It's my understanding that the old idea of "one gene, one protein" is no longer valid. In my personal experience with several genes, it has been well established that the gene product (a protein) is further chopped into smaller, and therefore different, proteins or polypeptides. Futhermore, different cells in the same organism may process the same protein into different functional pieces, depending on the needs of that cell. Also consider that many proteins are composed of several different proteins (gene products) that interact to form the quaternary structure of the protein. The absence of one of these proteins would therefore prevent the formation of the larger protein. The path from gene to protein is quite complex in itself. The path from gene to behavior must be vastly more complex.