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Sex-determination system

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❶If the egg receives another X chromosome from the sperm, the resulting individual is XX, forms ovaries, and is female; if the egg receives a Y chromosome from the sperm, the individual is XY, forms testes, and is male. This domain is found in several transcription factors and nonhistone chromatin proteins, and it induces bending in the region of DNA to which it binds Figure

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Primary and secondary sex determination
Developmental Biology. 6th edition.
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A Polymerase chain reaction followed by electrophoresis shows the presence of the Sry gene in normal XY males and in a transgenic XX Sry mouse. The gene is absent in a female XX littermate. B The external genitalia more When the Sry protein binds to its sites on DNA, it probably creates large conformational changes.

It unwinds the double helix in its vicinity and bends the DNA as much as 80 degrees Pontiggia et al. This bending may bring distantly bound proteins of the transcription apparatus into close contact, enabling them to interact and influence transcription.

The identities of these proteins are not yet known, but they, too, are needed for testis determination. SRY may have more than one mode of action in converting the bipotential gonads into testes. It had been assumed for the past decade that SRY worked directly in the genital ridge to convert the epithelium into male-specific Sertoli cells.

Recent studies Capel et al. SRY in the genital ridge cells induces the cells to secrete a chemotactic factor that permits the migration of mesonephric cells into the XY gonad. These mesonephric cells induce the gonadal epithelium to become Sertoli cells with male-specific gene expression patterns.

The researchers found that when they cultured XX gonads with either XX or XY mesonephrons, the mesonephric cells did not enter the gonads.

There was a strict correlation between the presence of Sry in the gonadal cells, mesonephric cell migration, and the formation of testis cords.

Tilmann and Capel showed that mesonephric cells are critical for testis cord formation and that the migrating mesonephric cells can induce XX gonadal cells to form testis cords. It appears, then, that Sry may function indirectly to create testes by inducing mesonephric cell migration into the gonad.

In the experiment diagrammed, urogenital ridges containing both the mesonephric kidneys and gonadal rudiments were collected from day embryonic mice. One of the mice was marked with more The mapping of the testis-determining factor to the SRY region took scientists more than 50 years to accomplish.

Moreover, other testis-forming genes have been found on the autosomes. One of the autosomal genes involved in sex determination is SOX9 , which encodes a putative transcription factor that also contains an HMG box. Individuals having only one functional copy of this gene have a syndrome called campomelic dysplasia, a disease involving numerous skeletal and organ systems. It appears that SOX9 is essential for testis formation. The mouse homologue of this gene, Sox9, is expressed only in male XY but not in female XX genital ridges.

Moreover, Sox9 expression is seen in the same genital ridge cells as Sry, and it is expressed just slightly after Sry expression Wright et al. The Sox9 protein binds to a promoter site on the Amh gene, providing a critical link in the pathway toward a male phenotype Figure Synergism of Sox9 and Sf1 to activate the expression of the Amh gene.

A The binding of Sox9 to the Amh promoter initiates transcription of the Amh gene in the Sertoli cells. While Sry is found specifically in mammals, Sox9 is found throughout the vertebrates. Sox9 may be the older and more central sex determination gene, although in mammals it became activated by its relative, Sry. Another protein that may be directly or indirectly activated by SRY is the transcription factor SF1 s teroidogenic f actor 1. Sf1 is necessary to make the bipotential gonad; but while Sf1 levels decline in the genital ridge of XX mouse embryos, the Sf1 gene stays on in the developing testis.

Sf1 appears to be active in masculinizing both the Leydig and the Sertoli cells. In the Leydig cells, Sf1 activates the genes encoding the enzymes that make testosterone. In , Bernstein and her colleagues reported two sisters who were genetically XY. Their Y chromosomes were normal, but they had a duplication of a small portion of the short arm of the X chromosome. Subsequent cases were found, and it was concluded that if there were two copies of this region on the active X chromosome, the SRY signal would be reversed Figure Bardoni and her colleagues proposed that this region contains a gene for a protein that competes with the SRY factor and that is important in directing the development of the ovary.

