What does scientific research say about whether dyslexia is inherited? The answer to this question requires an understanding of some background issues. First, genes rarely dictate that a particular characteristic is certain to be passed from one related person to another. Rather, shared genes most often increase the probability that a trait will be passed from one family member to another. Second, the probability that a trait is influenced by genetics is usually expressed by a heritability index. A heritability index of 1.0 would indicate that a family member who possessed a trait would be certain to pass that trait along to his or her progeny. In contrast, an index of 0.0 would indicate that genetics plays no role whatsoever in determining the existence of a particular trait among family members. There are virtually no circumstances where heritability indices of 1.0 or 0.0 are actually found in research so the majority of the indices actually found fall between 1 and 0.

There are two classic methods for estimating the influence of genetics and the environment on the development of dyslexia. The first is the study of identical and fraternal twins. Identical twins, of course, have exactly the same genetic makeup. Fraternal twins share only half of their genes. However, both identical and fraternal twins most often share the same environment. If genetics is most important in determining dyslexia then identical twins should have a higher heritability index than fraternal twins. In contrast, if environment is most important there should be little difference between the heritability index of identical and fraternal twins.

The three largest twin studies are those conducted in Colorado, in London, and in England and Wales (the studies are still ongoing). The typical methodology in the studies is that the researchers are notified of the existence of twin children in a particular geographical area and they then recruit the children as participants in the research study. At the same time the researchers recruit a matched sample (i.e., same age, same sex) of control children from schools the twins attend. A battery of tests are then administered to all of the children over a number of years. These tests allow the determination of the extent to which genetics and the environment are related to the development of reading skills.
The twin studies from the three major research projects yielded very similar results. The heritability index for reading skills was about .65 whereas the environment index was about .25.

The second classic method for examining the heritability of dyslexia involves the recruitment of participating families. One set of families (called at risk families) have at least one parent that has dyslexia. The second set of families (control families) are recruited to be as similar as possible to at risk families, but do not have a parent who has dyslexia. Again, the procedure is to administer tests to children in the families that will allow a determination of the extent to which reading development is proceeding normally.

The results of the family studies, like the twin studies, showed a substantial contribution of genetics in the development of reading skills, and only a moderate influence of shared environment.

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