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6 May 2007
XYY - One Chromosome Too Many
by Paul Aitken

Anyone under 40 and not British is unlikely to remember the excellent BBC series XYY Man. Based on a series of books by Kenneth Royce, it tapped into a belief that males born with an extra helping of the Y chromosome were genetically predisposed towards criminality. The series' protagonist, Spider Scott, was destined to lead a life on the wrong side of the law, but British Intelligence recruited Spider, hoping to harness his natural criminal disposition as a force for ultimate good. But even in his capacity as an agent for the good guys, Spider still felt the relentless pull of his genetic predisposition towards the dark side.

The theory linking XYY to criminality developed partly out of expectation. Scientists were already aware that those men born with a double X chromosome (known as Klinefelter's syndrome) tended to develop feminine physical characteristics such as breast growth. When the XYY karotype was discovered in 1961, there was immediate speculation that the phenotype would be extra masculine. That is to say bigger, stronger and really, really disinclined to ask for help with directions. And few things are more masculine than criminal aggression. Only the most die-hard nurturist could fail to notice that, of the two genders, men are by orders of magnitude more likely to commit violent crimes.

Several studies conducted amongst prison populations in the mid-sixties concluded there was indeed a link. Early reports claimed that the prevalence of XYY men in prison was between 10 and 60 times higher than in the general population. These early studies were subject to bias and sampling errors, and later studies tended to contradict these findings but at the time they were largely accepted. So much so, that in 1968, the notorious nurse killer Richard Speck claimed as part of his defense that he was an XYY karotype. The defense failed on its merits and it was later determined that Speck was in fact a normal XY male. Nice try anyway, Dick.

But this sensational defense tactic helped popularize the notion that XYY males are natural born psychopaths, destined for jail or the gallows. Magazines speculated about famous killers in the past that may have been XYY. Out of this milieu of speculation and misinformation came the character of Spider Scott, XYY Man.

The idea that genetics is destiny, if it was ever seriously advanced, has long since been discredited. Our genetic make-up may incline us towards certain behaviors - having sex, eating fast food and going postal - but we don't have to follow through. The problem with the early studies is that they were conducted amongst prison populations so it was hardly surprising that the XYY males found there were (gasp!) criminals. But even if the original studies associating XYY men with criminal behavior were not flawed and/or erroneous, the inescapable conclusion would still have to be that most XYY men are not criminals. Or if they are, they haven't yet been caught.

Today, it's estimated that about 1-in-1,000 males is born with two Y chromosomes, known officially as the 47,XYY chromosome pattern. The "47" represents the total number of chromosomes (the usual number is 46).

The XYY chromosome karotype is normally the result of errors that occur in gametes (sex cells). This is when the pairs of chromosomes line up at the center of the cell before they detach and are pulled to opposite sides (just prior to cell division). Occasionally one of the Y chromosomes refuses to let go of the other and the whole pair gets pulled to the side. This leaves one cell with two Y chromosomes and the other with none. Because some of the instructions for sperm formation are located on the Y chromosome, the cell with no Y chromosome fails to complete spermatogenesis and dies. Those with two Ys will go on to compete in the sperm marathon and some will win, although there is some evidence that sperm with two Y chromosomes are handicapped in the fertilization sweepstakes.

XYY can also result from post-fertilization errors that occur after the egg has begun to divide, although this is much rarer. Depending on what point in development this error occurs some men develop a "mosaic" chromosome constitution; that is, some of their cells are XY and others are XYY. If the proportion of XYY cells is small, you may never feel the effects.

Any deviation from the standard allotment of chromosomes in a cell is referred to as aneuploidy. Here's a quick vocabulary lesson: One set of chromosomes (germ cells such as egg and sperm) - Haploid. Two sets of chromosomes (all the other cells in your body) - Diploid. All fucked up - Aneuploid.

Aneuploidy is actually not all that rare. You probably have a few kicking around your body right now. Aneuploids that result during the division of diploid cells can sometimes turn cancerous (cancerous cells are always aneuploid). But before you start fretting about which one of your several trillion cells is getting scrambled, it's worth remembering that most aneuploid cells are not viable and die before doing damage. This is true for most aneuploid germ cells as well. But for some reason, cells with extra X and Y chromosomes, as well as those with an extra chromosome 21 (Down's syndrome), can survive and thrive. Because the Y chromosomes are supplied exclusively by the father, maternal age has no effect on the likelihood of XYY. Paternal age doesn't seem to have much affect either.

Due to the widespread availability of amniocentesis, and the effectiveness of this technique in determining chromosomal disorders, there has arisen a great deal of controversy regarding the XYY phenotype. People are having fewer children and they don't want these children to be defective. But is XYY a defect? It's been fairly well established that XYY males are not the natural born criminals they were thought to be but that doesn't mean that extra Y chromosome doesn't have an effect.

While the numerous studies of XYY males undertaken over the last few decades have often been conflicting, there is a kind of consensus emerging. XYY males generally exhibit normal physical development until puberty, although they are prone to delays in speech development and score 10 to 15 points lower on IQ tests than their siblings. During adolescence their double dose of masculinity is manifested in increased height and aggressiveness. They're also susceptible to acne throughout their adult lives. Strangely, the sons of XYY males are no more likely to be XYY than those of the general population. Somehow the extra Y is eliminated during spermatogenesis, although why this should be is not clearly understood.

So, if an extra Y chromosomes pops up in the amniocentesis results what should you do? The answer to this depends largely on your religious and ethical leanings, but it may help to consider this: If big, dumb, aggressive men are to be considered defective, one would have to write-off just about every male between the ages of 14 and 21. I've been there, zits and all. It wasn't so bad. And although I wouldn't want the whole world to be populated by unruly teenage males, an extra one-in-a-thousand isn't going to hurt anybody.

Related articles:
Intersex - The Third Gender
Will Science Make Men An Endangered Species?
A Penis Doesn't Always Make A Man
Choosing Gender: Unnatural Selection?
Mr. Sperm Goes To War

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