The "warrior gene" is a genetic variant of the monoamine oxidase A (MAOA) gene. It was named due to its association with aggressive behaviour, and is also associated with various other characteristics.
Monoamine oxidase A is an enzyme that breaks down neurotransmitters – chemical messengers involved in transmitting messages from the brain to the rest of the body. Dopamine, norepinephrine and epinephrine are a few examples of such neurotransmitters. Dopamine controls sensations involved in pleasure and reward and epinephrine and norepinephrine influence the flight-or-fight response.
The level of the monoamine oxidase A enzyme is controlled by the number of repeats of a 30-base pair segment in a regulatory region of the MAOA gene. Individuals with 3.5 or 4 repeats, have the MAOA-H allele and make high levels of monoamine oxidase A. Individuals with 2, 3, or 5 repeats have the MAOA-L allele and only produce low levels of monoamine oxidase A; hence the neurotransmitters are not broken down efficiently. The MAOA-L alleles are known as the "warrior gene".
The MAOA gene is located on the X-chromosome. Males are more likely to display the characteristics associated with the "warrior gene", as males only inherit one X chromosome. Females inherit two X chromosomes and the way that the warrior gene affects women is not well understood.