# Speech-Language Pathology/Stuttering/Genetics of Stuttering
## Dopaminergic Genes

Three genes associated with the neurotransmitter dopamine correlate with five disorders: ADD/ADHD, stuttering, Tourette's syndrome, obsessive compulsive disorder (OCD), and tics. All five disorders involve a combination of stress and movement:
* Persons with ADD/ADHD have normal brain activity when resting, but when they try to concentrate on a task, they have reduced prefrontal cortex blood flow, which reduces their ability to concentrate. Persons with ADHD (mostly children) then become hyperactive and can't sit still.
* Tics cause a muscle, often in your face, to move rapidly and uncontrollably. Stress usually makes the tic uncontrollable.
* Individuals with OCD repeatedly perform physical tasks, e.g., hand-washing. Stress usually makes this behavior uncontrollable.
* Individuals with Tourette's syndrome compulsively touch objects (e.g., floors or walls) or make stereotyped movements or noises. Stress usually makes the movements uncontrollable.
* Stuttering is excessive speech-production muscle activity resulting in blocked, prolonged, or repeated sounds. Stress usually makes the stuttering uncontrollable.
All five disorders manifest most strongly—and are most difficult to control—in high-stress situations. These disorders may all be stress reducers that work for a few moments but then cause greater stress, resulting in cyclical abnormal behaviors (see the section Stress-Related Changes).
Why do some individuals with these three genes develop stuttering, while others develop Tourette's, while still others develop other disorders, or no disorder at all? In a subgroup of individuals with Tourette's, a childhood autoimmune "trigger" leads to Tourette's. A childhood streptococcal infection causes a child's immune system to attack brain cells in the putamen area. The putamen controls gross (large) muscle movements. Excessive dopamine in the putamen area of the brain is associated with Tourette's. The child recovers from the fever, but then develops Tourette's.
Speculatively, a similar autoimmune disorder involving the left caudate nucleus speech motor control area could trigger childhood stuttering.

## Stuttering Families

In 2005, researchers announced that almost half the family members stutter in several extended families in Cameroon, Africa. The lead researcher said, "These families provide hope that we'll be able to identify the gene that appears to be at work to cause stuttering in these families."
A 1940 study investigated five generations of an Iowa family, with stuttering in all generations. In the third generation, three of four children in one family stuttered. When these children grew up, three moved to Kansas and lost contact with the sibling remaining in Iowa. Of the descendants of the woman remaining in Iowa, 40% stuttered. Of the descendants of the Kansas siblings, only 6% stuttered.
The researcher concluded that the Iowa family had a "tone" that was conducive to stuttering. The hypothesis was that this family believed that stuttering was inherited, so they anxiously watched their children looking for signs of stuttering, and this fear of stuttering in the parents caused the children to stutter.
In 1960, another researcher studied the next generation of the family. Only 2% of these children stuttered, and the researcher reported that there had been a change in the familial assumptions and attitudes about stuttering.
In 1940, speech-language pathologists believed that stuttering was caused by parental reactions to normal childhood dysfluencies. The Iowa family "proved" what the speech-language pathologists believed. In 2005, speech-language pathologists believe that stuttering has a genetic cause, and the Cameroon families are expected to prove this hypothesis. In 1986, a speech-language pathologist re-examined the 1940 Iowa study and concluded that, "genetic transmission has been judged to provide an equally viable explanation for the data."
At least 25 studies have investigated family patterns in the incidence of stuttering. The results vary widely. Most studies are contradicted by other studies.
Between 20% and 74% of stutterers say they have a relative who stutters. But other studies found that 1% to 42% of non-stutterers said they have a relative who stutters. In other words, many stutterers and many non-stutterers say they have relatives who stutter. One study found that if you stutter, you are about three times more likely to have a close relative who stutters.
Studies of the families of stutterers failed to find simple Mendelian types of inheritance, such as sex-linked, autosomal dominant, or recessive.
A study of children aged 2 to 6 found that more than two-thirds of stuttering children have relatives who stutter. This study found that male and female children were equally likely to have relatives who stutter. This study also found that stuttering was more likely in first-degree relatives than in second- or third-degree relations.

