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Y chromosome
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The Y chromosome is the sex-determining chromosome in most mammals, including humans. In mammals, it contains the gene SRY, which triggers testis development, thus determining sex. The human Y chromosome is composed of about 60 million base pairs.
mammals have one pair of sex chromosomes in each cell. Males have one Y chromosome and one X chromosome, while females have two X chromosomes. In mammals, the Y chromosome contains a gene which triggers embryonic development as a male.
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The Y chromosome is the sex-determining chromosome in most mammals, including humans. In mammals, it contains the gene SRY, which triggers testis development, thus determining sex. The human Y chromosome is composed of about 60 million base pairs.
Overview
Most mammals have one pair of sex chromosomes in each cell. Males have one Y chromosome and one X chromosome, while females have two X chromosomes. In mammals, the Y chromosome contains a gene which triggers embryonic development as a male. This gene is SRY. Other genes (in addition to SRY) on the Y chromosomes of men and other mammals are needed for normal sperm production.
There are exceptions, however. For example, the platypus relies on an XY sex-determination system based on five pairs of chromosomes . Although platypus sex chromosomes in fact appear to bear a much stronger homology (similarity) with the avian Z chromosome, and the SRY gene so central to sex-determination in most other mammals in apparently not involved in platypus sex-determination. Among humans, some men have two Xs and a Y ("XXY", see Klinefelter's syndrome), or one X and two Ys (see XYY syndrome), and some women have three Xs or a single X (and no Y, "X0", see Turner syndrome). There are other exceptions in which SRY is damaged (leading to an XY female), or copied to the X (leading to an XX male). For related phenomena see Androgen insensitivity syndrome and Intersex.
Presence or absence of the Y-chromosome is a method of sexual determination.
Origins and evolution
Before Y-chromosome
Many ectothermic vertebrates have no sex chromosomes. If they have different sexes, sex is determined environmentally rather than genetically. For some of them, especially reptiles, sex depends on the incubation temperature; others are hermaphroditic (meaning they contain both male and female gametes in the same individual).
Origin
The X and Y chromosomes are thought to have originated from a pair of identical chromosomes, termed autosomes, when an ancestral mammal developed an allelic variation, a so-called 'sex locus' - simply possessing this allele caused the organism to be male. The chromosome with this allele became the Y chromosome, while the other member of the pair became the X chromosome. Over time, genes which were beneficial for males and harmful to (or had no effect on) females either developed on the Y chromosome, or were acquired through the process of translocation..
Until recently, the X and Y chromosomes were thought to have diverged around 300 million years ago. However recent research, particularly that stemming from the sequencing of the platypus genome, has suggested that the XY sex-determination system wouldn't have been present more that 166 million years ago, at the split of monotreme from other mammals. This reduction in the age of the XY chromosomal system by around 150 million years is based on the finding that platypus X chromosomes show much stronger homology to the avian Z sex chromosome that with other mammalian X chromosomes . This suggests that the platypus X and avian Z chromosomes both evolved from the same chromosome of a common ancestor of the two.
Recombination inhibition
Recombination between the X and Y chromosomes proved harmful - it resulted in males without necessary genes formerly found on the X chromosome, and females with unnecessary or even harmful genes previously only found on the Y chromosome. As a result, genes beneficial to males accumulated near the sex-determining genes, and recombination in this region was suppressed in order to preserve this male specific region. Over time, the Y chromosome changed in such a way as to inhibit the areas around the sex determining genes from recombining at all with the X chromosome. As a result of this process 95% of the human Y chromosome is unable to recombine.
Shrinking
With time, larger and larger areas became unable to recombine with the X chromosome. This caused its own problems: without recombination, the removal of harmful mutations from chromosomes becomes increasingly difficult. With no ability to remove the harmful mutations, only fortuitous mutations that permanently turned these coding regions off would give their bearer an advantage over those with still-functioning, harmful genes. This gradually turned large swaths of the chromosome into genetic junk; this was eventually removed from the Y chromosome.
