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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.

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 [1]. Platypus sex chromosomes in fact appear to bear a much stronger homology (similarity) with the avian Z chromosome[2], and the SRY gene so central to sex-determination in most other mammals is apparently not involved in platypus sex-determination[3]. 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.

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 human cell[4]. The human Y chromosome contains 86[5] 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.[6] It is the Single nucleotide polymorphisms in this region which are used for tracing direct paternal ancestral lines.

Genes

Not including pseudoautosomal genes, genes include:

  • NRY, with corresponding gene on X chromosome
    • AMELY/AMELX (amelogenin)
    • RPS4Y1/RPS4Y2/RPS4X (Ribosomal protein S4)
  • NRY, other
    • AZF1 (azoospermia factor 1)
    • BPY2 (basic protein on the Y chromosome)
    • DAZ1 (deleted in azoospermia)
    • DAZ2
    • PRKY (protein kinase, Y-linked)
    • RBMY1A1
    • SRY (sex-determining region)
    • TSPY (testis-specific protein)
    • USP9Y
    • UTY (ubiquitously transcribed TPR gene on Y chromosome)
    • ZFY (zinc finger protein)

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

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

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.[7] See www.smgf.org for more information. Mitochondrial DNA, maternally inherited, is used in an analogous way to trace the maternal line.

See also

Notes

  1. ↑ 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: 913–917. doi:10.1038/nature03021. 
  2. ↑ Warren WC, Hillier LDW, Graves JAM, et al. (2008). "Genome analysis of the platypus reveals unique signatures of evolution". Nature 453: 175–183. doi:10.1038/nature06936. 
  3. ↑ 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: 965–973. doi:10.1101/gr.7101908. 
  4. ↑ National Library of Medicine's Genetic Home Reference
  5. ↑ Ensembl Human MapView release 43 (February 2007). Retrieved on 2007-04-14.
  6. ↑ ScienceDaily.com Apr. 3, 2008
  7. ↑ See www.smgf.org for more information.

References

  • Skaletsky, H.S., et al. (2003) The male-specific region of the human Y chromosome is a mosaic of discrete sequence classes. Nature, 423, 825-837
  • Rozen, S., et al. (2003) Abundant gene conversion between arms of palindromes in human and ape Y chromosomes. Nature, 423, 873-876.

External links

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