It has been 100 years since the first case of developmental dyslexia was described. Hitherto numerous researches had shed light on the causes and consequences of this disorder but the debate concerning its definition is still highly contented. In this essay, I will first answer the question of what is dyslexia and then move to the debate of whether it has a genetic basis.
- Firstly, I will introduce what has been done in the early research of dyslexia and then tried to find a definition for us to understand dyslexia properly. Secondly, I will introduce the research for supporting the view that dyslexia has a genetic basis.
1.1 Early history of research on dyslexia
Reading, a complex behavior that requires a set of cognitive skills, has been highly valued by society and is a key component to education. An inability to read has profound social and psychological consequences. Several scholars in the 19th century studied the loss of the ability to read or understand writing.
Kurrmaul in 1877 describe the reading difficulties of literate brain-damaged patients as word-blindness. It is only in 1887 that a German ophthalmologist, R Berlin, first used the word ‘dyslexia’ to describe reading difficulties caused by cerebral disease or injury. However, having read articles published by Hinshelwood in the 1890s and early 1900s, W.P Morgan (1895) points out that a patient can be suffering from ‘dyslexia’ without cerebral disease or injury. He quotes the case of a boy who has reading difficulties even though he has suffered no apparent brain damage. Though for a long time, the problem of dyslexia is widely studied, dyslexia was not a common knowledge for more than half a century and the concept of dyslexia was not familiar and unclear to many people. People need to understand what dyslexia is in order to help ones inflicted with the disease.
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1.2 The definition of dyslexia
Dyslexia is a neurological disorder with a genetic origin and behavioral signs which extend beyond problems of written language. Early research confined the dyslexia into a medical model and thus clinical practice fail to distinguish a dyslexia patient from normal readers. In the 1968 World Federation of Neurology meeting, a definition of dyslexia formally introduced and stated that dyslexia is a disorder manifested by difficulty in learning to read despite conventional instruction, adequate intelligence and socio-cultural opportunity. It is dependent upon fundamental cognitive disabilities which are mostly of constitutional origin. Many scholars criticize this definition. Firstly, the terms are vague as there are insufficient examples to illustrate conventional instruction or to point out the criteria of adequate intelligence and to explain the meaning of socio-cultural opportunity. These scholars point out that the biggest weakness of this definition is exclusion. The definition only states what a person with dyslexia should not be and does not include criteria for its positive diagnosis other than to state that it is a reading difficulty dependent on fundamental cognitive disabilities (Snowling, 2004). Although its definition debatable, it is applied by the diagnostic and statistical manual of mental disorders and the international classification of mental and behavioral disorders for many years.
Researchers have never stop on the medical model. Without positive diagnosis criterion, doctors cannot differentiate children with specific reading difficulties and children who have reading difficulties because of a more general learning problems. Scholars have adopted tests through the comparisons of verbal IQ (intelligence quotient) and performance on reading tests of children with reading retardation and skilled reader in a hope that it could identify the children with dyslexia. Nonetheless, a number of findings such as Morton and Frith (1995) highlighted that it is not correct to assume that literacy problems are the only symptoms of dyslexia. These tests are purely behavioral definitions and the diagnosis is relative. For instance, there are many examples that show discrepancies with the predictions carried out by those researches. Some dyslexia children after receiving highly effective training in decoding non-words would score well and many children with reading problems can improve their reading ability by having a better relationship with their teachers. Overly depending on these tests as a short cut to diagnosis would run the risk of excluding dyslexic children with reading problems and involve children who only show mild positive signs of dyslexia. The definition that concerns dyslexia as synonymous with specific reading difficulty has failed to be self evidence because it only focuses solely on reading and IQ-test performance and other tests.