## Twin Studies

A half-dozen studies of twins have found that concordance for stuttering (both twins either stutter or don't stutter, rather than one twin stuttering and the other not stuttering) is much more likely in identical twins than in fraternal twins. The exact numbers vary between studies, in part because some studies only looked at same sex twins (a male and female fraternal twin pair is unlikely to both stutter because males are more likely to stutter than females).
On the other hand, a study of 95 pairs of identical twins reared apart found 5 stutterers (2% of the subjects, the expected prevalence). None of their twins stuttered. This is in spite of finding similarities in talkativeness, pitch, and hoarseness, as well as tastes in clothing, books, etc. This suggests that stuttering is not genetic.

## Relatives of Female Stutterers

Several studies have found that female stutterers are more likely than males to have relatives who stutter. This suggested that the few females who stutter might have a stronger genetic component in their families. Analogously, men are more likely to be tall than women, but tall women are more likely to have tall relatives.
However, another study of adult stutterers "did not find that relatives of female [stutterers] were more likely to stutter than relatives of the male [stutterers]."

## Stuttering Related to Other Factors

Studies of the family incidence of stuttering have compared left-handed stutterers to right-handed stutterers, speech motor skills, anxiety, and reading ability. Familial differences were found only for speech motor skills.
Two studies of birth or developmental trauma had contradictory results. A study of adults suggested that there are two causes of stuttering: genetic, and trauma or illness. A similar study of children who stutter reached the opposite conclusion.
Two studies found that severe stutterers were no more likely than mild stutterers to have relatives who stutter. Two more studies found no familial differences between recovered and persistent stutterers.

### References

* ^ Comings, D., et al., "Polygenic Inheritance of Tourette Syndrome, Stuttering, Attention Deficit Hyperactivity, Conduct, and Oppositional Defiant Disorder," American Journal of Medical Genetics 67:264-288 (1996).
* ^ Amen, Daniel. Healing ADD, (2002, ISBN 0425183270).
* ^ Singer, H.S., J.D. Giuliano, B.H. Hansen, J.J. Hallett, J.P. Laurino, M .Benson, and L.S. Kiessling. "Antibodies against human putamen in children with Tourette syndrome", Neurology, June 1998; 50: 1618-1624, summary.
* ^ Drayna, D. "Newly discovered families give impetus to genetic research," Stuttering Foundation of America newsletter, Fall 2005, page 1. Viswanath, N., H.S. Lee, R. Chakraborty. "Evidence for a Major Gene Influence on Persistance Developmental Stuttering," Human Biology, June 2004, 76:3, 401-412.
* ^ }Bloodstein, Oliver (1995) A Handbook On Stuttering, 5th edition, San Diego: Singular Press.
* ^ Wingate, M. (1986) "Physiological and genetic factors." In G.H. Shames & H. Rubin, Eds.), Stuttering Then And Now (pp.49-69). Columbus: Charles E. Merrill Publishing Co.
* ^ Yairi, E. Ambrose, N., Cox, N. (1996) "Genetics of Stuttering: A Critical Review," Journal of Speech and Hearing Research, 39 771-784.
* ^ Andrews, et al., "Stuttering: a review of research ﬁndings and theories," Journal of Speech and Hearing Disorders, 48, 226-246, 1983.
* ^ Ambrose, N., Yairi, E., Cox, N. “Genetic Aspects of Early Childhood Stuttering.” Journal of Speech and Hearing Research, 36, 701-706, August 1993.
* ^ Yairi, E. Ambrose, N., Cox, N. (1996) "Genetics of Stuttering: A Critical Review," Journal of Speech and Hearing Research, 39 771-784.
* ^ Bloodstein, Oliver (1995) A Handbook On Stuttering, 5th edition, San Diego: Singular Press.
* ^ Yairi, E. Ambrose, N., Cox, N. (1996) "Genetics of Stuttering: A Critical Review," Journal of Speech and Hearing Research, 39 771-784.
* ^ Janssen, P., Kloth, S., Kraaimat, F., Brutten, G. "Genetic Factors In Stuttering: A Replication of Ambrose, Yairi, and Cox’s (1993) Study With Adult Probands," Journal of Fluency Disorders, 21 (June 1996), 105-106.
* ^ Yairi, E. Ambrose, N., Cox, N. (1996) "Genetics of Stuttering: A Critical Review," Journal of Speech and Hearing Research, 39 771-784.