Blocks of limited haplotype diversity revealed by high-resolution scanningof human chromosome 21

Global patterns of human DNA sequence variation (haplotypes) defined by com mon single nucleotide polymorphisms (SNPs) have important implications for identifying disease associations and human traits. We have used high-densit y oligonucleotide arrays, in combination with somatic cell genetics, to ide ntify a large fraction of all common human chromosome 21 SNPs and to direct ly observe the haplotype structure defined by these SNPs. This structure re veals blocks of limited haplotype diversity in which more than 80% of a glo bal human sample can typically be characterized by only three common haplot ypes.