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Author Topic:   DCDC2 as a gene of general intelligence (IQ)
Volkmar Weiss
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posted 11-20-2005 03:55 PM     Click Here to See the Profile for Volkmar Weiss   Click Here to Email Volkmar Weiss     Edit/Delete Message Reply w/Quote
DCDC2 - doublecortin domain containing 2 (6p22.1): More than a major gene of reading ability?

Two independent groups (see http://www.pnas.org/cgi/content/abstract/0508591102v1 and http://www.journals.uchicago.edu/AJHG/journal/issues/v78n1/42841/brief/42841.abstrac t.html ) have discovered DCDC2 to be associated with reading disability (developmental dyslexia). Reading disability is characterized by severe difficulties in learning to read and spell despite adequate schooling and normal visual acuity.

In the two studies, the probands have been selected by tests in such a way that DCDC2 has to be understood as a minor gene of general intelligence or even more. Since long, reading speed and reading ability are subtests of a number of IQ-tests with high validity, see for example http://www.volkmar-weiss.de/lehrl.html . A reliable IQ estimate can be obtained with the KAI test (by Siegfried Lehrl) which sums up only the scores of the two subtests reading speed and memory span. In Austria a reading test has been applied to the general population of schoolchildren during the nation-wide PISA 2000 testing. The distribution of reading speed found is not unimodal and suggests an underlying genetic polymporphism, see http://www.pisa-austria.at/pisa2000/national/kap3/index.htm (Abb.III. 5), where dyslexia represents only the lower tail of the distribution. In Behavior Genetics 24 (1994) 345-355 Gilger, J.W. et al., see http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=p ubmed&dopt=Abstract&list_uids=7993313&query_hl=7 published "Commingling and segregation analysis of reading performance in families of normal reading probands". They wrote: "The common allele accounted for 54% of the phenotypic variance in reading scores." On July 17, 1995, I commented Gilger et al. at the VIIth Meeting of the International Society for The Study of Individual Differences at Warsaw: "If one tests the mathematically gifted they have top scores of reading rate, too. When there is a gene accounting for 54% of the variance of reading performance, there is no room left for a second major gene of general intelligence."

The first genomwide scan for intelligence trait loci wide scan for general intelligence (see https://genepi.qimr.edu.au/staff/nick_pdf/CV411.pdf ) also hinted to 6p23. We are aware, that because of the limited density of markers such a first scan can only be a near hit. It could be DCDC2! (NQO2 suggested by me - see a former posting - could be nothing more than an informed guess.). What is urgently needed, are data on the distribution of all coding genetic polymorphisms of DCDC2 and their relationship with IQ in the general population. One of the subtests of IQ in such studies should be, of course, reading speed (as in the KAI test).

Volkmar Weiss
Leipzig, Germany
http://www.volkmar-weiss.de/intellig.html

[This message has been edited by Volkmar Weiss (edited 11-20-2005).]

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