Haplotype identification. (a) Positions in the human genome where single nucleotide polymorphisms occur are often identified in genomic samples. The SNPs can be in any part of the genome, whether or not part of a known gene. (b) Groups of these SNPs are compiled into a haplotype. The SNPs will vary in the overall human population, such as in the four fictional individuals shown here, but the SNPs chosen to define a haplotype are often the same in most individuals of a particular population. (c) A few single nucleotide polymorphisms are chosen as haplotype-defining SNPs (tag SNPs), which are then used to simplify the process of identifying an individual’s haplotype (by sequencing 3 loci instead of 20). If the positions shown here are sequenced, an A-T-C haplotype might be characteristic of a population native to a location in northern Europe, whereas a G-T-C haplotype might prevail in a population in Asia. Multiple haplotypes of this kind are used to trace prehistoric human migrations.