Friday, February 24, 2012

Disappearing Act?

Image courtesy of Indigo Instruments, Canada

Usually, the first question posed to an expectant mother is whether she is carrying a boy or a girl.  The difference in her answer is based on the presence or absence of the Y chromosome.  The genetic information carried on the Y chromosome act as instructions for making a male’s testes and sperm.  This important sex determining factor may actually be disappearing.  Researchers have been tracking the changes in the Y chromosome over many years of evolution.  This has developed into a controversy over whether the Y chromosome will eventually vanish altogether. 

Humans have 23 pairs of chromosomes, for a grand total of 46.  Twenty-two of these chromosome pairs are considered autosomal and are shared by both men and women.  The remaining chromosome set makes up the sex chromosomes, X and Y.  Women possess two X chromosomes, while men possess one X and one Y chromosome.  

Today, the X chromosome is much larger than the Y chromosome.  However, researchers agree that at some point long ago, the sex chromosomes used to be genetically identical.  They themselves were also autosomal chromosomes in some ancestral organism.  Then, one chromosome acquired a variation that made the organism male, which became the Y chromosome. Over millions of years, the chromosomes became increasingly different, and the Y chromosome became a degraded version of the X chromosome.  Whether the Y chromosome is finished shrinking is the source of the controversy

One side argues that the Y chromosome is doomed to extinction.  The human X chromosome has about 1000 genes that provide instructions for making proteins.  The human Y chromosome has only about 78 genes that do the same.  This same pattern of sex chromosome degradation has been observed in multiple other species, including both vertebrates and invertebrates.  This rate of degradation is very consistent in other mammals.  In some rodents, the Y chromosome has already seemed to disappear. 

However, others are not convinced that the Y chromosome will actually disappear completely in humans.  They believe that though the Y chromosome has decreased in size, its degradation has reached equilibrium.  After all, it has survived many millions of years of evolution.  The human Y chromosome has not been losing genetic information as consistently as other species.  In fact, our Y chromosome has been relatively stable for the past 6 million years.  Also, the Y chromosome has not only lost genes, but has acquired some genes over the years.  About 8 different genes have been added to what is considered the original human Y chromosome. 

These hypotheses remain in conflict and the disappearing Y chromosome controversy continues.   However, if the Y chromosome were to vanish, sexual reproduction would be drastically different.  The remaining genes on the Y chromosome are mainly active in the testes and are extremely important for the production of sperm.  If these genes were to disappear, males would be infertile.  Only time will tell if humans will see radical reproduction reform.

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