In testicular development, this gene would be suppressed, but having two active copies of the gene would override this suppression. This gene, DAX1 , has been cloned and shown to encode a member of the nuclear hormone receptor family Muscatelli et al.

Dax1 is expressed in the genital ridges of the mouse embryo, shortly after Sry expression. Indeed, in XY mice, Sry and Dax1 are expressed in the same cells. Thus, DAX1 is probably a gene that is involved in ovary determination. Phenotypic sex reversal in humans having two copies of the DAX1 locus. The WNT4 gene is another gene that may be critical in ovary determination.

This gene is expressed in the mouse genital ridge while it is still in its bipotential stage. Wnt4 expression then becomes undetectable in XY gonads which become testes , whereas it is maintained in XX gonads as they begin to form ovaries.

In transgenic XX mice that lack the Wnt4 genes, the ovary fails to form properly, and its cells express testis-specific markers, including AMH- and testosterone-producing enzymes Vainio et al.

Sry may form testes by repressing Wnt4 expression in the genital ridge, as well as by promoting Sf1. One possible model is shown in Figure Possible mechanism for primary sex determination in mammals. While we do not know the specific interactions involved, this model attempts to organize the data into a coherent sequence. Other models are possible. It should be realized that both testis and ovary development are active processes.

Although remarkable progress has been made in recent years, we still do not know what the testis- or ovary-determining genes are doing, and the problem of primary sex determination remains as it has since prehistory one of the great unsolved problems of biology.

Primary sex determination involves the formation of either an ovary or a testis from the bipotential gonad. This, however, does not give the complete sexual phenotype. Secondary sex determination in mammals involves the development of the female and male phenotypes in response to hormones secreted by the ovaries and testes.

Both female and male secondary sex determination have two major temporal phases. The first occurs within the embryo during organogenesis; the second occurs during adolescence. As mentioned earlier, if the bipotential gonads are removed from an embryonic mammal, the female phenotype is realized: This pattern also is seen in certain humans who are born without functional gonads.

Individuals whose cells have only one X chromosome and no Y chromosome originally develop ovaries, but these ovaries atrophy before birth, and the germ cells die before puberty. However, under the influence of estrogen, derived first from the ovary but then from the mother and placenta, these infants are born with a female genital tract Langman and Wilson The formation of the male phenotype involves the secretion of two testicular hormones.

The second is the steroid testosterone, which is secreted from the fetal Leydig cells. This hormone causes the Wolffian duct to differentiate into the epididymis, vas deferens, and seminal vesicles, and it causes the urogenital swellings to develop into the scrotum and penis.

The existence of these two independent systems of masculinization is demonstrated by people having androgen insensitivity syndrome. However, they lack the testosterone receptor protein, and therefore cannot respond to the testosterone made by their testes Meyer et al.

Because they are able to respond to estrogen made in their adrenal glands, they develop the female phenotype Figure However, despite their distinctly female appearance, these individuals do have testes, and even though they cannot respond to testosterone, they produce and respond to AMH. An XY individual with androgen insensitivity syndrome. Despite the XY karyotype and the presence of testes, such individuals develop female secondary sex characteristics.

Although testosterone is one of the two primary masculinizing hormones, there is evidence that it might not be the active masculinizing hormone in certain tissues. Testosterone appears to be responsible for promoting the formation of the male reproductive structures the epididymis, seminal vesicles, and vas deferens that develop from the Wolffian duct primordium.

However, it does not directly masculinize the male urethra, prostate, penis, or scrotum. These latter functions are controlled by 5a-dihydrotestosterone Figure Testosterone- and dihydrotestosterone-dependent regions of the human male genital system.

After Imperato-McGinley et al. These individuals lack a functional gene for this enzyme Andersson et al. Although XY children with this syndrome have functioning testes, they have a blind vaginal pouch and an enlarged clitoris. They appear to be girls and are raised as such. Their internal anatomy, however, is male: Thus, it appears that the formation of the external genitalia is under the control of dihydrotestosterone, whereas Wolffian duct differentiation is controlled by testosterone itself.