One way out of this dilemma is to consider dyslexia as a disorder that has multi-levels of description. Rutter and Yule (1975) pointed out that the specific reading retardation is usually multi-factorially determined opposed to the claim that dyslexia is a unitary condition. Being a developmental disorder, dyslexia can be expected to have behavioral features that will change with maturation and response to environmental interactions (cf.Bishop, 1997). It may therefore be unrealistic to agree upon a simple and unchanging definition of dyslexia. Frith (1997) argued that there are causal links from brain to mind to behavior that must be considered when attempting to understand dyslexia. It is important to seek explanations at the three different levels in this causal chain namely the biological, the cognitive and the behavioral, in order to develop a comprehensive theory of why some children fail ‘unexpectedly&tsquo; to learn to read(Morton and Frith, 1995). Moreover, the environmental factors will act as a stimulus to intensify or meliorate the condition of these three levels.
The common ground of the study of dyslexia, agrees that dyslexia is a neuro-development disorder with a biological origin and behavioral signs which extend far beyond problems of written language (Frith, 1997). The idea of dyslexia as a syndrome with a neurological basis springs from the work of Tim Miles, Elaine Miles and many intelligent students. It helps to solve the paradoxes that exist in defining dyslexia. Morton and Frith (1997) had developed a framework with three levels and environmental influence in a neutral view to describe a descriptive definition of dyslexia.
The past 15 years have seen a continuing increase in research effort aimed at identifying the biological underpinnings of dyslexia. Galaburda (1989) demonstrated abnormal symmetry in the structure of the planum temporal; Livingstone et al. (1991) identified cellular migration abnormalities in the magnocellular system of the brain which have been related to behavioral findings by Cornelissen et al.(1995). Genetic linkage studies with dyslexic families have identified regions on chromosomes 15, 1 and recently 6 (Cardon et al., 1994). Thus, restricting the discussion to behavioral observation is no longer necessary. This gap has been widened by cognitive neuro-science which insists that there is a space for the scientific study of the mind and brain and not just behavior (Frith, 1995). Cognitive level of explanation can be a bridge that links brain and behavior together. Cognitive abilities can be explained by Cognitive theories through observable behavior. The poor reading performance can be termed as a cognitive dysfunction which in turn can be explained by a brain dysfunction. In addition, this causal links chains from brain to mind to behavior has to be set within the context of environmental and cultural influences.
Figure2(Mortan and Frith 1995)
An illustration of the causal modeling of dyslexia with the hypothesis of a phonological deficit hypothesis now shows in figure 2. In this figure, Morton and Frith in 1995 argued that when we try to explain a developmental disorder, we have to make a distinction between different levels of description. In the biological level and environmental level, we can look for causes and cures and in the behavioral level we can observe and assess the patient. Then the cognitive level lies in between these levels and have links with the rest of the levels. Here, the intuitive clinical impression can be captured and that the presenting disorder is a distinct and recognizable entity despite variable symptoms. This notation enables different theories about a disorder to be represented in a neutral fashion (Frith, 1995). The proposal of a phonological deficit as the cognitive basis of dyslexia has a strong theoretical and empirical support that it has been widely accepted. Starting on the biological level of figure 2, it is supposed that there is a congenital dysfunction of left-hemisphere perisylvian brain areas which affects phonological processing (Galaburda, 1989; Paulesu et al., 1996; Rumsey et al., 1992). Furthermore, the evidence for a genetic origin of dyslexia is increasingly compelling (Pennington, 1990). However, this theory also has its pitfalls. One of the biggest weakness of the phonological theory is it does not effectively explain the occurrence of sensory and motor disorders in dyslexic individuals. People who support the phonological theory typically have dismissed these disorders as not part of the core features of dyslexia. They consider their co-occurrence with the phonological deficit as potential markers of dyslexia instead of treating them as a causal role in the aetiology of reading impairment (Snowling, 2000).
In the domain of neauro-cognitive causes study of dyslexia, there are two other theories: the cerebellar theory and the magnocellular theory. The former one is that the dyslexic’s cerebellum is mildly dysfunctional and that a number of cognitive difficulties ensue, whereas the latter one postulates that the magnocellular dysfunction is no restricted to the visual pathways but is generalized to all modalities (Ramus et al, 2003). These three theories do not contradicted each other but potentially compatible. When it refers to the cognitive level, three theories imply a processing deficit. Fast temporal processing may be a basic characteristic of all perceptual systems, visual as well as auditory, object-based as well as speech-based. On the other hand, the slower-than- normal perceptual processing might affect the development of a phonological system (Frith, 1997).
