Friday, 25 May 2012

The Y chromosome isn’t going anywhere… at least for a while

By Nathan Alexander, numerically known as 42894058.

Opposing the popular theory about the gradual mutation of the “Y” chromosome in the human male, scientists from the Whitehead Institute for Biomedical Research in Massachusetts have reportedly confirmed that the chromosome's genes are very unlikely to change for at least several million years. Hundreds of millions of years ago, the “X” and “Y” chromosomes in the human genetic makeup matched up very smoothly; in much the same way as the other 22 autosomal chromosome pairs in humans still do to this day.

Originally, the two human sex chromosomes exchanged genes as was necessary for appropriate gene repair, which effectively avoided many serious mutations in the DNA. Approximately 166 million years ago, however, a sizable portion “Y” chromosome was turned around and incorrectly inserted into one mammalian ancestor of humanity, which changed the very shape of the chromosome, giving it the characteristic “Y”-shape that it now possesses. This serious mutation rendered the human sex chromosomes incapable of trading genes, and this was inherited to the next generation. Previously, scientists had accepted the fact this genetic anomaly would result in the gradual elimination of the Y chromosome, leaving the male gender with an “X0” chromosome pair, which is more simply an “X” chromosome not attached to anything else (Hughes, et al, 2012).

Chromosomes reside within cellular genetic material. In a human somatic cell, there are normally 46 chromosomes existing as homologous pairs. In simple terms, a homologous pair is a couple of chromosomes with the same lengths, centromere positions and staining patterns (Reece, et al, 2012). In a homologous pair, both chromosomes possess identical genes for heritable characteristics. Human females carry a distinct homologous pair of sex chromosomes in their genetic makeup, often simply dubbed “XX” in reference to the overall shape of the chromosomes. Males do not possess homologous sex chromosomes, and instead carry one “X” and one “Y” chromosome. These two different chromosomes are only very slightly homologous, which was the primary source of the initial theory that the “Y” chromosome may, in due time, disappear as a genetic mutation (Hamzelou, 2012).

Homologous chromosomes, including the Y chromosome.
Dr Jennifer Hughes, the head scientist of the team at the research institute, has ultimately proven that this is not the case. It has been stated that although the “Y” chromosome did indeed lose a substantial amount of its original genes at the time of the first known mutation, there has no been no change in the remaining genes since. Hughes infers that these genes serve highly specific, vital functions in the body, which suggests that there is great doubt that any noticeable alterations to this chromosome will take place, at least for many million years. Currently, little has been confirmed regarding why this genetic anomaly first came about, although, naturally, several theories do exist. Upon comparing the human genome to that of the rhesus macaque, a primate species of which human evolutionary ancestry can be traced back approximately 25 million years, Dr Hughes was able to confirm that males of both species retained the exact same percentage of genes in the “Y” chromosome. This genetic breakthrough was able to prove that after 25 million years, there was no further deterioration since the first known anomaly in the “Y” chromosome, thus disproving the idea that, in time, the “Y” chromosome would cease to exist.


References

Hamzelou, J., 2012. Men's Y Chromosome is not about to go extinct, New Scientist, Accessed 17/03/12.

Hughes, J. et al., 2012. Strict evolutionary conservation followed rapid gene loss on human and rhesus Y chromosomes, Nature: International Weekly Journal of Science, Accessed 17/03/12.

Reece, J. B. et al., 2011. Campbell Biology, 9th edition, Australian version, Pearson Australia Group, Pty Ltd.

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