Follow this rule:
One gene one peptide.
From there multiple genes may work together to make multiple peptides that create a polypeptide protein or otherwise work independently to cause an expression of a single thing. It depends on what you are looking at. You could have a variety of different things going on such as epistasis or even genes working together to regulate one another and cause a finished product. In this topic there is no general umbrella you can put it all into.