Sex Chromosome Cross Over May Occur More Often Than Previously Thought

Sex chromosomes undergo division like all other cells but something makes them stand out from other chromosomes. Sex chromosomes X and Y switch only pieces of themselves when they cross over. It is this phenomenon that is responsible for individual variations between different people, even siblings.

A recent study done at the Arizona State University was led by Melissa Wilson Sayres, assistant professor in the School of Life Sciences and member of the Biodesign Institute's Center for Evolution and Medicine. This study was then published in the early online edition of the journal Genetics. It found out that the process of crossing over occurs much more often than was previously thought. The study has helped answer crucial questions about the diversity of the human race and sex chromosomal disorders.

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The research team analyzed the DNA sequences of X chromosomes taken from 26 unrelated females. They found out that the genetic diversity was far greater in a certain area of the sex chromosomes named PAR1 (the region of X chromosome that crosses over) as compared to the other regions of the X chromosome.

It was expected that this greater range of diversity in the PAR1 region of X chromosome would suddenly drop “like a cliff” upon transition with other regions. What the researchers observed was that this genetic diversity had a slow “rolling hill” pattern. This transition area was found to be quite fuzzy and might be responsible for various sex linked disorders.

Previously, it was thought that there is a strict recombination boundary that put a limit on the swapping between the X and Y. Contrary to the popular belief, the evolution of the human genome over millions of years resulted in bit by bit “inversion” of the Y chromosome. Over time, the sex determining region of the Y chromosome (SRY), that determines the gender of an individual, has come to lie right next to the boundary.

The close proximity of SRY to the boundary region can result in hopping of the SRY over to the X chromosome which increases the chances of sex linked disorders, for example, de la Chappelle syndrome (SRY positive XX males), Turner syndrome (a single X chromosome in females rather than XX chromosomes), Klinefelter Syndrome (males with an extra X chromosomes-XXY), Swyer Syndrome (the person is genetically male with a weak XY chromosomal pattern but develops ovaries too).

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During the course of this study, the researchers found 24 additional genes located within PAR1 and numerous others near the PAR1 boundary. These genes are important regulators of bone growth and melatonin production. These genes are also profoundly associated with psychiatric disorders, including bipolar affective disorder

The Future Implications

This study has made the made the blurry lines of human sex chromosome crossing over even blurrier. This study has prompted further research about how this frequent cross over is contributing towards the prevalence of sex linked disorders in humans.

According to Melissa Wilson Sayres, understanding the evolution of the X and Y chromosomes is imperative in order to understand the differences in the genetics of sex determination.