Interestingly, when the testes of these children produce more testosterone at puberty, the external genitalia are able to respond to the higher levels of the hormone, and they differentiate. The penis enlarges, the scrotum descends, and the person originally thought to be a girl is shown to be a young man. The drug finasteride, which inhibits the conversion of testosterone to dihydrotestosterone, is being used to treat prostate growth and male pattern baldness.

In addition to testosterone, the Leydig cells secrete another hormone, insulin-like hormone 3 Insl3. This hormone is required for the descent of the gonads into the scrotum. Males lacking this hormone are infertile because the testes do not descend. In females, lack of this hormone deregulates the menstrual cycle. Assay for AMH activity in the anterior segment of a B After 3 days in culture with more It may cause sex reversal in some mammals, and may become useful as an anti-tumor drug.

In mice, DES can cause the oviduct epithelium to take on the appearance of the uterus, and the uterine epithelium to resemble that of the cervix Ma et al. In males, estrogen is actually needed for fertility. This concentrates the sperm, giving them a longer lifespan and providing more sperm per ejaculate.

This absorption of water is regulated by estrogen. If estrogen or its receptor is absent in mice, this water is not absorbed, and the mouse is sterile Hess et al. While blood concentrations of estrogen are higher in females than in males, the concentration of estrogen in the rete testis is even higher than that in female blood.

Breast tissue has a sexually dimorphic mode of development. Testosterone inhibits breast development, while estrogen promotes it. Most breast development is accomplished after birth, and different hormones act during puberty and pregnancy to cause breast enlargement and differentiation. Diethylstilbesterol was a drug given to women to ease their pregnancies. Unfortunately, it was later found to alter the reproductive tract of female fetuses. Androgen insensitivity syndrome is one of several conditions called pseudohermaphroditism.

In a pseudohermaphrodite, there is only one type of gonad, but the secondary sex characteristics differ from what would be expected from the gonadal sex. In humans, male pseudohermaphroditism can be caused by mutations in the androgen receptor or by mutations affecting testosterone synthesis Geissler et al.

Female pseudohermaphroditism can be caused by an overproduction of testosterone. True hermaphrodites rare in humans, but the norm in some invertebrates such as nematodes and earthworms contain both male and female gonadal tissue.

Mammalian true hermaphrodites result from abnormalities of primary sex determination. Such abnormalities can occur when the Y chromosome is translocated to the X chromosome.

In those tissues where the translocated X chromosome is inactivated during dosage compensation, the SRY gene will be turned off. Some species such as various flowers and fish do not have a fixed sex, and instead go through life cycles and change sex based on genetic cues during corresponding life stages of their type. This could be due to environmental factors such as seasons and temperature.

Human fetus genitals can sometimes develop abnormalities during maternal pregnancies due to mutations in the fetuses sex-determinism system, resulting in the fetus becoming intersex.

Sex determination was discovered in the mealworm by the American geneticist Nettie Stevens in In this system, most females have two of the same kind of sex chromosome XX , while most males have two distinct sex chromosomes XY. The X and Y sex chromosomes are different in shape and size from each other, unlike the rest of the chromosomes autosomes , and are sometimes called allosomes. In some species, such as humans, organisms remain sex indifferent for a time after they're created; in others, however, such as fruit flies, sexual differentiation occurs as soon as the egg is fertilized.

Some species including humans have a gene SRY on the Y chromosome that determines maleness. In Y-centered sex determination, the SRY gene is the main gene in determining male characteristics, but multiple genes are required to develop testes.

In XY mice, lack of the gene DAX1 on the X chromosome results in sterility, but in humans it causes adrenal hypoplasia congenita. Some species, such as fruit flies , use the presence of two X chromosomes to determine femaleness. Some fish have variants of the XY sex-determination system , as well as the regular system. For example, while having an XY format, Xiphophorus nezahualcoyotl and X. At least one monotreme , the platypus , presents a particular sex determination scheme that in some ways resembles that of the ZW sex chromosomes of birds and lacks the SRY gene.

Although it is an XY system, the platypus' sex chromosomes share no homologues with eutherian sex chromosomes. 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.

