levels of serotonin, there is an observed increase in impulsivity and depression With neurotransmitters playing a central role in the development of aggressive behaviour, it follows that many of the gene mutations that have been implicated with aggressive behaviours are involved in the breakdown and/or receipt of neurotransmitters. Alexis Edwards and her team identified 59 mutations in 57 genes that affected aggressive behaviour in Drosophila melanogaster. The results of their research showed that 32 of the mutants displayed increased aggression and 27 of the mutants displayed less aggression than the control group. Several of the genes examined were found to affect nervous system development and function. Aggression was assessed in this experiment by depriving mutant Drosophila flies from food and then allowing them to defend a limited food source. The number of contest competitions between flies was recorded and compared to non-mutant flies to assess whether the mutants were more or less aggressive than the wild type. Examples of mutations that increase aggression are mutations in the fruitless or dissatisfaction genes which result in observable increases in male-male aggression. Amines Mutations involving amines have been shown to be a prevalent source of changes in behaviour. A point mutation in the structural gene for Monoamine Oxidase A, also known as MAO-A, is responsible for the breakdown of neurotransmitters. This mutation is X-linked, affecting only males, and eliminates the production of MAO-A. Males afflicted with this mutation are prone to mild mental retardation as well as violent and antisocial behaviour. Another amine affecting aggression is β-alanine which is a bioamine neurotransmitter that has been implicated in Drosophila aggression. A mutation known as the black mutation causes reduced levels of β-alanine and results in less reactive flies than the wild type. Testosterone In nearly all species, there is an obvious disequilibrium between frequency and severity of aggression in males versus females. Males are almost always the more aggressive sex and there are genetic differences that back up this observation. A common explanation for this phenomenon is the higher testosterone levels in males. Testosterone levels have a direct effect on neurotransmitter functioning contributing to physical aggression. Mutations affecting neurotransmitters, as stated above, are the dominant cause of changes in aggressive behaviour. Another contributor to the unequal male-female aggression ratio are the sex-linked gene mutations that affect only male behaviour, such as MAO-A mentioned above. These mutations could be the reason why males are nearly