A more thorough explanation:

Xylon (zI-lon), n. [from Greek xulon “wood”] is an archaic punishment apparatus similar to stocks.

XYY-chromosome defense is a criminal law defense, usually asserted as the basis for an insanity plea, whereby a male defendant argues that his criminal behavior is due to the genetic abnormality of having an extra Y chromosome, which causes him to have uncontrollable aggressive impulses.

For example, a man who commits a violent crime may claim that his extra Y chromosome caused him to act impulsively and uncontrollably. However, most courts have rejected this defense because its scientific foundations are uncertain.

As one commentator has suggested, an attorney defending an XYY individual will be required to call upon both a geneticist and a psychiatrist to give expert testimony. The geneticist's role would be to testify with respect to the individual's genetic structure, any distinguishing characteristics which are relevant to an insanity defense, and the result of family studies designated to determine the influence of genetics and environment on the development of this individual. The psychiatrist's testimony would focus upon the defendant's mental capacity or condition. But in the absence of sound medical support for an XYY defense, courts are understandably unsympathetic to defense efforts to obtain such expert testimony.