But researchers can draw conclusions from looking at only a fraction of the SNPs.
Peter Donnelly, professor of statistical science at the University of Oxford in England and a paper co-author, draws a useful analogy: "Imagine if you knew there was a group of five people who always caught the same bus," he said.
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"If you want to know which particular bus they're on, you don't have to find all of them. You just have to look for one particular individual The other four people are likely to be there as well."
"The motivation for the HapMap," Donnelly said, "is to create a resource which catalogs these correlations, and which researchers can then use with only five or ten percent of the effort and expense" of looking at all the possible SNPs.
The haplotype mapping project started in October 2002 with the creation of the International HapMap Consortium.
This large international group of scientists came together to create a public database of common human sequence variations that would be used to guide future genetic studies.
The consortium now has a map of more than a million of the most common SNPs grouped into haplotypes, and two million more will be added soon.
The early haplotype map uses DNA from 269 individuals spanning four different populations90 people from Ibadan, Nigeria; 90 Americans from Utah; 45 Han Chinese from Beijing; and 44 Japanese residents of Tokyo.
The authors caution that these populations, although varied geographically, should not be taken to be representative of larger populations, such as all of Africa, North America, or Asia.
The common variants in the four sampled populations are nearly complete.
But subtle structural differences need to be mapped before a complete picture of genetic variation among these four human populations can be drawn.
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