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Y-chromosome

 
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Y-chro·mo·some

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or Y chromosome (wī'krō'mə-sōm')
n.
The sex chromosome associated with male characteristics in mammals, not occurring in females and occurring with one X-chromosome in the male sex-chromosome pair.


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Gale Genetics Encyclopedia:

Y Chromosome

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The diploid human genome is packaged within 46 chromosomes, as two pairs of 23 discrete elements, into all cells other than the haploid gametic egg and sperm cells. During the reproductive process, each parent's gametes contribute 22 nonsex chromosomes and either one X or one Y chromosome.

Paternal Inheritance

The X and Y chromosomes are the sex chromosomes for mammals, including humans. Not only are the X and Y sex chromosomes in mammals physically distinctive, with the Y being smaller, the Y chromosome is exceptionally peculiar. The X chromosome contains considerably more genes than the Y, which has its functionality essentially limited to traits associated with being male. It is the Y chromosome that carries the major masculinity-determining gene (SRY, for sex-determining region Y), which dictates maleness. In a mating pair, if the paternal partner contributes a normal Y chromosome, male gonadal tissues (testes) develop in the offspring. Only males have the potential to transmit a Y chromosome to the next generation, and thus the father's contribution is decisive regarding an offspring's sex.

Since normally only one Y chromosome exists per cell, no pairing between X and Y occurs at meiosis, except at small regions. Normally, no crossing over occurs. Therefore, except for rare mutations that may occur during spermatogenesis, a son will inherit an identical copy of his father's Y chromosome, and this copy is also essentially identical to the Y chromosomes carried by all his paternal forefathers, across the generations. This is in contrast to the rest of his chromosomal heritage, which will be a unique mosaic of contributions from multiple ancestors created by the reshuffling process of recombination.

Sex Determination and Y Chromosome Genes

While SRY is the most dramatic gene affiliated with the Y chromosome, about thirty other genes have been identified. Some notable representatives include AZFa, b, and c, which are associated with spermatogenesis and male infertility, SMCY, associated with the immune response function responsible for transplantation rejection when male tissue is grafted to female tissue, and TSPY, which may play a role in testicular cancer.

Sex Chromosome Evolution and Peculiarities

Discussions of sex chromosome evolution raise the question of the biological risks and benefits of sexual differentiation in organisms. Overall, sexual dimorphism enhances diversity that, in turn, improves the chances for evolutionary change and potential survival during periods of environmental change.

There are risks in the specialization of the Y chromosome, however. Besides its absence in females, lack of recombination for most of its physical territory except at its tips, and the strict pattern of paternal inheritance, the solitary cellular existence of the Y chromosome reduces the opportunity for DNA repair, which normally occurs while pairing during mitosis. This may explain the prevalence of multicopy DNA sequences on the Y, and why many of its genes have lost functionality. In fact, while genes predominately specific to male function tend to accumulate on the Y chromosome, other genes that have functional counterparts elsewhere will atrophy over evolutionary time, through the accumulation of uncorrected mutations. Thus the Y chromosome is slowing evolving toward a composition with fewer and fewer essential genes.

Molecular Anthropology Using the Y Chromosome

The field of molecular anthropology is predicated on the concept that the genes of modern populations encode aspects of human history. By studying the degree of genetic molecular variation in modern organisms, one can, in principle, understand past events. The Y chromosome is uniquely suited to such studies. Secondary applications of Y chromosome variation studies include forensics (criminological investigations, such as determining whether or not an individual has been involved in a crime) and genealogical reconstruction (verifying membership in a particular family's ancestry).

DNA polymers (such as chromosomes) are composed of a four-letter alphabet of chemicals called nucleotide bases. Random unique event mutations in DNA sequences can change the identity of a single base in the DNA molecule. These "spelling changes" are the essential currency of genetic anthropological research.

What is central is the assumption that a particular mutation arose just once in human history, and all men that display such a mutation on their Y chromosome descend from a common forefather on whom the mutation first appeared. The sequential buildup of such mutational events across the generations can be readily determined and displayed as a gene tree. Informally, the last known mutation to accumulate on a particular chromosome can be used to define a particular lineage or branch tip in the tree. As long as the mutational change does not affect the individual's ability to reproduce, it may be preserved and handed down to each succeeding generation, eventually becoming widespread in a population. Such mutations are called polymorphisms or genetic markers.

