Why Study Twins

Identical and fraternal twins are uniquely informative about the relative contributions from genes and environment to individual differences in reading across the normal range, and to the severe reading deficits found in dyslexia. Identical twin pairs are derived from the same sperm and egg (the egg is called a "zygote," so identical twins are "Monozygotic" or MZ). Therefore, MZ twins share all their genes. Fraternal twins develop from two different sperm-egg fertilizations (i.e., they are "Dizygotic" or DZ). Therefore, they share, on average, half of their segregating genes (the small minority of genes that vary across individuals). Thus, DZ twins have the same degree of average genetic similarity as ordinary brothers and sisters, but like the MZ twins, they share their intrauterine environment, are born at the same time, and then share their family environment.

Of course growing up in the same family, schools, and community does not mean that the environments are the same for twins in a pair. Children help to create their own environments within and outside the family. DZ twins' different genes might lead them to greater differences in the way they interact with and select their environments, compared to MZ twins with identical genes. For example, one twin of a DZ pair might have normal genes related to reading development, while the other has genes that are related to dyslexia. The twin with genes related to dyslexia might have greater problems in learning to read, experience less pleasure in reading, and ultimately choose to read less than the twin who has normal or superior genes related to reading development. Thus, the hypothetical dyslexic twin's smaller amount of reading practice may contribute to their reading failure. However, less reading practice is not the only cause in most cases of dyslexia. Children with dyslexia typically require far more reading practice than normally developing children to reach a normal reading level.

Comparisons of large groups of MZ and DZ twins can yield quantitative estimates of the relative influence on individual differences in the population from genetic, shared environment, and nonshared environment factors. The detailed mathematical procedures are beyond the scope of this chapter, but a few points may help the reader understand the basic logic of the analyses. MZ twins share their genes and their family/school environment, so any differences within MZ twin pairs is logically due to nonshared environment influences. Such influences could include differential stress during gestation, accidents, or disease after birth, and unusual differences in educational opportunity. DZ twins within a pair will also have nonshared environmental influences, but in addition, they may have genetic differences relevant to the studied behavior. Thus, any greater behavioral differences between DZ twins compared to MZ twins may be due to genetic factors. In this chapter, we are focusing on the genetic contributions to extreme reading deficits or dyslexia. Thus, if MZ and DZ twins shared dyslexia equally often, we would conclude that there is no evidence for genetic influence in the population. In contrast, if nearly all MZ twins shared the disorder (thus with little influence from nonshared environmental factors), and DZ twins shared dyslexia in nearly half the pairs (remember that DZ twins share half their segregating genes on average), we would conclude that genes accounted for most cases of dyslexia in the population.

It is important to emphasize that estimates of genetic influence from twin studies are for the average influence in the population, and not for any individual in the study. For example, we might say that 60% of the reading deficit in a group of twins is due to genetic influence, but this does not mean that the genetic influence is 60% for each individual. For some individuals in the group, there might have been little or no genetic influence on their reading deficit, while for others, the genetic influence might have been very strong. When we are ultimately able to identify specific genes related to dyslexia, we will have more information about genetic contributions in the individual case.

A second qualification of behavioral genetic estimates of heritability or genetic influence is that the proportion of genetic influence in the population depends on the range of environmental variation in the population. If there are vast differences in schooling and cultural support for literacy in a population, environmental factors will account for most of the variation in literacy and dyslexia. In contrast, if schooling and cultural support for literacy are relatively constant across the population, genes are likely to play a proportionally greater role for dyslexia and for individual differences across the normal range.

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Adult Dyslexia

Adult Dyslexia

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