Sex-determination system
A sex-determination system is a biological
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...

 system that determines the development of sexual characteristics
In biology, sex is a process of combining and mixing genetic traits, often resulting in the specialization of organisms into a male or female variety . Sexual reproduction involves combining specialized cells to form offspring that inherit traits from both parents...

 in an organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...

. Most sexual organisms have two sexes
Gender is a range of characteristics used to distinguish between males and females, particularly in the cases of men and women and the masculine and feminine attributes assigned to them. Depending on the context, the discriminating characteristics vary from sex to social role to gender identity...

. In many cases, sex determination is genetic: male
Male refers to the biological 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 fertilization...

s and female
Female is the sex of an organism, or a part of an organism, which produces non-mobile ova .- Defining characteristics :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
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...

s or even different gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

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 .-Description:...

. In 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. Most animals are motile, meaning they can move spontaneously and...

s, this is often accompanied by chromosomal
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...

 differences. In other cases, sex is determined by environmental
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....

 variables (such as temperature
Temperature-dependent sex determination
Temperature-dependent sex determination is type of environmental sex determination in which the temperatures experienced during embryonic development determine the sex of the offspring. It is most prevalent and common among amniote vertebrates that are classified under the reptile class, but is...

) or social variables (the size of an organism relative to other members of its population
A population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...

). The details of some sex-determination systems are not yet fully understood.

Origin of sex chromosomes

The accepted hypothesis of XY and ZW sex chromosome evolution is that they evolved at the same time, in two different branches. However, there is some evidence to suggest that there could have been transitions between ZW and XY, such as in Xiphophorus maculatus, which have both ZW and XY systems in the same population, despite the fact that ZW and XY have different gene locations. The platypus' genes also back up the possible evolutionary link between XY and ZW, because they have the DMRT1
Doublesex and mab-3 related transcription factor 1, also known as DMRT1, is a protein which in humans is encoded by the DMRT1 gene.- Function :...

 gene possessed by birds on their X chromosomes. Regardless, XY and ZW follow a similar route. All sex chromosomes started out as an original autosome
An autosome is a chromosome that is not a sex chromosome, or allosome; that is to say, there is an equal number of copies of the chromosome in males and females. For example, in humans, there are 22 pairs of autosomes. In addition to autosomes, there are sex chromosomes, to be specific: X and Y...

 of an original reptile that relied upon temperature to determine the sex of offspring. After the mammals separated, the branch further split into Lepidosauria
The Lepidosauria are reptiles with overlapping scales. This subclass includes Squamata and Sphenodontidae. It is a monophyletic group and therefore contains all descendents of a common ancestor. The squamata includes snakes, lizards, tuataras, and amphisbaenia. Lepidosauria is the sister taxon...

 and Archosauromorpha
Archosauromorpha is an infraclass of diapsid reptiles that first appeared during the late Permian and became more common during the Triassic. Included in this infraclass are the groups Rhynchosauria, Trilophosauridae, Prolacertiformes, Archosauriformes, and, tentatively, Choristodera...

. These two groups both evolved the ZW system separately, as evidenced by the existence of different sex chromosomal locations. In mammals, one of the autosome pair, now Y, mutated its SOX3
Transcription factor SOX-3 is a protein that in humans is encoded by the SOX3 gene.This gene encodes a member of the SOX family of transcription factors involved in the regulation of embryonic brain development and in determination of cell fate. The encoded protein acts as a transcriptional...

 gene into the 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 SOX gene family of DNA-binding proteins...

 gene, causing that chromosome to designate sex. After this mutation, the SRY-containing chromosome inverted
Chromosomal inversion
An inversion is a chromosome rearrangement in which a segment of a chromosome is reversed end to end. An inversion occurs when a single chromosome undergoes breakage and rearrangement within itself. Inversions are of two types: paracentric and pericentric.Paracentric inversions do not include the...

 and could no longer swap genes with its partner. The Y chromosome continued to receive both positive and negative mutations throughout the years, and without a partner to combine with, it slowly decayed. However, because both the chromosomes were necessary for reproduction, the Y chromosome remained, despite its inability to pair with the X.

XX/XY sex chromosomes

The XX/XY sex-determination system is the most familiar, as it is found in human
Humans are the only living species in the Homo genus...

s. In the system, females have two of the same kind of sex chromosome (XX), while males have two distinct sex chromosomes (XY). The XY sex chromosomes are different in shape and size from each other, unlike the autosomes, and are termed 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.The x chromosome is present in the ovum, while x or y chromosomes can be present in sperm.-References:...

s. Some species (including humans) have a gene SRY on the Y chromosome that determines maleness; others (such as the fruit fly
Drosophila melanogaster
Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...

