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Given that the evidence of a substantial genetic contribution to cognitive ability is now convincing, the next challenge is to locate and identify the genes involved. It is likely that there are several genes, each of only small effect. Detection of such so-called quantitative trait loci contributing to polygenic traits in plants and animals is facilitated by the ability to carry out experimental crosses. However, in humans detection of quantitative trait loci depends on studies of siblings or other relative pairs or on allelic association studies aimed at detecting linkage disequilibrium in populations. The allelic association approach is currently being applied to investigate cognitive ability by using both candidate genes and DNA pooling methods that potentially allow rapid screening of the whole genome for quantitative trait loci.