Genetic Studies: Possibilities and Limitations Related to Stuttering
Commentary by Ronald L. Webster, Ph.D., Founder and President of Hollins Communications Research Institute (HCRI), a nonprofit center dedicated to the investigation and treatment of stuttering.
The issue of genetics in stuttering is important and it deserves to be understood in terms of what genetic analyses can and cannot do in improving our understanding of stuttering. The scientific and popular enthusiasm for modern DNA analyses is generating a possibly problematic situation with the seemingly magical expectation that the human condition can be understood almost solely through genetic analyses.
It is clear, however, that the information resulting from genetic analyses must be integrated with other forms of knowledge about how organisms are organized and how they function.
The coherent formulation of knowledge about fundamental properties of organisms will lead to better understanding of the specific mechanisms that are embodied within them and the activities accomplished by these mechanisms. This will be true for the disorder identified as stuttering.
A few recent studies illustrate the nature of research being conducted on genetics and stuttering. Following are links to abstracts.
- New complexities in the genetics of stuttering: significant sex-specific linkage signals.
- Genomewide significant linkage to stuttering on chromosome 12.
- Evidence for a major gene influence on persistent developmental stuttering.
- The genetic basis of persistence and recovery in stuttering.
- Genetic studies of stuttering in a founder population.
- Genetic etiology in cases of recovered and persistent stuttering in an unselected, longitudinal sample of young twins.
These reports show scattered and inconsistent results when different types of genetic linkage studies are conducted. The main point is that there is still relatively little evidence for specific genetic factors in the broad population of stutterers. Evidence improves slightly when more closely related samples are examined.
There is the long-standing general observation that stuttering tends to run in families. Approximately half of the people who stutter can identify someone in their family background who stuttered. That finding implicates the notion that genetic factors are relevant in some way to the problem of stuttering.
There is preliminary evidence that aspects of stuttering within family members can be accounted for by advanced genetic models. However, when geneticists have conducted genetic linkage studies in which they searched for genetic markers that differentiated family groups with stuttering represented with family groups without stuttering represented, only weak relationships were found.
One of the conclusions that can be drawn about the probable role of genetics in stuttering is that slightly more that 50 percent of the cases will have a genetic component present. That means there will be a considerable proportion of stuttering cases where a genetic component will not be present. There is considerable importance to this interpretation when it comes to searching for the cause or causes of stuttering.
It seems likely that there is a mechanism or mechanisms that can produce stuttering in a individual even when there is no specific genetic element present. If this is indeed the case, then, in those instances of stuttering where a genetic component is identified, it would be expected that the physical manifestations of the genetic material are acting upon components of an existing body system that is primarily responsible for the disfluent speech events.
The implication that I draw from the genetics research is that we should intensify the search for physical mechanisms within the body that govern speech motor output. A good place to begin the search is to examine the broad array of physical variables that enhance fluency in stutterers and degrade fluency in normal speakers. Nature is presenting us with sets of clues that are almost certainly pointing to the arena where the causal factors in stuttering will be found.
For information about HCRI’s 39 years of research and therapy delivery, visit www.stuttering.org.