Any definition should be seen as a hypothesis and to be rejected if future findings disprove it. As Tim Miles said that, a diagnosis of dyslexia is, in effect, a sort of bet. The definition in the framework of biological, cognitive and behavioral level within the interaction of cultural influences depict the dyslexia as a neuro-developmental disorder with a biological origin, which affects speech processing with a range of clinical manifestations (Frith, 1997). In this definition, it appears that the cognitive level of description provides a unifying theory of dyslexia. Such a theory is necessary to pool together the numerous different observational strands in this most intriguing and subtle disorder.
After the discussion of definition issues in dyslexia, we turn to focus on the approval that dyslexia has its genetic basis. We will first look at the study of heritability in dyslexia and then to talk about the genetic findings for supporting dyslexia has a genetic basis.
The rapidly accumulating evidence suggests that developmental dyslexia is one of many common familial disorders. The genetic explanations of dyslexia are rather convinced by research that uses the newly genetic techniques and statistical methods in the genetic study of dyslexia. Although most findings cannot be replicated as there are many variants need to be identified. We still can believe that dyslexia has a genetic basis by the evidence of the genetic study of dyslexia.
2.1 Famaliality of Dyslexia
The question of whether dyslexia has a genetic basis has been studied for a very long time. Numerous researches have been conducted. Among them, there are a number of findings that suggest developmental dyslexia is hereditary. Orton in 1925 hypothesizes that children born in a family of dyslexia have great chance of being dyslexia. According to a recent estimation made by Gilger, Pennington and Deferies in 1991, the risk of a son with a dyslexia father to be a dyslexia is approximate 40% and about 36% if the mother is dyslexia. Moreover, if both parents are affected, the risk and severity of dyslexia in the child would greatly increase. Nevertheless, for the girls, this ratio is relatively lower, at about 20% regardless of the gender of the affected parent (Childs&finucci,1983;Deferies&Decker,1982;Pennington,1991).However,the higher familial aggregation of reading problems is insufficient to prove that dyslexia has genetic basis. The environment shared by families are strongly influence their reading ability.
2.2 Twins Studies
The twins studies can help us understand the complexity of the interaction between genes and environment in some degree. The first kind of twin studies is the comparison of concordance rates that could evaluate the hereditary basis of dyslexia as a clinical condition. The second evaluates the reading performance of twins for estimating heritability coefficient by analyzing various indicators of reading performance. Thus, it is important to diffrentiate these two types of twin studies.
In the first kind of twin study of dyslexia, researchers compared the concordance rates in monozygotic (MZ) twin pairs the identical twin pairs and dizygotic(DZ) twin pairs the fraternal pairs. Regression counted in the research dues to the assessment of environmental factors and its interaction with genes in reading disabled. The results show that at least one member of every pair had reading problems. Moreover, MZ has a higher concordance for reading disability than in DZ twin pairs (Hermann, 1959; Zerbin-Rudin, 1967;Decker and Vandenberg,1985). By comparing the findings of the concordance rates in twin pairs we can imply that developmental dyslexia has a genetic aetiology.
In the second type of twin pairs, a vast number of studies have reported MZ and DZ twin correlations for various measures of reading performance (Grigorenko, 1996). MZ correlations implied the presence of genetic influence through the comparison with DZ correlations. However, heritability estimates are varied. Some of the variability can be due to the fact that the sample size of those main researchers was relatively small. In addition, some twin studies suggest that only certain reading-related skills are inherited. Thus it has been shown that word recognition, phonological coding show important genetic influence, whereas reading comprehension and orthographic coding do not (Olson, Wise,Conners,Rack,&Fulker, 1989). Because the latter one significantly influenced by the environmental factors.