Since most of the Y chromosome has the special property of not recombining during meiosis, no shuffling of DNA from different ancestors occurs. As a consequence, any Y chromosome accumulates all the mutations that have occurred during its lineal life span and thus preserves the paternal genetic legacy that has been transmitted from father to son over the generations. The discovery of numerous Y chromosome polymorphisms has allowed us to deduce a reliable genealogy composed of numerous distinctive lineages. This concept is analogous to the genealogical relationships maintained by the traditional transmission of surnames in some cultures, although the gene tree approach provides access to a prehistorically deeper set of paternal relationships.

Molecular anthropologists have exploited this knowledge in an attempt to understand the history and evolutionary relationships of contemporary populations by performing a systematic survey of Y-chromosome DNA sequence variation. The unique nature of Y-chromosome diversification provides an elegant record of human population histories allowing researchers to reconstruct a global picture, emblematic of modern human origins, affinity, differentiation, and demographic history. The evidence shows that all modern extant human Y chromosomes trace their ancestry to Africa, and that descendants left Africa perhaps less than 100,000 years (or approximately 4,000 generations) ago.

While variation in any single DNA molecule can reflect only a small portion of human diversity, by merging other genetic information, such as data from the maternally transmitted mitochondrial DNA molecule, and nongenetic knowledge derived from archeological, linguistic, and other sources, we can improve our understanding of the affinities and histories of contemporary peoples.

Bibliography

Cavalli-Sforza, Luigi L. Genes, Peoples, and Languages, Mark Seielstad, trans. New York: North Point Press, 2000.

Jobling, Mark, and Christopher Tyler-Smith. "New Uses for New Haplotypes: The Human Y Chromosome, Disease, and Selection." Trends in Genetics 16 (2000): 356-362.

Strachan, Tom, and Andrew P. Read. Human Molecular Genetics. New York: Wiley-Liss, 1996.

—Peter A. Underhill

Oxford Dictionary of Sports Science & Medicine:

Y chromosome

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The smaller of the two sex chromosomes. It is normally found in males only and seems to carry few genes. See also gender verification.

Oxford Dictionary of Biochemistry:

Y chromosome

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a sex chromosome found only in the heterogametic sex and usually different in size from the X chromosome. In many animals it carries the testis-determining factor that triggers male embryonic development.

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Saunders Veterinary Dictionary:

Y chromosome

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The chromosome which causes the medulla of the embryonic gonad to form a testis. If there is one other chromosome present and it is X the newborn animal will be a fertile male. If there are two other X chromosomes, giving an XXY configuration, it will be a phenotypic male but sterile. Autosomal genes can have the same effect, creating an intersex newborn in an animal with XX chromosomes.

Mosby's Dental Dictionary:

Y chromosome

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n

A sex chromosome that in humans and many other species is present only in the male, appearing singly in the normal male. It is carried as a sex determinant by one-half of the male gametes. None of the female gametes contain a Y chromosome.

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Wikipedia on Answers.com:

Y chromosome

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Human Y chromatid

The Y chromosome is one of the two sex-determining chromosomes in most mammals, including humans. In mammals, it contains the gene SRY, which triggers testis development if present. The human Y chromosome is composed of about 60 million base pairs. DNA in the Y chromosome is passed from father to son, and Y-DNA analysis may thus be used in genealogy research. With a 30% difference between humans and chimpanzees, the Y chromosome is one of the fastest evolving parts of the human genome.[1]

Contents

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, SRY, which triggers embryonic development as a male. The Y chromosomes of humans and other mammals also contain other genes 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.[2] Platypus sex chromosomes in fact appear to bear a much stronger homology (similarity) with the avian Z chromosome,[3] and the SRY gene so central to sex-determination in most other mammals is apparently not involved in platypus sex-determination.[4] 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 instead of a double X ("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.

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 evolved from a pair of identical chromosomes,[5][6] 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.[7] 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.[8]

Until recently, the X and Y chromosomes were thought to have diverged around 300 million years ago. However, recent research,[9] particularly that stemming from the sequencing of the platypus genome,[3] has suggested that the XY sex-determination system wouldn't have been present more than 166 million years ago, at the split of the monotremes from other mammals.[4] This reestimation of the age of the therian XY system is based on the finding that sequences that are on the X chromosomes of marsupials and eutherian mammals are present on the autosomes of platypus and birds.[4] The older estimate was based on erroneous reports that the platypus X chromosomes contained these sequences.[2][10]