) use the presence of two X chromosomes to determine femaleness. In Y-centered sex determination, the SRY gene is not the only gene to have an influence on sex. In XY mice, lack of the gene DAX1
DAX1 is a nuclear receptor protein that in humans is encoded by the NR0B1 gene...

 on the X chromosome results in sterility, but in humans it causes adrenal hypoplasia congenita. However, when an extra DAX1 gene is placed on the X, the result is a female, despite the existence of SRY.

The XX/XY system is also found in most other mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s, as well as some insects. However, at least one monotreme
Monotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals...

, the platypus, presents a particular sex determination scheme that in some ways resembles that of the ZW sex chromosomes of birds, and also lacks the SRY gene, whereas some rodent
Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....

s, such as several Arvicolinae
The Arvicolinae are a subfamily of rodents that includes the voles, lemmings, and muskrats. They are most closely related to the other subfamilies in the Cricetidae . Some authorities place the subfamily Arvicolinae in the family Muridae along with all other members of the superfamily Muroidea...

 (voles and lemmings), are also noted for their unusual sex determination systems. The platypus has ten sex chromosomes; males have an XYXYXYXYXY pattern while females have ten X chromosomes. Although it is an XY system, the platypus' sex chromosomes share no homologues with eutherian sex chromosomes. Instead, homologues with eutherian sex chromosomes lie on the platypus chromosome 6, which means that the eutherian sex chromosomes were autosomes at the time that the monotremes diverged from the therian mammals (marsupials and eutherian mammals). However, homologues to the avian DMRT1 gene on platypus sex chromosomes X3 and X5 suggest that it is possible the sex-determining gene for the platypus is the same one that is involved in bird sex-determination. However, more research must be conducted in order to determine the exact sex determining gene of the platypus.

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
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

s, including the grasshoppers and crickets of order Orthoptera
Orthoptera is an order of insects with paurometabolous or incomplete metamorphosis, including the grasshoppers, crickets 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
Cockroaches are insects of the order Blattaria or Blattodea, of which about 30 species out of 4,500 total are associated with human habitations...


The nematode
The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...

 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 and developmental biology of C. elegans was begun in 1974 by Sydney Brenner and it has since been used extensively as a model...

is male with one sex chromosome (X0); with a pair of chromosomes (XX) it is a hermaphrodite
In biology, a hermaphrodite is an organism that has reproductive organs normally associated with both male and female sexes.Many taxonomic groups of animals do not have separate sexes. In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which both...


A small number of mammals also lack a Y chromosome. These include the Amami spiny rat (Tokudaia osimensis) and the Tokunoshima spiny rat (Tokudaia tokunoshimensis). Voles also have a form of XO determination in which both genders lack a second sex chromosome. The mechanism of sex determination is not yet understood.

ZW sex chromosomes

The ZW sex-determination system is found in bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s, 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). In the chicken, this was found to be dependent on the expression of DMRT1. However, not all species depend upon the W for their gender. For example, there are moths and butterflies that are ZW, but some have been found female with ZO, as well as female with ZZW. Also, while mammals inactivate one of their extra X chromosomes when female, it appears that in the case of Lepidoptera
Lepidoptera is a large order of insects that includes moths and butterflies . It is one of the most widespread and widely recognizable insect orders in the world, encompassing moths and the three superfamilies of butterflies, skipper butterflies, and moth-butterflies...

, the males produce double the normal amount of enzymes, due to having two Z's. Because the use of ZW sex determination is varied, it is still unknown how exactly most species determine their sex. Despite the similarities between ZW and XY, the sex chromosomes do not line up correctly and evolved separately. In the case of the chicken, their Z chromosome is more similar to humans' autosome 9. The chicken's Z chromosome also seems to match with the X chromosomes of the platypus.


Haplodiploidy is found in insects belonging to Hymenoptera
Hymenoptera is one of the largest orders of insects, comprising the sawflies, wasps, bees and ants. There are over 130,000 recognized species, with many more remaining to be described. The name refers to the heavy wings of the insects, and is derived from the Ancient Greek ὑμήν : membrane and...

