Phenotypic variation is ubiquitous. Diversity in behavioral response within a single genotype is ubiquitous. Sources, mechanisms and innovations in variation within a genotype are hardly understood. In neuroscience, the problem of genotype-phenotype relationship is notoriously difficult due to many intermediate layers that mediate between the DNA, the physical structures of cells, and the dynamics of neuroglial and vascular interactions in development and response to environmental stimuli. Neuroangiogenesis in mice retina provides a mechanism that lends itself to detailed study of the relationship between genotype and phenotype.