Sunday, April 29, 2007

No shared environmental influence!!!!

At last, I can report of an interesting piece of research. The authors Dworzynski, Remington, Rijsdijk, Howell, and Plomin have used data on 1000s of twins to study genetic relationships in recovered and persistent stuttering: see Am J Speech Lang Pathol. 2007 May;16(2):169-78 or here. Remember twins can be monozygotic (the same DNA), or dizygotic (share roughly half the DNA like siblings do). If the monozygotic twins share a disorder more often than dyzygotic twins at statistical significance, then the disorder is genetic because the only difference between them are the fact that they share either all the genes or roughly half the genes. Though the interesting aspect about twin studies is that you keep the environment relatively fixed, i.e. twin grow up in the same environment. A second advantage is that technically speaking this is simple statistical computation, that even non-quantitative researchers should not be able to mess up. There are surely negatives but I am not sure, probably the inability to locate genes.

Building up a twin pool is hard work but very lucrative scientifically speaking and ensures divine reverence by other scientists. Every research field undertakes a pilgrimage to these pools, and after worshipping the holders of the holy pool are allowed access to study the genetic influence.

They found that
Stuttering appears to be a disorder that has high heritability and little shared environment effect in early childhood and for recovered and persistent groups of children, by age 7.


JZ said...

I tend to believe it is more influenced by genetics. My stuttering seems to be inherited. One of my uncles from mother's side stuttered severely, I used to have frequent silent blocks, and my niece showed fixed blocks on 2.5 years of age. Thanks God she has passed over that period. I wonder if her gender helped her to recover.

Maybe my niece's stutter at age 2.5 is another evidence of the authors' finding that even at age 3, there is high influence of genetics with little impact of environment.

JZ said...

Talking about the paper,as their best-fit model revealed, at age 3, stuttering is influence by genetics for 58% and 42% for unique environmental influence. The shared environmental effect is said "little" because it was not suggested and analyzed by the authors. As the authors wrote, it was "Because the DZ concordances
were less than half the MZ concordances at ages 3
and 4".

I just don't know why the authors did not analyze the covariance of genetic and environmental effects, and gave out the results to demonstrate the "little shared environmental effect". Statistically it should be applicable.