"Because humans follow a Lygaean reproduction model as opposed to the Protenor model, an individual with a YY genotype cannot be naturally made. There are cases in humans where the genotypes go beyond XY and XX (there exists XXY, XXX, XXXX- the problem is that due to a concept called X-inactivation of Barr bodies, every X except those necessary for sex determination are inactivated). There does exist however, in RARE cases, the XYY genotype in males. I will get into that in a moment. It is also worth noting that strictly YY genotypes exist in other organisms, but not in humans.
As I'm sure you know, under normal circumstances, females have an XX genotype and males have an XY genotype, where they inherit an X from their mother, and the Y from their fathers. XYY genotypes (scroll down to the bottom) result from a phenomenon known as aneuploidy, or the loss or addition of an extra chromosome. It's a random event that happens during the formation of sperm cells, where an extra Y chromosome is produced on accident (nondisjunction). If this happens to be the sperm that fertilizes the egg, then this is the genotype that develops. However, it has been suggested these men are handicapped in several ways in that they are usually of stunted intelligence and prone to aggressive behavior. There is not yet a definitive correlation."
