Genes can affect behaviour. In some cases we can begin to understand why because the gene in question is responsible for the production of a neurotransmitter or receptor. Other cases are so bizarre that we cannot even begin to guess what is the underlying cause. The symptoms vary from no more than minor patterns of repetitive behaviour, motor ticks, to the most notorious symptom, an uncontrollable involuntary urge to utter swearwords and obscenities. They have an exaggerated startle reflex and will jump to obey any abruptly uttered command however dangerous or foolish it may be.
Monoamine oxidase is an enzyme which acts to destroy certain neurotransmitters such as dopamine.
Ultimately, because almost all genes code for enzyme products, mutant genes give rise to their effects through the altered action of an enzyme. In some of the cases above we can guess what the responsible enzyme might be but in others we have no idea. In the cases below we have a clear understanding of the biochemical defect. In some cases this has led to either a cure or an alleviation. Phenylketonuria - is caused by the absence of an enzyme which converts phenylalanine to tyrosine.