, such as ant
Ants are social insects of the family Formicidae and, along with the related wasps and bees, belong to the order Hymenoptera. Ants evolved from wasp-like ancestors in the mid-Cretaceous period between 110 and 130 million years ago and diversified after the rise of flowering plants. More than...

s and bee
Bees are flying insects closely related to wasps and ants, and are known for their role in pollination and for producing honey and beeswax. Bees are a monophyletic 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 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 is a term used for the highest level of social organization in a hierarchical classification....

, as it increases the significance of kin selection
Kin selection
Kin selection refers to apparent strategies in evolution that favor the reproductive success of an organism's relatives, even at a cost to the organism's own survival and reproduction. Charles Darwin was the first to discuss the concept of group/kin selection...

, but it is debated. Most females in the Hymenoptera order can decide the sex of their offspring by holding received sperm in their spermatheca
The spermatheca , also called receptaculum seminis , is an organ of the female reproductive tract in insects, some molluscs, oligochaeta worms and certain other invertebrates and vertebrates...

 and either releasing it into their oviduct or not. This allows them to create more workers, depending on the status of the colony.

Non-genetic sex-determination systems

Many other sex-determination systems exist. In some species of reptiles, including alligator
An alligator is a crocodilian in the genus Alligator of the family Alligatoridae. There are two extant alligator species: the American alligator and the Chinese alligator ....

s, some turtle
Turtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...

s, and the tuatara
The tuatara is a reptile endemic to New Zealand which, though it resembles most lizards, is actually part of a distinct lineage, order Sphenodontia. The two species of tuatara are the only surviving members of its order, which flourished around 200 million years ago. Their most recent common...

, sex is determined by the temperature at which the egg is incubated. Some species, such as some snail
Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...

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, and bluehead wrasses (Thalassoma bifasciatum) are the reverse. In the marine worm Bonellia viridis
Bonellia viridis
The Green Spoonworm is a marine worm noted for displaying exceptional sexual dimorphism and for the biocidal properties of a pigment in its skin....

, larvae become males if they make physical contact with a female, and females if they end up on the bare sea floor.

Some species, however, have no sex-determination system. Hermaphrodites include the common earthworm and certain species of snails. A few species of fish, reptiles, and insects reproduce by parthenogenesis
Parthenogenesis is a form of asexual reproduction found in females, where growth and development of embryos occur without fertilization by a male...

 and are female altogether.

In some arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...

s, sex is determined by infection, as when Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 of the genus Wolbachia
Wolbachia is a genus of bacteria which infects arthropod species, including a high proportion of insects , as well as some nematodes. It is one of the world's most common parasitic microbes and is possibly 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 is still being researched):
  • Swordtail fish
  • The Chironomus
    Chironomidae are a family of nematoceran flies with a global distribution. They are closely related to the Ceratopogonidae, Simuliidae, and Thaumaleidae...

     midge species
  • The Platypus
    The platypus is a semi-aquatic mammal endemic to eastern 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 eggs instead of giving birth to live young...

     has 10 sex chromosomes but lacks the mammalian sex-determining gene SRY, meaning that the process of sex determination in the Platypus remains unknown.

See also

  • Clarence Erwin McClung who discovered the role of chromosomes in sex determination.
  • Testis-determining factor
  • Barr body
    Barr body
    A Barr body is the inactive X chromosome in a female somatic cell, rendered inactive in a process called lyonization, in those species in which sex is determined by the presence of the Y or W chromosome rather than the diploidy of the X or Z...

  • Maternal influence on sex determination
    Maternal influence on sex determination
    In humans and several other species of animals, the father determines the sex of the child. In the XY sex-determination system, the female-provided ovum contributes an X chromosome and the male-provided sperm contributes either an X chromosome or a Y chromosome, resulting in female or male ...

  • 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.-The Jost Paradigm:Under typical circumstances, the sex of an individual will be determined and expressed through the following mechanisms:...

    • 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; flowers are the reproductive organs of flowering plants, cones are the reproductive...

      , or primary sexual characteristic
    • Secondary sex characteristic
      Secondary sex characteristic
      Secondary sex characteristics are features that distinguish the two sexes of a species, but that are not directly part of the reproductive system. They are believed to be the product of sexual selection for traits which give an individual an advantage over its rivals in courtship and aggressive...

    • Shettles Method
      Shettles Method
      The Shettles Method is a child conception idea that is reputed to help determine a baby's 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, giving a total of 47 chromosomes instead of the more usual 46. This produces a 47,XYY karyotype...

Further reading

Sex chromosomes and sex determination in amniotes

Sex determination in insects

Sex determination in plants

External links

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