Today, the human Y chromosome itself contains only 78 working genes, compared to close to 1500 working genes on the X chromosome. In some animals, Y degradation is even more severe. The dunnart, a marsupial carrying a 10-12 Mb Y chromosome, has only four characterised genes; among them the SRY gene, is the smallest known mammalian Y chromosome.
Gene conversion
In 2003, researchers from MIT discovered a process which may slow down the process of degradation.
They found that human Y chromosome is able to "recombine" with itself, using palindrome base pair sequences. Such a "recombination" is called gene conversion or recombinational loss of heterozygosity (RecLOH).
In the case of the Y chromosomes, the palindromes are not junk DNA; these strings of bases contain functioning genes important for male fertility. Most of the sequence pairs are greater than 99.97% identical. The extensive use of gene conversion may play a role in the ability of the Y chromosome to edit out genetic mistakes and maintain the integrity of the relatively few genes it carries. In other words, since the Y chromosome is single, it has duplicates of its genes on itself instead of having a second, homologous, chromosome. When errors occur, it can use other parts of itself as a template to correct them.
Findings were confirmed by comparing similar regions of the Y chromosome in humans to the Y chromosomes of chimpanzees, bonobos and gorillas. The comparison demonstrated that the same phenomenon of gene conversion appeared to be at work more than 5 million years ago, when humans and the non-human primates diverged from each other.
Future evolution
After only an SRY (or other sex-determining) gene remains from the whole Y chromosome, there are the following possibilities:
- The gene is connected to X chromosome or some autosome, making it the new Y chromosome. The whole process starts again. This has happened in the Transcaucasian Mole Vole (Ellobius lutescens), the Northern Mole Vole (E. talpinus) and the Zaisan Mole Vole (E. tancrei). In E. lutescens, both sexes have unpaired X chromosomes; in E. talpinus and E. tancrei, both females and males have XX. A similar situation to that of the Transcaucasian Mole Vole seems to exist in the Tokudaia osimensis species complex of spinous country-rats. Among Eumuroida, Ellobius and Tokudaia are not particularly closely related. Consequently unusual mechanisms of sex determination may well be far more common among these rodents than generally assumed.
- Part of some autosome is connected to both the X and Y chromosomes. This happened with one species of Drosophila.
- The Y chromosome remains, containing only the SRY gene.
Human Y chromosome
In humans, the Y chromosome spans 58 million base pairs (the building blocks of DNA) and represents approximately 0.38% of the total DNA in a human cell. The human Y chromosome contains 86 genes, which code for only 23 distinct proteins. Traits that are inherited via the Y chromosome are called holandric traits.
The human Y chromosome is unable to recombine with the X chromosome, except for small pieces of pseudoautosomal regions at the telomeres (which comprise about 5% of the chromosome's length). These regions are relics of ancient homology between the X and Y chromosomes. The bulk of the Y chromosome which does not recombine is called the "NRY" or non-recombining region of the Y chromosome. It is the SNPs in this region which are used for tracing direct paternal ancestral lines.
Genes
Not including pseudoautosomal genes, genes include:
Y-Chromosome-linked diseases
Y-Chromosome-linked diseases can be of more common types, or very rare ones. Yet, the rare ones still have importance in understanding the function of the Y-chromosome in the normal case.
More common
No vital genes reside only on the Y chromosome, since roughly half of humans (females) do not have Y chromosomes. The only well-defined human disease linked to a defect on the Y chromosome is defective testicular development (due to deletion or deleterious mutation of SRY). However, having two X-chromosomes and one Y-chromosome has similar effects. On the other hand, having Y-chromosome polysomy has other effects than masculinization.
Defective Y-chromosome
This results in the person presenting a female phenotype even though that person possesses an XY karyotype (i.e., is born with female-like genitalia). The lack of the second X results in infertility. In other words, viewed from opposite direction, the person goes through defeminization but fails to complete masculinization.
