As with mice, it appears that a major portion of the genetic contribution to human fear and anxiety involves neurotransmitters and their receptors, and again GABA and its receptors play a key role. But perhaps the most important neurotransmitter mediating anxiety in humans is serotonin. Variability in the receptors responsible for clearing serotonin from the synaptic space between two communicating neurons correlates quite well with variation in anxiety among different individuals. Anxiety is closely connected with depression in humans, and drugs that modulate serotonin levels in neuronal synapses also affect both depression and anxiety. Serious depression also has a marked genetic component.
Fear and anxiety are influenced by many genes; there is no such thing as a simple "fear" gene that is inherited from one generation to the next. The genes controlling neurotransmitters and their receptors are all present in several different forms in the general population. The particular combinations of these different forms that we receive from our parents will predispose us to respond with greater or lesser degrees of anxiety to events in our environment. But the degree to which our lives are affected by this inherited predisposition will depend to a very large extent on our individual histories--the number, strength, type and duration of events that elicit such reactions in the first place.