The effect of a neurotransmitter on a postsynaptic neuron is determined by the

The effect of a neurotransmitter on a postsynaptic neuron is determined by the RECEPTORS ON THE POSTSYNAPTIC MEMBRANE.
A receptor is a molecule, usually protein that is found on the surface of every cell membrane and which is responsible for receiving signals that are coming from the outside of a cell into the cell. When chemical signals bind to receptors they bring about a change in the activity of the cell depending on the effects they are suppose to exert on the cells.
Thus, the receptors on the postsynaptic membrane will determine the effect of a neurotransmitter on a postynpatic neuron.

Mutations are changes in the dna