Parents ask me this question frequently, but to answer it accurately a brief review of genetics helps. Most people believe that a trait like eye color or a disease that is “genetic” is passed down directly from parent to child (if you have the gene you have the problem). This form of heritability is referred to as a dominant transmission. Dyslexia, and for that matter the vast majority of the developmental problems children have, dominant transmission is exceedingly rare. It is more like eye color (where at least eight genes are involved in determining your child’s eye color). There are rare families where dyslexia has been shown to have a more dominant-like sort of transmission, but rare is the key word.
Dyslexia from a genetic model is often referred to a multifactorial or polygenetic mode of transmission again in the vast majority of families. This means that for children to have dyslexia multiple gene anomalies (i.e., glitches) have to be present. It is also important to understand that gene glitches produce a liability (a risk) for the reading problem that can be affected positively or negatively by the child’s environment (the big one there is quality of early reading instruction).
Thus, a child with a significant liability for dyslexia (multiple genes with glitches) who receives poor quality reading instruction is most likely to develop dyslexia.
Studies involving twins are an important method in genetic heritability research. Remember that identical twins (called monozygotic twins) have 100% overlap in gene copies were as fraternal twins (called dyzogotic twins) share 50% overlap in gene copies. By studying twins who have grown up together and apart we can determine the heritability of dyslexia. In several large studies researchers find that reading disorder is about 60-70% heritable and environmental factors account for about 30% of the expression of the disorder (about 10% of the heritability is unknown). Heritability also increases a lot in children of higher socioeconomic backgrounds and decreases in children who come from poverty or from less educated parents (for example in twins of well-educated parents the heritability is about 70% and from less well educated parents about 50%).
Lastly, there are several genes that have been identified in dyslexia. The names and location of these genes are less important as to an understanding of what they all seem to do. The majority of genes identified in dyslexia affect something called neuronal migration. This is a process where genes direct the formation and location of synapses across the brain. What researchers have found is that this process is misdirected and less well organized. In uncommon cases, clumps of brain matter don’t form at all.
There are some important take homes for parents. Yes dyslexia is “genetic” and if you have one child with dyslexia, your other children are likely at higher risk. So in those children keep an eye out for early reading problems and intervene early. Remember the research shows that in most children the quality of reading instruction is just as important as the genes, including those you passed down for his/her beautiful eyes.