More research must be conducted in order to determine the exact sex determining gene of the platypus. 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. Generally in this method, the sex is determined by amount of genes expressed across the two chromosomes. This system is observed in a number of insects, including the grasshoppers and crickets of order Orthoptera and in cockroaches order Blattodea.

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 and Sorex araneus , a shrew species. Transcaucasian mole voles Ellobius lutescens also have a form of XO determination, in which both sexes lack a second sex chromosome.

These genes reduce male gene activation and increase it, respectively. The ZW sex-determination system is found in birds, some reptiles, and some insects and other organisms. The ZW sex-determination system is reversed compared to the XY system: In the chicken, this was found to be dependent on the expression of DMRT1. In the case of the chicken, their Z chromosome is more similar to humans' autosome 9. This is due to the fact that the haploid eggs double their chromosomes, resulting in ZZ or WW.

The ZZ become males, but the WW are not viable and are not brought to term. In some Bryophyte and some algae species, the gametophyte stage of the life cycle, rather than being hermaphrodite, occurs as separate male or female individuals that produce male and female gametes respectively.

When meiosis occurs in the sporophyte generation of the life cycle, the sex chromosomes known as U and V assort in spores that carry either the U chromosome and give rise to female gametophytes, or the V chromosome and give rise to male gametophytes.

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. Males cannot have sons or fathers. This may be significant for the development of eusociality , as it increases the significance of kin selection , but it is debated. This allows them to create more workers, depending on the status of the colony. Many other 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 during a temperature-sensitive period. There are no examples of temperature-dependent sex determination TSD in birds. Megapodes had formerly been thought to exhibit this phenomenon, but were found to actually have different temperature-dependent embryo mortality rates for each sex. The specific temperatures required to produce each sex are known as the female-promoting temperature and the male-promoting temperature.

It is unknown how exactly temperature-dependent sex determination evolved. For example, a warmer area could be more suitable for nesting, so more females are produced to increase the amount that nest next season.

There are other environmental sex determination systems including location-dependent determination systems as seen in the marine worm Bonellia viridis — larvae become males if they make physical contact with a female, and females if they end up on the bare sea floor. This is triggered by the presence of a chemical produced by the females, bonellin. 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.

Some species, however, have no sex-determination system. Hermaphrodite species include the common earthworm and certain species of snails. A few species of fish, reptiles, and insects reproduce by parthenogenesis and are female altogether. There are some reptiles, such as the boa constrictor and Komodo dragon that can reproduce both sexually and asexually, depending on whether a mate is available. Other unusual systems include those of the swordtail fish [ clarification needed ] ; [11] the Chironomus midges [ clarification needed ] [ citation needed ] ; the platypus , which has 10 sex chromosomes [12] but lacks the mammalian sex-determining gene SRY, meaning that the process of sex determination in the platypus remains unknown; [13] the juvenile hermaphroditism of zebrafish , with an unknown trigger; [11] and the platyfish , which has W, X, and Y chromosomes.

The accepted hypothesis of XY and ZW sex chromosome evolution is that they evolved at the same time, in two different branches. All sex chromosomes started out as an original autosome of an original amniote that relied upon temperature to determine the sex of offspring.

After the mammals separated, the branch further split into Lepidosauria and Archosauromorpha.

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Any mechanism of sex determination (q.v.) in which a genetic factor (such as the nature of the sex chromosomes in the fertilized egg or the X:A ratio (q.v.) of the embryo) is the primary sex-determining signal. Also called genetic sex determination. Compare with environmental sex determination.

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Genotypic sex determination any mechanism of sex determination which a factor such as the genotypic sex determination temperature dependent sex determination nature of the sex chromosomes in the fertilized egg or the genotypic sex determination definition .

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Genotypic sex determination definition any mechanism of sex determination which a factor such as the nature sex determination in mammals of the genotypic sex sex determination determination definition sex chromosomes in the fertilized egg or the x a ratio anybody. Primary sex determination is the determination of the gonads. In mammals, primary sex determination is strictly chromosomal and is not usually influenced by the environment. In most cases, the female is XX and the male is XY. Every individual must have at least one X chromosome. Since the female is XX, each of her eggs has a single X chromosome.