Recombination inhibition

Recombination between the X and Y chromosomes proved harmful—it resulted in males without necessary genes formerly found on the Y 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.[7] 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

The human Y chromosome has lost 1,393 of its 1,438 original genes over the course of its existence. With a rate of genetic loss of 4.6 genes per million years, the Y chromosome may potentially lose complete function within the next 10 million years.[11] Comparative genomic analysis, however, reveals that many mammalian species are experiencing a similar loss of function in their heterozygous sex chromosome. Degeneration may simply be the fate of all nonrecombining sex chromosomes due to three common evolutionary forces: high mutation rate, inefficient selection and genetic drift.[7] On the other hand, recent comparisons of the human and chimpanzee Y chromosomes show that the human Y chromosome has not lost any genes since the divergence of humans and chimpanzees between 6–7 million years ago,[12] providing direct evidence that the linear extrapolation model is flawed.

High mutation rate

The human Y chromosome is particularly exposed to high mutation rates due to the environment in which it is housed. The Y chromosome is passed exclusively through sperm, which undergo multiple cell divisions during gametogenesis. Each cellular division provides further opportunity to accumulate base pair mutations. Additionally, sperm are stored in the highly oxidative environment of the testis, which encourages further mutation. These two conditions combined put the Y chromosome at a risk of mutation 4.8 times greater than the rest of the genome.[7]

Inefficient selection

Without the ability to recombine during meiosis, the Y chromosome is unable to expose individual alleles to natural selection. Deleterious alleles are allowed to "hitchhike" with beneficial neighbors, thus propagating maladapted alleles in to the next generation. Conversely, advantageous alleles may be selected against if they are surrounded by harmful alleles (background selection). Due to this inability to sort through its gene content, the Y chromosome is particularly prone to the accumulation of "junk" DNA. Massive accumulations of retrotransposable elements are scattered throughout the Y.[7] The random insertion of DNA segments often disrupts encoded gene sequences and renders them nonfunctional. However, the Y chromosome has no way of weeding out these "jumping genes". Without the ability to isolate alleles, selection cannot effectively act upon them.

A clear, quantitative indication of this inefficiency is the entropy rate of the Y chromosome. Whereas all other chromosomes in the human genome have entropy rates of 1.5–1.9 bits per nucleotide (compared to the theoretical maximum of exactly 2 for no redundancy), the Y chromosome's entropy rate is only 0.84.[13] This means the Y chromosome has a much lower information content relative to its overall length; it is more redundant.

Genetic drift

Even if a well adapted Y chromosome manages to maintain genetic activity by avoiding mutation accumulation, there is no guarantee it will be passed down to the next generation. The population size of the Y chromosome is inherently limited to 1/4 that of autosomes: diploid organisms contain two copies of autosomal chromosomes while only half the population contains 1 Y chromosome. Thus, genetic drift is an exceptionally strong force acting upon the Y chromosome. Through sheer random assortment, an adult male may never pass on his Y chromosome if he only has female offspring. Thus, although a male may have a well adapted Y chromosome free of excessive mutation, it may never make it in to the next gene pool.[7] The repeat random loss of well-adapted Y chromosomes, coupled with the tendency of the Y chromosome to evolve to have more deleterious mutations rather than less for reasons described above, contributes to the species-wide degeneration of Y chromosomes through Muller's ratchet.[14]

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.[15] Such a "recombination" is called gene conversion.

In the case of the Y chromosomes, the palindromes are not noncoding 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

In the terminal stages of the degeneration of the Y chromosome, other chromosomes increasingly take over genes and functions formerly associated with it. Finally, the Y chromosome disappears entirely, and a new sex-determining system arises.[7] Several species of rodent in the sister families Muridae and Cricetidae have reached these stages,[16][17] in the following ways:

  • The Transcaucasian mole vole, Ellobius lutescens, the Zaisan mole vole, Ellobius tancrei, and the Japanese spinous country rats Tokudaia osimensis and Tokudaia muenninki, have lost the Y chromosome and SRY entirely.[7][18][19] Tokudaia spp. have relocated some other genes ancestrally present on the Y chromosome to the X chromosome.[19] Both genders of Tokudaia spp. and Ellobius lutescens have an XO genotype,[19] whereas all Ellobius tancrei possess an XX genotype.[7] The new sex-determining system for these rodents remains unclear.
  • The wood lemming Myopus schisticolor, the arctic lemming, Dicrostonyx torquatus, and multiple species in the grass mouse genus Akodon have evolved fertile females who possess the genotype generally coding for males, XY, in addition to the ancestral XX female, through a variety of modifications to the X and Y chromosomes.[16][20][21]
  • In the creeping vole, Microtus oregoni, the females, with just one X chromosome each, produce X gametes only, and the males, XY, produce Y gametes, or gametes devoid of any sex chromosome, through nondisjunction.[22]

