25 September 2010

Beyond XX and XY

Since 1905, when Nettie Stevens and E.B. Wilson first identified the XY sex-determination system found in humans and many other species, it has been recognized that males and females differ in their sex chromosome arrangement. While males are heterogamic with two distinctive sex chromosomes (XY), females are homogamic with two of the same type of chromosome (XX). In approximately one out of 160 live births however, chromosomal anomalies occur with children being born with an abnormal number of chromosomes (aneuploidy). Among the more well-known sex-chromosomal disorders including Turner Syndrome (single X chromosome), Klinefelter's Syndrome (XXY syndrome), and Triple-X Syndrome (XXX chromosome karyotype).

When the 47,XYY karyotype was first identified in 1959, geneticists questioned whether it could be considered a disorder since there were no clear indications of pathology.

Most XYY males develop normally and have no idea that their genetic structure is abnormal. Although research into XYY males has shown no significant difference in testosterone levels, differences have been found on other physical characteristics including physical height, impulsivity, short-temperedness, and enuresis. XYY males are also more prone to certain types of learning difficulties as well as delayed speech and language skills. . . . research into the prevalence of XYY males in forensic populations has not yielded consistent evidence of a clear role for the karyotype in violent behaviour. Given that most XYY males in the general population never commit violent crimes (and, in fact, often remain unaware of their chromosomal abnormality), critics of research focusing on incarcerated males point out the obvious selection bias at work. These same critics also argued that XYY research led to potential stigmatizing of XYY males. While the controversy rages on, researchers continue to report significant findings for XYY males in terms of increased mortality, physical height, neurocognitive deficits and autism. Although behavioural differences are still being reported (including incidence of sexual violence), no clear consensus on the XYY-violence link seems likely to emerge at this point.

From here.

Notably, none of the most common sex chromosome anomolies have any clear link to sexual orientation.

Indeed, while being one sex chromosome short (as in Turner syndrome) produces some definitely noteworthy symptoms, an extra sex chromosome's effects are quite subtle, with many most clearly impairing fertility. Some of the symptoms associated with extra sex chromosomes in individuals with Y chromosomes seems very similar to conditions like ADHD, which are not uncommon in males generally.

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