This is a defense that is asserted in Criminal law. It is based on a theory that a defendant's XYY chromosomal abnormality is a condition that should relieve him or her of legal responsibility for his or her criminal act.
The male defendant takes a defense that his criminal behavior is due to the genetic abnormality of having an extra Y chromosome which causes him to have uncontrollable aggressive impulses. However, the defense has never been successfully used in the United States. Though the abnormality can be easily diagnosed using a blood test, the courts have rejected the defense as its scientific foundations are uncertain.