After sequencing the human genome, scientists believed it would be possible to draw up a list of diseases, morphological characteristics and behavioral traits linked to each gene, but the post-genome era has shown that while links between genes and phenotypes, including behavioral phenotypes, do exist, they are more complex than was previously thought. There is no linear connection between genotype and brain and between brain and behavior; consequently, genomic and behavioral levels of organization are not isomorphous. There is no isomorphism because one gene plays many different roles, which means that the integrative processes needed for the development and functioning of an organism inevitably occurs in situations of non-linear multiple causality. Pleiotropy and epistasis, interactions between genes and the environment, alternative splicing and neuronal integration are all crucial mechanisms contributing to the many and varied aspects of brain-related genes. (C) 2011 Elsevier Ltd. All rights reserved.