Linkage disequilibrium in the human genome

With the availability of a dense genome-wide map of single nucleotide polym orphisms (SNPs)(1), a central issue in human genetics is whether it is now possible to use linkage disequilibrium (LD) to map genes that cause disease . LD refers to correlations among neighbouring alleles, reflecting 'haploty pes' descended from single, ancestral chromosomes. The size of LD blocks ha s been the subject of considerable debate. Computer simulations(2) and empi rical data(3) have suggested that LD extends only a few kilobases (kb) arou nd common SNPs, whereas other data have suggested that it can extend much f urther, in some cases greater than 100 kb(4-6). It has been difficult to ob tain a systematic picture of LD because past studies have been based on onl y a few (1-3) loci and different populations. Here, we report a large-scale experiment using a uniform protocol to examine 19 randomly selected genomi c regions. LD in a United States population of north-European descent typic ally extends 60 kb from common alleles, implying that LD mapping is likely to be practical in this population. By contrast, LD in a Nigerian populatio n extends markedly less far. The results illuminate human history, suggesti ng that LD in northern Europeans is shaped by a marked demographic event ab out 27,000-53,000 years ago.