Y chromosome

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The Y chromosome is one of the sex-determining chromosomes in humans and most other mammals. In mammals, it contains the gene SRY, which triggers testis development, thus determining maleness.

Contents

[edit] 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 the gene that 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. Among humans, some men have two X's and a Y ("XXY", see Klinefelter 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.

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 fruit flies (Drosophila melanogaster and relatives), some other insects, some fish, some reptiles, and some plants. In fruit flies, the Y chromosome does not trigger male development. Instead, sex is determined by the number of X chromosomes. So XXY fruit flies are female, and fruit flies with a single X (X0), are male but sterile.

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.

A closely related topic is Sex determination.

[edit] Origins and evolution

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.

The X and Y chromosomes diverged around 300 million years ago[1] when some reptile, the distant ancestor of all mammals, developed a so-called 'male gene' - simply possessing this gene caused the organism to be male. The chromosome with this gene became the Y chromosome, and a similar chromosome without it became the X chromosome. So initially, X and Y chromosomes were nearly identical. Over time, genes which were beneficial for males and harmful to (or had no effect on) females either moved to or developed on the Y chromosome.

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 assembled near the sex-determining genes in order to make this less probable. Eventually, 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.

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. These harmful mutations continued to damage Y-unique genes until several finally stopped functioning and became genetic junk; this was eventually removed from the Y chromosome.

As a result of this process 95% of the human Y chromosome is unable to recombine, the chromosome itself contains only 83 working genes[2]; compare this to close to 1000 working genes on the X chromosome. In some animals, Y degradation is even more severe. For example, the kangaroo Y chromosome contains only the SRY gene. [citation needed]

[edit] Gene conversion

In 2003, researchers from MIT discovered another process which may slow down the process of degradation.[3] 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.

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.

This recombination phenomenon RecLOH is also observed in Genetic Genealogy when multicopy Y-STR markers located at adjacent palindromic arms change from different repeat counts to twin alleles of equal length. Often 2, 3 or more Y-STR markers are involved in the same recombinational event and change to twin alleles at once.

[edit] 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 with two species of mole vole (Ellobius tancrei and E. lutescens). In one species, both sexes have unpaired X chromosomes; in the other, both females and males have XX.
  • Part of some autosome is connected to both the X and Y chromosomes. This happened with one species of Drosophila.

[edit] 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 78 genes, which code for only 23 distinct proteins. This relationship is typical in that most species' Y chromosomes contain the fewest genes of any of the chromosomes.[citation needed]

Because the Y chromosome changes relatively slowly over time and is only passed along the direct male line, it may be used to trace paternal lineage. It is the cause of the fewest number of known genetic diseases in humans (44 in total).[citation needed]

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). About 56, or 72%, of the Y chromosome genes are in this area; as a result, these genes are common between both sex chromosomes.[citation needed]

[edit] Genes

  • AMELY (amelogenin,Y-chromosomal)
  • ANT3Y (adenine nucleotide translocator-3 on the Y)
  • ASMTY (which stands for acetylserotonin methyltransferase)
  • AZF1 (azoospermia factor 1)
  • AZF2 (azoospermia factor 2)
  • BPY2 (basic protein on the Y chromosome)
  • CSF2RY (granulocyte-macrophage colony-stimulating factor receptor, alpha subunit on the Y chromosome)
  • DAZ (deleted in azoospermia)
  • IL3RAY (interleukin-3 receptor)
  • PRKY (protein kinase, Y-linked)
  • RBM1 (RNA binding motif protein, Y chromosome, family 1, member A1)
  • RBM2 (RNA binding motif protein 2)
  • SRY (sex-determining region)
  • TDF (testis determining factor)
  • TSPY (testis-specific protein)
  • UTY (ubiquitously transcribed TPR gene on Y chromosome)
  • ZFY (zinc finger protein)

[edit] Chromosome-linked diseases

No vital genes reside only on the Y chromosome, since 50% of humans 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. This results in sex reversal, so that a person with an XY karyotype has a female phenotype (i.e., is born a female). The lack of the second X results in infertility.

However, 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. Males with 47,XYY syndrome have one X chromosome and two Y chromosomes, for a total of 47 chromosomes per cell. Researchers are not yet certain why an extra copy of the Y chromosome is associated with tall stature and learning problems in some boys and men. These effects are variable and often minimal or undetectable. When chromosome surveys were first done in the 1960s, it was reported that a higher than expected number of men in prisons were found to have an extra Y chromosome, so that for a while it was thought to predispose a boy to antisocial behavior (and was dubbed the "criminal karyotype"). Better population surveys have since demonstrated that the association was simply that boys and men with learning problems are more likely statistically to spend time in prison and that there is no other independent statistical association with extra Y. The "criminal karyotype" concept is inaccurate and obsolete.

Greater degrees of Y chromosome polysomy (e.g., XYYYY) are very rare. Rarely, males may have more than one extra copy of the Y chromosome in every cell (polysomy Y). The extra genetic material in these cases can lead to skeletal abnormalities, decreased IQ, and delayed development, but the features of these conditions are variable.

There are also problems that arise from having 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 but a risk of malignancy.

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

[edit] 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.[4]

[edit] See also

[edit] References

  1. ^ http://www.abc.net.au/science/news/stories/s63100.htm
  2. ^ http://www.ensembl.org/Homo_sapiens/mapview?chr=Y
  3. ^
  4. ^ See www.smgf.org for more information.
  • Cordum, 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.

[edit] External links

Human chromosomes

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