The cause can be seen as an incomplete Y chromosome: the usual karyotype in these cases is 46X, plus a fragment of Y. This usually results in defective testicular development, such that the infant may or may not have fully formed male genitalia internally or externally. The full range of ambiguity of structure may occur, especially if mosaicism is present. When the Y fragment is minimal and nonfunctional, the child usually is a girl with the features of Turner syndrome or mixed gonadal dysgenesis.
XXY
Klinefelter's syndrome (47, XXY) is not an aneuploidy of the Y chromosome, but a condition of having an extra X chromosome. It usually results in defective postnatal testicular function, but as the extra X does not seem to be due to direct interference with expression of Y genes. The mechanism is not fully understood.
XYY
It is possible for an abnormal number (aneuploidy) of Y chromosomes to result in problems.
47,XYY syndrome is caused by the presence of a single extra copy of the Y chromosome in each of a male's cells. 47,XYY males have one X chromosome and two Y chromosomes, for a total of 47 chromosomes per cell. Researchers have found that an extra copy of the Y chromosome is associated with increased stature and an increased incidence of learning problems in some boys and men, but the effects are variable, often minimal, and the vast majority do not know their karyotype. When chromosome surveys were done in the mid-1960s in British secure hospitals for the developmentally disabled, a higher than expected number of patients were found to have an extra Y chromosome. The patients were mischaracterized as aggressive and criminal, so that for a while an extra Y chromosome was believed to predispose a boy to antisocial behavior (and was dubbed the "criminal karyotype"). Subsequently, in 1968 in Scotland the only ever comprehensive nationwide chromosome survey of prisons found no overrepresentation of 47,XYY men, and later studies found 47,XYY boys and men had the same rate of criminal convictions as 46,XY boys and men of equal intelligence. Thus, the "criminal karyotype" concept is inaccurate and obsolete.
Rare
The following Y-Chromosome-linked diseases are rare, but notable because of their elucidating of the nature of the Y-chromosome.
More than two Y chromosomes
Greater degrees of Y chromosome polysomy (having more than one extra copy of the Y chromosome in every cell, e.g., XYYYY) are rare. The extra genetic material in these cases can lead to skeletal abnormalities, decreased IQ, and delayed development, but the severity features of these conditions are variable.
XX male syndrome
XX male syndrome occurs when there has been a recombination in the formation of the male gametes, causing the SRY-portion of the Y chromosome to move to the X chromosome. When such an X chromosome contributes to the child, the development will lead to a male, because of the SRY gene.
Genetic genealogy
In human genetic genealogy (the application of genetics to traditional genealogy) use of the information contained in the Y chromosome is of particular interest since, unlike other genes, the Y chromosome is passed exclusively from father to son. See for more information. Mitochondrial DNA, maternally inherited, is used in an analogous way to trace the maternal line.
Non-mammal Y-chromosome
Many groups of organisms in addition to mammals have Y chromosomes, but these Y chromosomes do not share common ancestry with mammalian Y chromosomes. Such groups include Drosophila, some other insects, some fish, some reptiles, and some plants. In Drosophila melanogaster, the Y chromosome does not trigger male development. Instead, sex is determined by the number of X chromosomes. The D. melanogaster Y chromosome does contain genes necessary for male fertility. So XXY D. melanogaster are female, and D. melanogaster with a single X (X0), are male but sterile. There are some species of Drosophila in which X0 males are both viable and fertile.
ZW-chromosomes
Other organisms have mirror image sex chromosomes: the female is "XY" and the male is "XX", but by convention biologists call a "female Y" a W chromosome and the other a Z chromosome. For example, female birds, snakes, and butterflies have ZW sex chromosomes, and males have ZZ sex chromosomes.
See also
External links
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- — Human Chromosome Y Launchpad
- — The Y Chromosome - From the Whitehead Institute for Biomedical Research
- — focus on the Y chomosome
- — Use of Novel Mechanism Preserves Y Chromosome Genes
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