Outside of the rodent family, the black muntjac, Muntiacus crinifrons, evolved new X and Y chromosomes through fusions of the ancestral sex chromosomes and autosomes.[23] Primate Y chromosomes, including in humans, have degenerated so much that primates will also evolve new sex determination systems relatively soon, in about 14 million years in humans.[7][24]

Human Y chromosome

In humans, the Y chromosome spans about 58 million base pairs (the building blocks of DNA) and represents approximately 2% of the total DNA in a male cell.[25] The human Y chromosome contains 86[26] 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.[27] 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.[citation needed]

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 the 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 44X, 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 is usually a girl with the features of Turner syndrome or mixed gonadal dysgenesis.

XXY
Main article: Klinefelter's syndrome

Klinefelter's syndrome (47, XXY) is not an aneuploidy of the Y chromosome, but a condition of having an extra X chromosome, which usually results in defective postnatal testicular function. The mechanism is not fully understood; the extra X does not seem to be due to direct interference with expression of Y genes.

XYY
Main article: XYY

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

Main articles: Mitochondrial Eve and Y-chromosomal Adam

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. 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

References

  1. ^ Wade, Nicholas (January 13, 2010). "Male Chromosome May Evolve Fastest". New York Times. http://www.nytimes.com/2010/01/14/science/14gene.html. 
  2. ^ a b Grützner F, Rens W, Tsend-Ayush E, et al. (2004). "In the platypus a meiotic chain of ten sex chromosomes shares genes with the bird Z and mammal X chromosomes". Nature 432 (7019): 913–917. doi:10.1038/nature03021. PMID 15502814. 
  3. ^ a b Warren WC, Hillier LDW, Graves JAM, et al. (2008). "Genome analysis of the platypus reveals unique signatures of evolution". Nature 453 (7192): 175–183. doi:10.1038/nature06936. PMC 2803040. PMID 18464734. http://www.nature.com/nature/journal/v453/n7192/full/nature06936.html. 
  4. ^ a b c Veyrunes F, Waters PD, Miethke P, et al. (2008). "Bird-like sex chromosomes of platypus imply recent origin of mammal sex chromosomes". Genome Research 18 (6): 965–973. doi:10.1101/gr.7101908. PMC 2413164. PMID 18463302. http://genome.cshlp.org/content/18/6/965.abstract. 
  5. ^ Muller, HJ (1914). "A gene for the fourth chromosome of Drosophila". Journal of Experimental Zoology 17 (3): 325–336. doi:10.1002/jez.1400170303. 
  6. ^ Lahn B, Page D (1999). "Four evolutionary strata on the human X chromosome". Science 286 (5441): 964–7. doi:10.1126/science.286.5441.964. PMID 10542153. 
  7. ^ a b c d e f g h i j Graves, J. A. M. (2006). "Sex chromosome specialization and degeneration in mammals". Cell 124 (5): 901–914. doi:10.1016/j.cell.2006.02.024. PMID 16530039. 
  8. ^ Graves J.A.M., Koina E., Sankovic N. (2006). "How the gene content of human sex chromosomes evolved". Curr Opin Genet Dev 16 (3): 219–24. doi:10.1016/j.gde.2006.04.007. PMID 16650758. 
  9. ^ Hamilton, Jon (January 13, 2010). "Human Male: Still A Work in Progress". NPR. http://www.npr.org/blogs/health/2010/01/human_male_still_a_work_in_pro.html. 
  10. ^ Graves, Jaclyn M.; Riggs, Arthur (1992). "Gene mapping studies confirm the homology between the platypus X and echidna X1 chromosomes and identify a conserved ancestral monotreme X chromosome". Chromosoma 101 (10): 596–601. doi:10.1007/BF00360536. 
  11. ^ Graves, J. A. M. (2004). "The degenerate Y chromosome—can conversion save it?". Reproduction Fertility and Development 16 (5): 527–534. doi:10.1071/RD03096. PMID 15367368. 
