2.5.2 Genetic explanations
Earliest investigators noted that dyslexia tends to run in families, and studies involving extended families or twins have confirmed this. The heritability of dyslexia is estimated at around 50 per cent ‘about half of the variability in dyslexic traits found in the general population could be attributable to genetic variation’. However, the mode of inheritance is not known, and as with most behaviourally defined conditions, genetic studies of dyslexia are complicated by a number of factors. The most obvious of these is the difficulty in arriving at a satisfactory definition of dyslexia, as we have seen in Section 1. Another complication arises when individuals with the genetic tendency for dyslexia fail to develop the condition, or when individuals without the genetic predisposition show problems that resemble dyslexia. Moreover, the same condition in different people may each be ‘caused’ by a different gene. All these factors can make heritability estimates unreliable. It is most likely that a combination of different genes are involved in dyslexia.
Why there can be no such thing as ‘a gene for dyslexia’?
In terms of human evolution, reading is a recent acquisition. The demand for literacy has largely arisen within the last 100 years, and is still confined to ‘developed’ societies. We can therefore be confident that no specific neural mechanism can have evolved for dealing with written language. Skilled reading is also a highly complex process, drawing on a wide range of abilities and requiring integration across many different domains of perception and cognition. Whatever genes are involved, they may include those associated with the structures necessary for language development in general, certain aspects of visual perception and cross-modal integration. Given the wide range of component processes involved, it seems evident that variation in reading ability (to the extent that this is genetically determined at all) will depend on interactions between many different genes. At most, what is inherited is a predisposition towards reading difficulties, but to explore this further we need to understand much more about the actual brain processes involved in such difficulties.
Box 10: Definitions
Highly unsaturated fatty acids: Lipid molecules which make up 20 per cent of the brain's dry weight, and are crucial for normal brain development and function.