Wiring the Brain

Following a stimulating discussion with Jon Brock on the variability of the phenotype in autism and dyslexia, I thought I would explore a little more the influence of randomness on the phenotypic expression of mutations affecting neurodevelopmental processes. I have written about this before , in general terms, but here want to discuss one particular aspect – how probabilistic inheritance of a defect at a cell biological level is played out across the brain and how this influences the emergent phenotype of the individual.   Developmental neurobiologists are well used to the scenario where mutation of a gene leads to an anatomical defect, but only some of the time. Depending on the scale at which the defect is defined that can mean “in only some individuals” – for example, whether the connections between the two hemispheres of the brain form.   In genetically identical people (or animals) carrying a mutation affecting this process these connections sometimes for