Sex chromosome
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Sex chromosomes (also referred to as allosomes, heterotypical chromosome, gonosomes, heterochromosomes,[1][2] or idiochromosomes[1]) are chromosomes that carry the genes that determine the sex of an individual. The human sex chromosomes are a typical pair of
Nettie Stevens and Edmund Beecher Wilson both independently discovered sex chromosomes in 1905. However, Stevens is credited for discovering them earlier than Wilson.[3]
Differentiation
In humans, each
An X chromosome is always present as the 23rd chromosome in the
Sex determination
All diploid organisms with allosome-determined sex get half of their allosomes from each of their parents. In most mammals, females are XX, and can pass along either of their Xs; since males are XY they can pass along either an X or a Y. Females in such species receive an X chromosome from each parent while males receive an X chromosome from their mother and a Y chromosome from their father. It is thus the male's sperm that determines the sex of each offspring in such species.
However, a small percentage of humans have a divergent sexual development, known as intersex. This can result from allosomes that are neither XX nor XY. It can also occur when two fertilized embryo fuse, producing a chimera that might contain two different sets of DNA one XX and the other XY.[citation needed] It could also result from exposure, often in utero, to chemicals that disrupt the normal conversion of the allosomes into sex hormones and further into the development of either ambiguous outer genitalia or internal organs.[6]
There is a gene in the Y chromosome that has regulatory sequences that control genes that code for maleness, called the
Other vertebrates
Diverse mechanisms are involved in the determination of sex in animals.[9] For mammals, sex determination is carried by the genetic contribution of the spermatozoon. Many lower chordates, such as fish, amphibians and reptiles, have systems that are influenced by the environment. Fish and amphibians, for example, have genetic sex determination but their sex can also be influenced by externally available steroids and incubation temperature of eggs.[10][11] In some reptiles, e.g. sea turtles, only the incubation temperature determines sex (temperature-dependent sex determination).
Plants
Many scientists argue that sex determination in plants is more complex than that in humans. This is because even flowering plants have a variety of mating systems, their sex determination primarily regulated by MADS-box genes. These genes code for proteins that form the sex organs in flowers.[12]
Plant sex chromosomes are most common in
The high prevalence of
Sequence composition and evolution
Amplification of transposable elements,
Non-vascular plants
Ferns and lycophytes have bisexual gametophytes, so there is no evidence for sex chromosomes.[13] In the bryophytes, including liverworts, hornworts and mosses, sex chromosomes are common. The sex chromosomes in bryophytes affect what type of gamete is produced by the gametophyte, and there is wide diversity in gametophyte type. Unlike seed plants, where gametophytes are always unisexual, in bryophytes they may produce male, female, or both types of gamete.[19]
Bryophytes most commonly employ a UV sex-determination system, where U produces female gametophytes and V produces male gametophytes. The U and V chromosomes are heteromorphic with U larger than V and are frequently both larger than the autosomes. There is variation even within this system, including UU/V and U/VV chromosome arrangements. In some bryophytes, microchromosomes have been found to co-occur with sex chromosomes and likely impact sex determination.[19]
Gymnosperms
Dioecy is common among
Angiosperms
Cosexual
In the domesticated papaya (
Medical applications
Allosomes not only carry the genes that determine male and female traits, but also those for some other characteristics as well. Genes that are carried by either sex chromosome are said to be sex linked. Sex linked diseases are passed down through families through one of the X or Y chromosomes. Since usually men inherit Y chromosomes, they are the only ones to inherit Y-linked traits. Men and women can get the X-linked ones since both inherit X chromosomes.[21]
An allele is either said to be
- Color blindness or color vision deficiency is the inability or decreased ability to see color, or perceive color differences, under normal lighting conditions. Color blindness affects many individuals in the population. There is no actual blindness, but there is a deficiency of color vision. The most usual cause is a fault in the development of one or more sets of retinal cones that perceive color in light and transmit that information to the optic nerve. This type of color blindness is usually a sex-linked condition. The genes that produce photopigments are carried on the X chromosome; if some of these genes are missing or damaged, color blindness will be expressed in males with a higher probability than in females because males only have one X chromosome.
- Hemophilia refers to a group of bleeding disorders in which it takes a long time for the blood to clot. This is referred to as X-Linked recessive.[24]Hemophilia is much more common in males than females because males are hemizygous. They only have one copy of the gene in question and therefore express the trait when they inherit one mutant allele. In contrast, a female must inherit two mutant alleles, a less frequent event since the mutant allele is rare in the population. X-linked traits are maternally inherited from carrier mothers or from an affected father. Each son born to a carrier mother has a 50% probability of inheriting the X chromosome carrying the mutant allele.
