Allelic associations between 100 DNA markers and high versus low IQ

Abstract

For DNA markers in or near genes of neurological relevance, allelic frequencies were compared for groups of White children high and low in IQ in an attempt to identify specific genes responsible for the substantial heritability of IQ scores. We previously reported results for 60 DNA markers and we now describe results for 40 additional markers. One sample consisted of high- and low-IQ groups with average IQs of 130 ( N = 24) and 82 ( N = 18), respectively. A replication sample was more extreme, including groups with average IQs of 142 ( N = 27) and 59 ( N = 17). Three of the 40 markers yielded significant allelic frequency differences between the high- and low-IQ groups in the original sample. In the replication sample, two of these markers (alcohol dehydrogenase 5 and the beta polypetide of nerve growth factor) yielded results in the same direction but were not significant. The third marker (EST00083), derived from a cDNA hippocampal library, was also significant in the replication sample. As described in another article (Skuder et al., 1995) in this issue, this marker was found to involve mitochondrial DNA (mtDNA) rather than nuclear DNA. The unexpected nature of this marker suggests caution in claiming that the replicated association for EST00083 is indeed a quantitative trait loci (QTL) for IQ until the association receives additional support. This study provides statistical power to detect associations that account for about 2% of the IQ variance in the population. We are currently obtaining samples four times larger that will provide statistical power to detect allelic associations that account for considerably less than 1% of the variance.