which lives in temperate soil environments. Research into the molecular biology and developmental biology of C....
 is male with one sex chromosome (X0); with a pair of chromosomes (XX) it is a hermaphrodite
Hermaphrodite

A hermaphrodite is an organism having both male and female reproductive organs. In many species, hermaphroditism is a common part of the life-cycle, enabling a form of sexual reproduction in which partners are not separated into distinct male and female types of individual....
. hermaphrodites have both sex chromosomes

ZW sex chromosomes

The ZW sex-determination system is found in bird
Bird

Birds are wing, Bipedalismal, endothermic , vertebrate animals that lay egg . There are around 10,000 living species, making them the most numerous tetrapod vertebrates....
s and some insect
Insect

Insects are the biggest class of arthropods and the only ones with wings. They are the most diverse group of animals on the planet. They are most diverse at the equator and their diversity declines toward the poles....
s and other organism
Organism

In biology, an organism is any life thing . In at least some form, all organisms are capable of response to stimulus , reproduction, growth and developmental biology, and maintenance of homeostasis as a stable whole....
s. 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).

Haplodiploidy

Haplodiploidy
Ploidy

Ploidy is the number of complete sets of non-homologous chromosomes in a biological cell. In humans, the somatic cells that comprise the body are diploid , but sex cells are haploid....
 is found in insects belonging to Hymenoptera
Hymenoptera

Hymenoptera is one of the larger order s of insects, comprising the sawfly, wasps, bees, and ants. The name refers to the membranous wings of the insects, and is derived from the Ancient Greek language wikt:???? : membrane and wikt:pte??? : wing....
, such as ant
Ant

Ants are Eusociality insects of the family Formicidae, and along with the related wasps and bees, they belong to the order Hymenoptera. Ants evolution from wasp-like ancestors in the mid-Cretaceous period between 110 and 130 million years ago and Evolutionary radiation after the rise of flowering plants....
s and bee
Bee

Bees are flying insects closely related to wasps and ants. Bees are a monophyly lineage within the superfamily Apoidea, presently classified by the unranked taxon name Anthophila....
s. 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
Bee

Bees are flying insects closely related to wasps and ants. Bees are a monophyly lineage within the superfamily Apoidea, presently classified by the unranked taxon name Anthophila....
 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
Eusociality

Eusociality is a term used for the highest level of social organization in a hierarchical classification. The term "eusocial" was introduced in 1966 by Suzanne Batra and given a more definitive meaning by E....
, as it increases the significance of kin selection
Kin selection

Some organisms tend to exhibit strategies that favor the reproductive success of their relatives, even at a cost to their own survival and/or reproduction....
. This is common also in wasps that are parasitic and in the male greenflies.

Non-genetic sex-determination systems

Many other sex-determination systems exist. In some species of reptiles, including alligator
Alligator

An Alligator is a crocodilian in the genus Alligator of the family Alligatoridae. The name alligator is an anglicization form of the Spanish language el lagarto , the name by which early Spain explorers and settlers in Florida called the alligator....
s, some turtle
Turtle

Turtles are reptiles of the Order Testudines , most of whose body is shielded by a special bone or cartilage animal shell developed from their ribs....
s, and the tuatara
Tuatara

The tuatara is a reptile endemism to New Zealand which, though it resembles most lizards, is actually part of a distinct lineage, order Sphenodontia....
, sex is determined by the temperature at which the egg is incubated. Other species, such as some snail
Snail

The word snail is a common name for almost all members of the molluscan class Gastropoda that have coiled animal shells in the adult stage. When the word snail is used in a general sense, it includes sea snails, land snails and freshwater snails....
s, 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. Earthworm
Earthworm

Earthworm is the common name for the largest members of Oligochaeta in the phylum Annelida. The earthworm is the most known worm in America, and other countries....
s and some snails are hermaphrodite
Hermaphrodite

A hermaphrodite is an organism having both male and female reproductive organs. In many species, hermaphroditism is a common part of the life-cycle, enabling a form of sexual reproduction in which partners are not separated into distinct male and female types of individual....
s; a few species of lizard, fish, and insect are all female and reproduce by parthenogenesis
Parthenogenesis

Parthenogenesis is an asexual form of reproduction found in females where growth and development of embryos or seeds occurs without fertilization by a male....
.

In some arthropod
Arthropod

Arthropods are animals belonging to the Scientific classification Arthropoda , and include the insects, arachnids, crustaceans, and others....
s, sex is determined by infection, as when Bacteria of the genus Wolbachia
Wolbachia

Wolbachia is a genus of inherited bacterium which infects arthropod species, including a high proportion of insects. It is one of the world's most common parasitic microbes and is potentially the most common reproductive parasite in the biosphere....
 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]:
  • Swordtail fish?
  • The Chironomus
    Chironomidae

    Chironomidae are a family of nematoceran Fly with a global distribution. They are closely related to the Ceratopogonidae, Simuliidae, and Thaumaleidae....
     midge species
  • The Platypus
    Platypus

    The Platypus is a semi-aquatic mammal Endemic to Eastern states of Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay Egg instead of giving birth to live young....
     lacks the mammalian sex-determining gene SRY
    SRY

    SRY is a sex-determining gene on the Y chromosome in the therians .This intronless gene encodes a transcription factor that is a member of the high mobility group -box family of DNA-binding proteins....
    , meaning that the process of sex determination in the Platypus remains unknown.


See also

  • Clarence Erwin McClung
    Clarence Erwin McClung

    Clarence Erwin McClung was an United States Biologist who discovered the role of chromosomes in sex determination.References ...
     who discovered the role of chromosomes in sex determination.
  • Testis-determining factor
  • Barr body
    Barr body

    In those species in which sex is determined by the presence of the Y or ZW sex-determination system chromosome rather than the Haplodiploid sex-determination system of the X or Z, a Barr body is the inactive X chromosome in a female cell, or the inactive Z in a male , rendered inactive in a process called Lyonization....
  • Protandry
  • Protogyny
  • For humans:
    • Human sex determination and differentiation
      Sex determination and differentiation (human)

      Human sex refers to the processes by which an individual becomes either a male or female during development....
    • Gender verification in sports
    • Sex organ
      Sex organ

      A sex organ, or primary sexual characteristic, as narrowly defined, is any of the anatomical parts of the body which are involved in sexual reproduction and constitute the reproductive system in a complex organism; in mammals, these include:...
      , or primary sexual characteristic
    • Secondary sex characteristic
      Secondary sex characteristic

      Secondary sex characteristics are traits that distinguish the two sexes of a species, but that are not directly part of the reproductive system....
    • Shettles Method
      Shettles Method

      The Shettles Method is a child fertilisation idea that is reputed to help determine a Infant sex. It was developed by Landrum B. Shettles in the 1960s and was publicized in the book How to Choose the Sex of Your Baby, coauthored by Shettles and David Rorvik....
    • XYY syndrome
      XYY syndrome

      XYY syndrome is an aneuploidy of the sex chromosomes in which a human male receives an extra Y chromosome, producing a 47,XYY karyotype....


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