 The contribution of gene – environment interaction to reading disability has been intensely studied using twin studies , which estimate the proportion of variance associated with a person 's environment and the proportion associated with their genes . Studies examining the influence of environmental factors such as parental education and teacher quality have determined that genetics have greater influence in supportive , rather than less optimal , environments . However , more optimal conditions may just allow those genetic risk factors to account for more of the variance in outcome because the environmental risk factors have been minimized . As environment plays a large role in learning and memory , it is likely that epigenetic modifications play an important role in reading ability . Animal experiments and measures of gene expression and methylation in the human periphery are used to study epigenetic processes ; however , both types of study have many limitations in the extrapolation of results for application to the human brain . 
