Dyslexia is an SLD that encompasses a variety of symptoms.
Current research has been limited in its understanding of this disorder due to inconsistent terminology and changing definitions, and therefore, has not supported one mainstream theory.
Possible hypotheses suggest a genetic predisposition (heritability), neurochemical differences, and morphological anomalies (i.e. corpus callosum, magnocellular pathway, hemisphere symmetry, etc).
These causes may contain only a minor common thread. Thus, an analysis of case studies for common patterns and a compilation of these projects is still in progress, and so our understanding the biological evidence for SLDs is just beginning.
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)
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 StudiesThe 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 groupsÕ 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.
Another study done was a genealogical pedigree for one German family with the RD criteria partially based on the familyÕs self-assessment for dyslexia. The results suggested a QTL on Chromosomes 1 and 2 as a mutation through translocation. The problems with this study were the limited sample size, preliminary criteria including self-assessment without prior testing, and this paperÕs focus on Chromosome 6 and 15. Finally, the problem of being a study from Germany could incorporate cultural bias in defining dyslexia and that heritability may differ for each population and each environment. "In humans, however, it is very difficult to determine whether a particular trait is heritable." (Griffiths et al., 1996)
Linkage/heterogeneity StudiesThis paper's main focus was on Chromosome 6 and 15 due to the limited scope of evidence for inheriting RDs. Some of these studies, Rabin and Froster, were mentioned in the above Family section.
Some of the linkage analysis studies seeking a single-gene hypothesized that RDs are autosomal dominant as research begun by Hallgren in the 1950s (DeFries et al., 1991). Later, this study was revisited by Finucci in 1976 and Stevenson in 1987. Both these studies found that RD have more than one mode of transmission that depends on the family. For instance, Finucci found that these possible modes of transmission were either autosomal dominant, autosomal recessive, or multifactoral inheritance based on the complexities associated with defining SLDs and determining human heredity for behavioral traits. Stevenson also found that the inheritance of RDs was either autosomal dominant or a QTL on Chromosome 15. The problem with these studies was the use of HallgrenÕs data set from the 1950s when SLD was an unknown behavioral trait and the limited sample sizes. This limited sample size was problematic in StevensonÕs study due to only one of nine families showed the QTL on Chromosome 15 and the other families did not all demonstrate autosomal dominant inheritance. Pennington also studied autosomal dominance which was verified for only 20% of the families.
These studies of autosomal dominance started a debate on penetrance, especially for females. The debate was whether RDs were fully or partially penetrant but no study has shown that dyslexia is sex-linked. This lack of sex-linked data suggests that RDs are not a simple Mendelian trait and that the phenotypic variance of more males with RDs is probably a bias on the subjective diagnosis for SLD assessment.
A final note on Finucci's study was this research began further hypothesis on the possible heterogeneity of RDs and seeking QTLs on Chromosomes 1,6, and 15. This paper focused on Chromosomes 6 (C6) and 15 (C15) due to more research with less complexity as mentioned in the introduction. As previously mentioned, Stevenson found C15 as a QTL for one family. Another study like FinucciÕs was done in Denmark by Bisgaard with the first hypothesis being to study the heterogeneity of RDs through C15. The null hypothesis was accepted which led to the next hypothesis of homogeneity in RDs. yet, there were problems with this study having a cultural bias like Froster's study mentioned in the above section. Another problem with suggesting RDs being homogenous was if that meant to the RD population only or if this study was meant to represent the entire population; thus, another debate on which populations these studies were representing.Another C15 QTL study was done by Smith et al. that also considered C6. Again, this research showed only one family with C15 heritability while some of the other families showed C6 heritability. This study also had limited sample sizes and represented the complexities of human heredity especially in a controversially defined disorder such as dyslexia.
Behavioral genetic studiesTo understand the genetics of a behavioral trait, Olson's study has been the main research in reading processes. This study used DeFries twin studies to select RD groups to test for phonological and orthographic coding scores. The results of this study showed 47% of the twins had deficits in phonological coding (i.e. phonics) but only 28% had orthographic coding deficits (i.e. spelling). The problem with this study was the suggestion that these groups represented the entire population but other studies followed.
Another study was done by Plomin et al. with the additional consideration of segmental language skills (i.e. word recognition) for DeFries cotwins. The results for this study were 93% phonological genetic covariance to -16% orthographic coding. These twins had 54% word recognition and phonological coding deficits and only 28% had orthographic deficits. The low orthographic coding score showed a low heritability and genetic covariance suggestive of environmental factors.
One study that has great implications for future research was Cardon's study11 that found families with RDs had a QTL on C6. This finding suggests that C6 may be an area causing phonological coding deficits for families with this type of SLD. Yet, there is more reset to be done for understanding the other QTLs, C15 and Chromosome 1+2 (C1,2), for those families that may inherit this trait. Also, it is important not to exclude the possibility that a potential C6 parent may have a C15 child who may have a C1,2 child.
| Name | Study Area | Specifics | Findings |
| DeFries, Olson | family | twins-id./frat. cot. | 73% id. cot. herit. 47% frat. cot. herit. |
| DeFries, Alarcon | family | twins-id/frat | 68% id. cot. herit. 38% frat. cot. herit. |
| Rabin, Feldman | family | sib. pair | QTL C15, C1 |
| Froster, Schulte-Korne | family | sib. pair | QTL C1+2 |
| Hallgren (in DeFries) | linkage | Mendelian | autosomal dom. |
| Finucci | linkage | modes of transmit homog. or heterog. | autosomal rec. or dom. multifactoral |
| Stevenson | linkage | modes of transmit homog. or heterog. | autosomal dom. or QTL C15 |
| Pennington | linkage | Mendelian | autosomal dom. 20% |
| Bisgaard | linkage | heterog. | QTL C15->homog.>aut. dom. |
| Smith | linkage | heterog. | QTL C6 |
| Olson (with DeFries) | behavioral | coding deficits | 47% twins phonological 28% twins orthographic |
| Plomin, McClearn | behavioral | segmental language | 54% twins phonological 28% twings orthographic |
| Gayan, Cardon | behavioral | QTL and coding deficits | QTL C6+phonological family |
| id. : identical frat. : fraternal cot. : cotwins herit. : heritability sib. : sibling dom. : dominant rec. : recessive homog. : homogeous heterog. : heterogeous |
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