Y Chromosome
The Y chromosome is one of two sex chromosomes present in many organisms. It never occurs alone but is paired with an X chromosome. The Y is usually the smaller of the two, though the size and shape may vary.
In mammals, there is a significant homology between the Y chromosomes of different species, but the human Y chromosome is somewhat unique in having a large region of heterochromatin, non-coding DNA, that makes up most of the long arm. This heterochromatin provides a useful tool in chromosome identification, since it is the brightest region of the human genome when the chromosomes are stained with a fluorescent dye such as quinicrine. A quick determination about the presence or absence of a Y chromosome in a newborn with ambiguous genitalia can be made by examining metaphase cells for this bright staining region.
The short arm of the Y chromosome also has a unique structure. The distal end is called the pseudoautosomal region because it shares DNA sequences and genes with the tip of the short arm of the X chromosome, and recombination occurs between these two areas in exactly the same way as with recombination on the autosomes. The homology also serves as a point of association during chromosome pairing and cell division.
A comprehensive physical map of the human Y is available, and the DNA sequence is nearly complete with 34 genes mapped along the length of the chromosome. The Human Genome Project has confirmed that the Y chromosome is the smallest of the set, with the fewest number of base pairs, only 21.8 Mb, and less than 30% of the known genes of the second shortest chromosome. But included in its gene complement are some of the most critical genes for human development. SRY, sex-determining region of the Y, located on the short arm of the Y is the trigger for male development. For males, the azoospermia genes on the long arm are important in determining fertility. The gonadoblastoma gene is involved in gonadal tumors. Other known genes include such characters as stature, tooth size, blood group determination, and histocompatibility. Although some prominent scientists have postulated that genes for traits typically associated with males, such as, lack of recall of dates (OOPS), selective hearing loss (HUH?), channel flipping (FLP), preference for the sports page (BUD-E), and ability to identify aircraft (DC-1), must reside on the Y chromosome, there is currently no evidence to support this theory.
Genes located uniquely on the Y chromosome are said to be Y-linked. These genes are passed exclusively from father to son. Early in the history of medical genetics, such traits as baldness were thought to be Y-linked because males express the condition fair more commonly than females. Additional study has shown that baldness and other similar characteristics are in fact sex-influenced. This means that rather than a specific Y-linked gene for baldness, it is the higher production of male hormones that is the contributing factor to baldness. There are very few confirmed Y-linked disorders. There is one report of male to male only transmission of a form of retinitis pigmentosa (eye disorder) that has been tentatively assigned Y linkage. Hairy pinnae, or hair growing from the ear lobe, has also been suggested as Y linked, though some reports suggest there may be an autosomal form in some families.
Although there are few genes on the Y chromosome that allow clear analysis of male to male transmission, investigators have identified a number of unique DNA sequences that have contributed to population and family studies. Evaluation of these sequences has produced interesting data on evolutionary patterns of different populations. By charting the frequency, distribution, and different mutations found in these DNA regions, investigators have been able to better understand the dynamics of various populations including migration and invasion. On a smaller scale, DNA analysis of male descendents of the slave Sally Hemmings has suggested that one or more of her children may have been fathered by Thomas Jefferson. DNA patterns in the male Hemmings line bear a close resemblance to the same sequences in male descendents of Jefferson's brother (Jefferson himself has no confirmed direct male descendents).
The Y chromosome, despite its small size, remains a chromosome with a very high profile primarily due to its association with sex determination.
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