Pierre L. Roubertoux, CNRS – Université de la Méditerranée, Marseille, France
Genes modulate brain functioning and behavioural outputs. Replicated studies have uncovered human genes linked with anxiety, cognitive processes and certain psychiatric disorders. Studies of the mouse have found more than 2000 genes to be linked to behavioural phenomena [motor, emotional and social behaviour], and cognitive performance. Given advances in proteomics, and the development of gene targeting, gene over-expression and fine mapping, by 2010, some 10 000 mouse genes will be identified and linked to behavioural traits.
When seen in relation to the findings of genome sequencing, these results present a paradox. There are no more than 30 000 genes in mammalian genomes, and it is generally assumed that no more than 20% of all genes are expressed in the brain; this would mean that 6000 genes could play a role in brain function and behavioural output. It has also been established that half of these 6000 genes code for proteins needed for sensory functions. The logical conclusion is then quite simply that there is no room left for all the genes involved in behaviour. How can this paradox be resolved?
After sequencing the human genome, scientists believed it would be possible to draw up a list of diseases, morphological characteristics and behavioural traits linked to each gene, but the post-genome era has shown that while links between genes and phenotypes, including behavioural phenotypes, do exist, they are more complex than was previously thought. There is no linear connection between genotype and brain and between brain and behaviour; consequently, genomic and behavioural 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 occur 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.
Post-genomic discoveries have forced us to reconsider approaches establishing links between genes and behavioural phenomena and therefore to revisit the phenomenological classification of behaviour, including psychiatric classifications, "positive and negative" eugenics, and the genetic approach to natural selection. |
