A polymorphism in mitochondrial DNA associated with IQ?

In an allelic association study of 100 DNA markers in or near genes of neurological relevance, one restriction fragment length polymorphism (RFLP) yielded significant differences between high- and low-IQ groups in two independent samples. The goal of this article is to describe how we tracked down the specific gene marked by the RFLP and to introduce some current techniques used to apply molecular genetics to complex traits like IQ. The RFLP, EST00083, is a brain-expressed sequence tag site (BESTS) derived from a cDNA hippocampal library. The cDNA clone was shown to involve a chimera between genomic DNA on Chromosome 6 and mitochondrial DNA (mtDNA). The RFLP was localized in the mtDNA rather than the genomic DNA. The RFLP is an Mspl restriction site (CCGG) at 15,925 base pairs of the complete mitochondrial genome in a gene that codes for the transfer RNA for threonine. The mitochondrial origin of the EST00083 RFLP explains why the RFLP is maternally transmitted and never yields heterozygotes. Although mtDNA could be associated with IQ, such an unusual result requires further replication.