Biosocial criminologist Kevin Beaver and his colleagues at Florida State University have a new paper that looks at the link between genetics and violence. According to their study, boys who have a particular variation of the gene Monoamine oxidase A (MAOA) are more likely to join gangs, behave violently, and use weapons. (While previous studies have linked this variant to violent behavior, this is the first to show it can predict gang membership.) The findings do not apply to girls who have the same variation of the gene, however.
Beaver explains why:

What's interesting about the MAOA gene is its location on the X-chromosome. As a result, males, who have one X-chromosome and one Y-chromosome, possess only one copy of this gene, while females, who have two X-chromosomes, carry two. Thus, if a male has an allele (variant) for the MAOA gene that is linked to violence, there isn't another copy to counteract it. Females, in contrast, have two copies, so even if they have one risk allele, they have another that could compensate for it. That's why most MAOA research has focused on males, and probably why the MAOA effect has, for the most part, only been detected in males.