XY sex-determination system
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The XY sex-determination system is a
In humans, the presence of the Y chromosome is responsible for triggering male development; in the absence of the Y chromosome, the fetus will undergo female development. There are various exceptions, such as individuals with
The XY system contrasts in several ways with the
A temperature-dependent sex determination system is found in some reptiles and fish.
Mechanisms
All animals have a set of DNA coding for genes present on chromosomes. In humans, most mammals, and some other species, two of the chromosomes, called the X chromosome and Y chromosome, code for sex. In these species, one or more genes are present on their Y chromosome that determine maleness. In this process, an X chromosome and a Y chromosome act to determine the sex of offspring, often due to genes located on the Y chromosome that code for maleness. Offspring have two sex chromosomes: an offspring with two X chromosomes (XX) will develop female characteristics, and an offspring with an X and a Y chromosome (XY) will develop male characteristics.
Mammals
In most mammals, sex is determined by presence of the Y chromosome. This makes individuals with XXY and XYY karyotypes males, and individuals with X and XXX karyotypes females.[1]
In the 1930s,
SRY is a sex-determining gene on the Y chromosome in the therians (placental mammals and marsupials).[8] Non-human mammals use several genes on the Y chromosome.[citation needed]
Not all male-specific genes are located on the Y chromosome. The platypus, a monotreme, use five pairs of different XY chromosomes with six groups of male-linked genes, AMH being the master switch.[9]
Humans
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A single gene (
Other animals
Some species of turtles have convergently evolved XY sex determination systems, specifically those in Chelidae and Staurotypinae.[11]
Other species (including most Drosophila species) use the presence of two X chromosomes to determine femaleness: one X chromosome gives putative maleness, but the presence of Y chromosome genes is required for normal male development. In the fruit fly individuals with XY are male and individuals with XX are female; however, individuals with XXY or XXX can also be female, and individuals with X can be males.[12]
Plants
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Very few dioecious angiosperm species have XY sex determination,[13] such as the Silene latifolia.[14] In these species sex determination is similar to mammals where male is XY and female is XX.[15]
Other systems
Whilst XY sex determination is the most familiar, since it is the system that humans use, there are a range of alternative systems found in nature. The inverse of the XY system (called ZW to distinguish it) is used in birds and many insects, in which it is the females that are heterogametic (ZW), while males are homogametic (ZZ).[16]
Many insects of the order
Influences
Genetic
In an interview for the Rediscovering Biology website,[18] researcher Eric Vilain described how the paradigm changed since the discovery of the SRY gene:
For a long time we thought that SRY would activate a cascade of male genes. It turns out that the sex determination pathway is probably more complicated and SRY may in fact inhibit some anti-male genes.
The idea is instead of having a simplistic mechanism by which you have pro-male genes going all the way to make a male, in fact there is a solid balance between pro-male genes and anti-male genes and if there is a little too much of anti-male genes, there may be a female born and if there is a little too much of pro-male genes then there will be a male born.
We [are] entering this new era in molecular biology of sex determination where it's a more subtle dosage of genes, some pro-males, some pro-females, some anti-males, some anti-females that all interplay with each other rather than a simple linear pathway of genes going one after the other, which makes it very fascinating but very complicated to study.
In an interview by Scientific American in 2007, Vilian was asked: "It sounds as if you are describing a shift from the prevailing view that female development is a default molecular pathway to active pro-male and antimale pathways. Are there also pro-female and antifemale pathways?"[19] He replied:
Modern sex determination started at the end of the 1940s—1947—when the French physiologist Alfred Jost said it's the testis that is determining sex. Having a testis determines maleness, not having a testis determines femaleness. The ovary is not sex-determining. It will not influence the development of the external genitalia. Now in 1959 when the karyotype of Klinefelter [a male who is XXY] and Turner [a female who has one X] syndromes was discovered, it became clear that in humans it was the presence or the absence of the Y chromosome that's sex determining. Because all Klinefelters that have a Y are male, whereas Turners, who have no Y, are females. So it's not a dosage or the number of X's, it's really the presence or absence of the Y. So if you combine those two paradigms, you end up having a molecular basis that's likely to be a factor, a gene, that's a testis-determining factor, and that's the sex-determining gene. So the field based on that is really oriented towards finding testis-determining factors. What we discovered, though, was not just pro-testis determining factors. There are a number of factors that are there, like WNT4, like DAX1, whose function is to counterbalance the male pathway.
In mammals, including humans, the SRY gene triggers the development of non-differentiated
We take it for granted that we maintain the sex we are born with, including whether we have testes or ovaries. But this work shows that the activity of a single gene, FOXL2, is all that prevents adult ovary cells turning into cells found in testes.
Implications
Looking into the genetic determinants of human sex can have wide-ranging consequences. Scientists have been studying different sex determination systems in fruit flies and animal models to attempt an understanding of how the genetics of sexual differentiation can influence biological processes like reproduction, ageing[22] and disease.
Maternal
In humans and many other species of animals, the
Hormone levels in the male parent affect the sex ratio of sperm in humans.[23] Maternal influences also impact which sperm are more likely to achieve conception.