  12. ^ Hughes, Jennifer F.; et al. (2005). "Conservation of Y-linked genes during human evolution revealed by comparative sequencing in chimpanzee". Nature 437 (7055): 100–103. doi:10.1038/nature04101. PMID 16136134. 
  13. ^ Liu, Zhandong; Venkatesh, Santosh S.; Maley, Carlo C. (2008). "Sequence space coverage, entropy of genomes and the potential to detect non-human DNA in human samples". BMC Genomics 9: 509. doi:10.1186/1471-2164-9-509. PMID 18973670.  Fig. 6, using the Lempel-Ziv estimators of entropy rate.
  14. ^ Charlesworth, B.; Charlesworth, D. (2000). "The degeneration of Y chromosomes". Philosophical Transactions of the Royal Society of London Series B-Biological Sciences 355 (1403): 1563–1572. doi:10.1098/rstb.2000.0717. 
  15. ^ Rozen S, Skaletsky H, Marszalek J, Minx P, Cordum H, Waterston R, Wilson R, Page D (2003). "Abundant gene conversion between arms of palindromes in human and ape Y chromosomes". Nature 423 (6942): 873–6. doi:10.1038/nature01723. PMID 12815433. 
  16. ^ a b Marchal, J. A.; Acosta, M. J.; Bullejos, M.; de la Guardia, R. D.; Sanchez, A. (2003). "Sex chromosomes, sex determination, and sex-linked sequences in Microtidae". Cytogenetic and Genome Research 101 (3–4): 266–273. doi:10.1159/000074347. 
  17. ^ Wilson, M. A.; Makova, K. D. (2009). "Genomic analyses of sex chromosome evolution". Annual Review of Genomics and Human Genetics 10: 333–354. doi:10.1146/annurev-genom-082908-150105. PMID 19630566. 
  18. ^ Just, W.; Baumstark, A.; Suss, A.; Graphodatsky, A.; Rens, W.; Schafer, N.; Bakloushinskaya, I.; et al. (2007). "Ellobius lutescens: Sex determination and sex chromosome". Sexual Development 1 (4): 211–221. doi:10.1159/000104771. PMID 18391532. 
  19. ^ a b c Arakawa, Y.; Nishida-Umehara, C.; Matsuda, Y.; Sutou, S.; Suzuki, H. (2002). "X-chromosomal localization of mammalian Y-linked genes in two XO species of the Ryukyu spiny rat". Cytogenetic and Genome Research 99 (1–4): 303–309. doi:10.1159/000071608. PMID 12900579. 
  20. ^ Hoekstra, H. E.; Edwards, S. V. (2000). "Multiple origins of XY female mice (genus Akodon): phylogenetic and chromosomal evidence". Proceedings of the Royal Society B 267 (1455): 1825–1831. doi:10.1098/rspb.2000.1217. PMC 1690748. PMID 11052532. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1690748. 
  21. ^ Ortiz, M. I.; Pinna-Senn, E.; Dalmasso, G.; Lisanti, J. A. (2009). "Chromosomal aspects and inheritance of the XY female condition in Akodon azarae (Rodentia, Sigmodontinae)". Mammalian Biology 74 (2): 125–129. doi:10.1016/j.mambio.2008.03.001. 
  22. ^ Charlesworth, B.; Dempsey, N. D. (2001). "A model of the evolution of the unusual sex chromosome system of Microtus oregoni". Heredity 86 (4): 387–394. doi:10.1046/j.1365-2540.2001.00803.x. PMID 11520338. 
  23. ^ Zhou, Q.; Wang, J.; Huang, L.; Nie, W. H.; Wang, J. H.; Liu, Y.; Zhao, X. Y.; et al. (2008). "Neo-sex chromosomes in the black muntjac recapitulate incipient evolution of mammalian sex chromosomes". Genome Biology 9 (6): R98. doi:10.1186/gb-2008-9-6-r98. PMID 18554412. 
  24. ^ Goto, H.; Peng, L.; Makova, K. D. (2009). "Evolution of X-degenerate Y chromosome genes in greater apes: conservation of gene content in human and gorilla, but not chimpanzee". Journal of Molecular Evolution 68 (2): 134–144. doi:10.1007/s00239-008-9189-y. PMID 19142680. 
  25. ^ National Library of Medicine's Genetic Home Reference
  26. ^ "Ensembl Human MapView release 43". February 2007. http://www.ensembl.org/Homo_sapiens/mapview?chr=Y. Retrieved 2007-04-14. 
  27. ^ ScienceDaily.com Apr. 3, 2008

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Sex chromosome

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