”dyslexia” has become more widely recognized in recent years as a special biological and educational problem in reading development. nevertheless, i often hear educators express doubts that difficulty in learning to read is due to anything more than poor environmental support for reading. of course environmental factors are responsible for many failures in learning to read, but the concept of dyslexia suggests that there are unique biological causes and special educational needs for many children with reading problems. in this article, i will briefly review some of the evidence on the biological side for genetic contributions to dyslexia, and for the special instructional needs of children with dyslexia. in a related article on this web site titled dyslexia and computer technology, i focus on the important role that computer technology and the web can play in diagnosing and meeting the special needs of children with dyslexia in their schools and homes.
research on dyslexia at the colorado center for the study of learning disabilities has been a highly collaborative effort across several laboratories. it has included investigations of children’s specific reading deficits, related perceptual and language skills, brain activity, brain structure, and their genetic and environmental origins (defries et al., 1997). genetic and environmental influences have been studied by comparing behavioral similarities between identical and same-sex fraternal twins, and by the direct examination of dna from children with dyslexia and from their parents and siblings.
data from identical and fraternal twins can be uniquely informative about the relative contributions from genes and environment to individual differences in reading, and to the severe reading deficits found in dyslexia. identical-twin pairs are derived from the same sperm and egg, so they share all their genes. fraternal twins develop from two different sperm-egg fertilizations, so they share, on average, half of their segregating genes (the small minority of genes that vary across individuals). thus, fraternal twins have the same degree of average genetic similarity as ordinary brothers and sisters, but like the identical twins, they share the same intrauterine environment, are born at the same time, and then share their family environment.
of course children help to create their own environment both within and outside the family, so fraternal twins’ different genes might lead them to greater environmental differences than those experienced by identical twins. for example, one fraternal twin 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 dyslexic twin’s smaller amount of reading practice may contribute to their reading failure, but less reading practice is not the only cause in most cases of dyslexia. in fact, children with dyslexia typically require far more reading practice than normally developing children to reach a normal reading level (ehri & saltmarsh, 1995; reitsma, 1983). so, what is the unique reading difficulty in dyslexia, and what is the cause of that difficulty?
in our behavior-genetic approach to these two questions, we identify twins in colorado with reading problems based on their school records. then we invite the twins to the laboratory for further testing in specific reading skills, iq, attention, memory, and language. we have found that most children with reading problems are uniquely deficient in a component reading skill called phonological decoding or ”word attack” (rack, snowling, & olson, 1992). this skill is measured by having children read pronounceable non-word letter strings aloud (framble, tegwop). we have also found that dyslexic children are uniquely deficient in a specific language skill called phoneme awareness, defined as the ability to isolate and manipulate the abstract phonemes in speech. for example, we ask children to delete sounds from spoken non words and then say the word that would result from the remaining sounds. if asked to say ”prot” without the ”rrr” sound, the correct response would be ”pot”. (no letters are shown in this task.) similar phonological deficits have been reported for children with dyslexia in sweden and other countries (lundberg, 1999). these deficits constrain the development of accurate word recognition in independent reading because good phonological skills provide a ”self-teaching” mechanism for decoding unfamiliar words in print (share, 1995).
our comparisons of identical and fraternal twins in colorado have revealed strong genetic influences on the dyslexic group’s deficit in phoneme awareness. the evidence for this genetic influence comes from the fact that if one twin has a deficit in phoneme awareness, it is far more likely that the other twin of the pair will have that deficit if the pair is genetically identical than if the pair is fraternal and sharing only 50% of their genes. genetic influence on phoneme awareness averages about 60%-70% across different measures, leaving 30%-40% of the group’s deficit due to environmental factors (gayan and olson, in press). similarly, the genetic influences on group deficits in phonological decoding (non-word reading) also average 60% to 70%, and we have shown that deficits in phonological decoding and phoneme awareness are largely due to the same genes. word-reading group deficits tend to have a slightly lower genetic influence (50-60%), but most of this genetic influence is shared with that for deficits in phoneme awareness and phonological decoding.
the above estimates of genetic influence are qualified in two ways. first, the environmental range in our twin sample was deliberately restricted by excluding children with obvious environmental deprivations such as lack of normal schooling, and learning english as a second language. had we included children with these environmental constraints on reading development, environmental influence would have played a greater role and genes a lesser role in the group’s reading deficit. a second qualification is that our estimates of genetic and environmental influence are for average effects across the group. within the dyslexic group, some individuals with dyslexia may have largely or completely environmental causes for their reading difficulties (i.e., subtle brain damage from a birth accident or illness). for other children in the dyslexic group, the cause may be strongly related to genetic factors. their dyslexia may be severe in spite of normal environmental support for reading and related cognitive skills, and the absence of any influence from accidents or illness. to know about genetic influences in a specific individual, we need direct evidence from their genes or dna.
linkage analyses of dyslexics’ and their siblings’ dna has revealed a region on the short arm of chromosome 6 that contains a gene or genes affecting the group deficit in phonological skills (gayan and olson, 1999). other studies have found evidence for genetic influences on dyslexia that are linked to regions on chromosomes 2, 15, and 18, so it is likely that there are different genetic pathways to dyslexia in different individuals. however, no specific genes have yet been identified in any of these regions that are related to dyslexia. in future research, we hope to identify specific genes and understand how variation in these genes leads to dyslexia in different individuals and in different environments. once specific genes are identified, their presence in an individual’s dna can be used to diagnose a genetic risk for dyslexia even before birth. environmental intervention such as enriched language and print experience could then be given to reduce the risk of reading failure when formal reading instruction begins in school. more direct gene therapy might eventually be possible in some cases.
when children begin formal reading instruction, computer technology can be employed to diagnose specific reading deficits and provide extraordinary environmental support for reading development. results from our research on computer-based remediation for reading disabilities are described in a companion article on this web site, titled dyslexia and computer technology. these studies show that even though there is a strong genetic contribution to many cases of dyslexia, extraordinary environmental intervention can significantly improve reading skills in children with dyslexia.
references
defries, j.c., filipek, p.a., fulker, d.w., olson, r.k., pennington, b.f., smith, s.d., & wise, b.w. (1997). colorado learning disabilities research center. learning disabilities, 8, 7-19.
ehri, l.c., & saltmarsh, j. (1995). beginning readers outperform older disabled readers in learning to read words by sight. reading & writing: an interdisciplinary journal, 7, 295-326.
gayan, j., & olson, r.k. (in press). genetic and environmental influences on orthographic and phonological skills in children with reading disabilities. developmental neuropsychology.
gayan, j., & olson, r.k. (1999). reading disability: evidence for a genetic etiology. european child & adolescent psychiatry, 8, 52-55 suppl. 3.
lundberg, i. (1999). learning to read in scandinavia. in m. harris, g. hatano et al. (eds.), learning to read and write: a cross-linguistic perspective, pp. 157-172. new york: cambridge university press.
rack, j.p., snowling, m.j., & olson, r.k. (1992). the nonword reading deficit in developmental dyslexia: a review. reading research quarterly, 27(1), 28-53.
reitsma, p. (1983). word-specific knowledge in beginning reading. journal of research in reading, 6, 41-56.
share, d. l. (1995). phonological recoding and self-teaching: sine qua non of reading acquisition. cognition, 55(2), 151-218.