Torsion Dystonia and IQ
Torsion dystonia is caused by a dominant gene with low penetrance.. The symptoms involve inappropriate contractions of muscles. In a mild case, that might mean a tendency to writer's cramp: in a severe case, it means uncontrollable contractions that leave your limbs twisted and useless. About 30% of the individuals with this gene have some noticeable symptoms, about 10% have very serious symptoms that can leave them in a wheel chair. The problem is not in the muscles, but in areas of the brain that control muscles. Torsion dystonia does not cause retardation... not hardly.
Unlike most genetic diseases, it [torsion dystoni] is dominant. You only need one copy of the mutant gene to have problems. That also means that any benefit must be large. When a recessive mutation is rare, there are many more carriers than homozygotes, and even a small advantage among heterozygotes can balance serious bad effects in the rare homozygtes. A dominant has to give a hefty advantage, even more so if it has any costs, which the torsion dystonia gene surely does. So if torsion dystonia is [...] [a] hereditary neurological disease [...] [that] pays off in IQ, it must make a big difference, and that difference will probably show up in patients. ( Note that in diseases like Tay-Sachs, nobody even studies carriers. Doctors are not geneticists.) Apparently it does. I found several reports of materially increased IQ among Ashkenazi torsion dystonia patients. . The difference is apparently so striking that it is mentioned in the very first scientific article on the disease, by Flatau back in 1911. Many other physicians made the same observation. And if you think that plenty that being crippled makes you smarter, think again: nobody every said that about polio victims. Roswell Eldridge, in a small group of patients, found that the average IQ was 122, 10 points higher than their controls matched for age, sex, ethnic background, and school. .
Quoted on Sun Nov 4th, 2012