2.3 Pattern of Transmission of Dyslexia
Researchers had conducted a number of segregation analyses, fitting different statistical models corresponding to various patterns to investigate the transmission of genes in families with reading disability. Some observers have concluded that familial dyslexia is transmitted in an autosomal (not sex-linked) dominant mode (Childs& Finucci, 1983; Hallgren, 1950), whereas others have found only partial (Pennington et al.,1991) or no support for an autosomal or codominant pattern of transmission. These findings were interpreted as suggesting that specific reading disability is genetically heterogeneous (Finucci et al.,1976; Lewitter, DeFries, &Elston, 1980). In here, Quantitative trait loci (QTL) mapping also has been applied (Cardon et al., 1994;Fulker et al.,1991) in order to localize individual genes that contribute to the development of dyslexia.
2.4 Genetic Localization
The researchers passionately set an ultimate goal of genetic study that is to locate and isolate the responsible gene for dyslexia. Once the genes responsible for dyslexia is located, the protein product encode by the gene may permit a physiological explanation for its role in normal processes or diseases and finally contributed to a gene therapy for dyslexic. However, some researchers like Snowling (2000) consider the location of genes is a wide goose chase. The human genome has a rough estimation of about 35 000 genes which distributed over 3 billion bp of DNA and half of them is related to brains. Even when researchers limited the number of candidate genes to screen by using different biological hypotheses, they still need to work with thousands of genes. Thus, considering the risk of failing to match any given hypothesis, researches adopt the linkage and association analysis these two types of mapping strategies. The principle underlying both genetic linkage and association mapping is to test for non-random relations between phenotypic similarity across many individuals and haplotype sharing between them. With more generations the analysis become more powerful and accurate because each meiosis provides another opportunity for spurious genotype-phenotype relations to decompose. Linkage analysis refers to the analysis of individuals for whom family relations are known, whereas association analysis is used for large samples of unrelated individuals. Now, linkage analysis is generally less effective than association analysis in detecting genotype-phenotype relations within a study sample size. However, linkage mapping can be done with much fewer genetic markers and is hence easier to use in practice than association analysis. Genome-wide linkage can be carried out by analysis of about 400 highly polymorphic DNA markers. By contrast, association mapping has the power to focus on the specific causal DNA variants that influence phenotype variability but in most case it must use much more times that use to analyse DNA polymorphisms then linkage mapping used.
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Using current molecular techniques of linkage analysis to carefully study selected family trees of dyslexic individuals in which developmental dyslexia reoccurs in different generations, some early results showed that a major gene for dyslexia was located on the short arm of chromosome 15 (Pennington et al.,1991;Smith, Pennington, Kimberling,& Ing,1990). Fulker and his colleagues in 1991 replicated the same result of chromosome 15 though selecting a sample of siblings with reading problems in the study of original extended-family. Others like Lubs in 1991, Rabin in 1993 and Cardon in 1994 did not find the same results.
From a recently review of genetic study of dyslexia, we can see that the candidate genes DCDC2 the double cortin doman containing protein 2 and K1AA0319 show strongest links to the dyslexia among severely affected individuals. However, the candidate genes chromosome 15 and ROBO1 roundabout Drosophila Homolog of 1, which were identified through breakpoint mapping in Finnish patients, seem to be less involved in the development of dyslexia across different populations. However, their research is limited to a few families in the Finnish population and to date, no specific cognitive processes are known to be influenced by the proposed susceptibility genes. Some studies have already started to include neurophysiological and imaging procedures in their phenotype characterization of patients. The molecular genetic studies conducted so far have not considered gender-specific genetic effects. A satisfactory power to detect such effects can be provided only when gender is taken into account during the analysis of results, and this should be a feature of future studies (Schumacher et al, 2008)
Although, scientific research has yet to prove that dyslexia is a gentic disorder, many researchers and evidence have show that it is a high possibility. In my opinion, dyslexia is a genetic disease and its symtoms can be aggravated or mitigated by the environment. Nevertheless, more research into the correlationship of the genetic factor and the environment needs to be conducted to verify this claim.
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Poor decoding and spelling abilities along with difficulties in precise and fluent recognition of words characterise the learning disability of dyslexia. Despite the extensive scientific attention that dyslexia has received there is still much debate about its causal explanation.
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