Human ova, like those of other mammals, are covered with a thick translucent layer called the
Recent research indicates that human ova may produce a chemical which appears to attract sperm and influence their swimming motion. However, not all sperm are positively impacted; some appear to remain uninfluenced and some actually move away from the egg.[25]
Maternal influences may also be possible that affect sex determination in such a way as to produce
The time at which insemination occurs during the
Sex-specific mortality of embryos also occurs.[23]
History
Ancient ideas on sex determination
Aristotle believed incorrectly that the sex of an infant is determined by how much heat a man's sperm had during insemination. He wrote:
... the semen of the male differs from the corresponding secretion of the female in that it contains a principle within itself of such a kind as to set up movements also in the embryo and to concoct thoroughly the ultimate nourishment, whereas the secretion of the female contains material alone. If, then, the male element prevails it draws the female element into itself, but if it is prevailed over it changes into the opposite or is destroyed.
Aristotle claimed in error that the male principle was the driver behind sex determination,[27] such that if the male principle was insufficiently expressed during reproduction, the fetus would develop as a female.
20th century genetics
The first clues to the existence of a factor that determines the development of testis in mammals came from experiments carried out by Alfred Jost,[32] who castrated embryonic rabbits in utero and noticed that they all acquired a female phenotype.[33]
In 1959, C. E. Ford and his team, in the wake of Jost's experiments, discovered
All these observations led to a consensus that a dominant gene that determines testis development (
See also
- Sexual differentiation (human)
- Secondary sex characteristic (human)
- Y-chromosomal Adam
- Sex Determination in Silene
- Sex-determination system
- Haplodiploid sex-determination system
- Z0 sex-determination system
- ZW sex-determination system
- Temperature-dependent sex determination
- X chromosome
- Y chromosome
References
- ^ a b Hake, Laura; O'Connor, Clare (2008). "Genetic Mechanisms of Sex Determination | Learn Science at Scitable". Nature. Archived from the original on 2021-04-28. Retrieved 2021-04-13.
- ^ Callaway, Ewen (9 April 2009). "Girl with Y chromosome sheds light on maleness". New Scientist. Retrieved 2023-02-22.
- ^ Sherwood, Susan. "Can a Zygote Survive Without an X Sex Chromosome?". Education - Seattle PI. Retrieved 2020-11-08.
- ^ Sherwood, Susan (April 25, 2017). "What Occurs When the Zygote Has One Fewer Chromosome than Usual?". Sciencing. Retrieved 2021-04-29.
- (PDF) from the original on Nov 26, 2023 – via e-Publications@Marquettee.
- ^ Olena, Abby (July 6, 2017). "Snake Sex Determination Dogma Overturned". The Scientist. Archived from the original on Dec 8, 2023.
- PMID 4399057.
- S2CID 31675679.
- (PDF) from the original on Aug 10, 2017.
- ISBN 978-0-07-147693-5.
- S2CID 14434440.
- ISBN 978-1-108-49985-9.
- ISBN 978-3-940344-23-6.
- PMID 19704689.
- ISBN 978-3-319-31703-8.
- PMID 14745830.
- ^ "5 Types of Sex Determination in Animals". genetics.knoji.com. Archived from the original on 5 February 2017. Retrieved 3 May 2018.
- ^ Rediscovering Biology, Unit 11 - Biology of Sex and Gender, Expert interview transcripts, Link Archived 2010-08-23 at the Wayback Machine
- ^ Lehrman, Sally. "When a Person Is Neither XX nor XY: A Q&A with Geneticist Eric Vilain". Scientific American. Retrieved 2021-08-08.
- ^
Uhlenhaut, N. Henriette; et al. (2009). "Somatic Sex Reprogramming of Adult Ovaries to Testes by FOXL2 Ablation". Cell. 139 (6): 1130–42. PMID 20005806.
- ^ Scientists find single 'on-off' gene that can change gender traits Archived 2011-08-14 at the Wayback Machine, Hannah Devlin, The Times, December 11, 2009.
- ^
Tower, John; Arbeitman, Michelle (2009). "The genetics of gender and life span". Journal of Biology. 8 (4): 38. PMID 19439039.
- ^ S2CID 27957961.
- ^ Suzanne Wymelenberg, Science and Babies, National Academy Press, 1990, page 17
- ^ Richard E. Jones and Kristin H. Lopez, Human Reproductive Biology, Third Edition, Elsevier, 2006, page 238
- ^ Familial recurrence of gender-balanced twins Archived October 2, 2015, at the Wayback Machine
- ^ De Generatione Animalium, 766B 15‑17.
- S2CID 1919033.
- ^ "Specialized chromosomes determine sex. - Nettie Maria Stevens". DNA from the Beginning. Archived from the original on 2012-10-01. Retrieved 2016-07-07.
- ^ John L. Heilbron (ed.), The Oxford Companion to the History of Modern Science, Oxford University Press, 2003, "genetics".
- ^ Glass, Bentley (1990) Theophilus Shickel Painter 1889—1969: A Biographical Memoir, National Academy of Sciences, Washington DC. Retrieved 24 Jan 2022.
- ^ Jost A., Recherches sur la differenciation sexuelle de l'embryon de lapin, Archives d'anatomie microscopique et de morphologie experimentale, 36: 271 – 315, 1947.
- PMC 6791057. Retrieved 2024-04-12.
- PMID 13642858.
- S2CID 38349997.
- ^ ISBN 9780443068119.
- S2CID 4364032.
External links
- Sex Determination and Differentiation
- Can Mammalian Mothers Control the Sex of their Offspring? (KQED Science article on San Diego Zoo research.)
- Maternal Diet and Other Factors Affecting Offspring Sex Ratio: A Review, published in Biology of Reproduction
- Sex Determination and the Maternal Dominance Hypothesis
- Sperm-Ovum Interactions at WikiGenes