- Fragile X syndrome is a genetic condition involving changes in part of the X chromosome. It is the most common form of inherited intellectual disability (mental retardation) in males. It is caused by a change in a gene called FMR1. A small part of the gene code is repeated on a fragile area of the X chromosome. The more repeats, the more likely there is to be a problem. Males and females can both be affected, but because males have only one X chromosome, a single fragile X is likely to affect them more. Most fragile-X males have large testes, big ears, narrow faces, and sensory processing disorders that result in learning disabilities.[25]
Other complications include:
- 46,XX testicular disorder of sex development, also called XX male syndrome, is a condition in which individuals with two X chromosomes in each cell, the pattern normally found in females, have a male appearance. People with this disorder have male external genitalia. In most people with 46,XX testicular disorder of sex development, the condition results from an exchange of genetic material between chromosomes (translocation). This exchange occurs as a random event during the formation of sperm cells in the affected person's father. The SRY gene (normally on the Y chromosome) is misplaced in this disorder, onto an X chromosome. Any person with an X chromosome that carries the SRY gene will develop male characteristics despite not having a Y chromosome.[26]
Evolution
Sex chromosomes evolve from standard pairs of autosomal chromosomes.[27] In a large number of organisms, the sex-determination systems presently observed are products of sex chromosome turnover. Sex chromosome turnover is a process defined as when the type of the sex chromosome changes as a product of a change in the identity of the sex-determining genes (such as by mutation) or by a change in their location.[28] In other cases, sex chromosomes may grow substantially with respect to their ancestral forms as a result of fusion events with autosomes, and autosome-sex chromosome fusions result in what are called neo-sex chromosomes. Five examples of this are now known in the songbird superfamily Sylvioidea.[29] There is one experimentally documented case of sex chromosome turnover occurring during a 30-year evolutionary experiment involving teleost fish (specifically the swordtails), in which hybridization experiments resulted in a translocation of the sex-determiner region of a sex chromosome into an autosome. This resulted in the autosome becoming a novel W sex chromosome.[30]
See also
- Fisher's principle
- Haldane's rule
- XY sex-determination system
- ZW sex-determination system
- X0 sex-determination system
References
- ^ a b "Allosome - Biology-Online Dictionary". www.biology-online.org. Archived from the original on 2018-02-11. Retrieved 2018-02-22.
- ^ "the definition of allosome". Dictionary.com. Archived from the original on 2016-02-04. Retrieved 2018-02-22.
- S2CID 1919033.
- ^ "How many chromosomes do people have?". Genetics Home Reference. U.S. National Library of Medicine. Archived from the original on 2013-04-02. Retrieved 2013-04-03.
- PMID 28947664.
- from the original on 2022-08-11. Retrieved 2022-03-29.
- ^ Graves, Jennifer A. Marshall (2015) Twenty-five years of the sex-determining gene Archived 2022-01-11 at the Wayback Machine, Nature 528, 343–344. https://doi.org/10.1038/528343a Retrieved 24 Jan 2022.
- Scitable. Archivedfrom the original on 2019-04-13. Retrieved 2019-10-24.
- ISBN 9780521462181. Archivedfrom the original on 2018-06-10. Retrieved 2019-11-04.
- PMID 18996493.
- ISSN 0044-8486.
- OCLC 50174640.
- ^ PMID 28267976.
- PMID 29074958.
- ^ PMID 26653795.
- ^ S2CID 14956007.
- S2CID 45809762.
- S2CID 23042305.
- ^ .
- PMID 25762551.
- ^ "Biological Basis of Heredity: Sex Linked Genes". Archived from the original on 2013-04-11. Retrieved 2013-04-07.
- ^ "Sex-linked recessive". PubMed. U.S. National Library of Medicine.
- ^ "Sex-Linked Traits". Heredity and genetics. Khan Academy. Archived from the original on 2013-04-08. Retrieved 2013-04-07.
- ^ "Hemophilia". PubMed Health. U.S. National Library of Medicine. Archived from the original on 2013-12-22. Retrieved 2017-11-01.
- ^ "Fragile X Syndrome - Symptoms, Diagnosis, Treatment of Fragile X Syndrome". NY Times Health Information. Archived from the original on 6 July 2013.
- ^ "46,XX testicular disorder of sex development". Genetics Home Reference. U.S. National Library of Medice. Archived from the original on 2013-03-30. Retrieved 2013-04-07.
- S2CID 10026903.
- S2CID 208574438.
- S2CID 252543767.
- PMID 30510159.