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| Genetic History The cause of word-blindness has been assumed to be due to hereditary transmission. In 1950, Halgren studied the family histories of approximately 300 children who displayed substantial learning deficits. He found that 88 percent of the children had an immediate family member with a reading disorder, and therefore suggested it was a hereditary trait. Evidence continued to build. In 1959, Hermann conducted three studies on dyslexia in identical and fraternal twins. The studies showed a 100 percent correlation in identical twins, while only a 33 percent correlation among fraternal twins. These studies led Hermann to redefine dyslexia as, "a defective capacity for acquiring, at the normal time, a proficiency in reading and writing, corresponding to average performance." He also concluded that the deficiency is hereditary, is often accompanied by difficulties with other symbols, and exists in absence of intellectual defect of the sensory organs. This might retard the normal accomplishment of those skills, in the absence of past or present appreciable inhibitory influences in the internal and external environments. (Piorzzado, Wittrock) Genetic Evidence There are three main research areas: familial studies, linkage/heterogeneity studies, and behavioral genetic studies of the reading process. Also, the major research stemmed from the data sets in the Colorado Research Project in 1985. Some of the individual projects analyzed a simple Mendelian trait like autosomal dominance while other projects analyzed potential heritability of reading behaviors or QTLs possibly involved in RD. (For a summary of the results, see table at bottom of page). Familial Studies The main project in this research area is DeFries et al. twin study to determine the heritability of cotwins for RD. Most of this data set was re analyzed in other projects found in this paper. Originally, DeFries studied identical versus fraternal twins, cotwins, cotwins of identical versus fraternal twins, and twins versus nontwins. Cotwins were the other twin that was undiagnosed for a RD which assumes that the "main" twin was diagnosed with dyslexia prior to the study. A problem with the sample groups was the age difference to compensate for the RD groups� preliminary test scores; therefore, this study used younger children with similar scores to the children with RD for the control group. The results for the cotwins of identical versus fraternal twins showed a 73% heritability for identical cotwins and 47% heritability for fraternal cotwins (DeFries et al., 1991). To support these findings, Stevenson's study is mentioned in another of DeFries's articles (DeFries & Alarcon, 1996), with his project finding 68% heritability for identical twins and 38% heritability for fraternal twins. Thus, showing a high heritability within cotwins and twins which suggests a high heritability for individual differences in RD for the entire population. |
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