When it comes to genome mapping, the spotlight has largely been on "physical" maps: those that list the millions of DNA bases embedded in an organism's genome. But many biologists also believe that genetic maps, which mark spots where DNA differs from one person to another, will prove critical in finding gene mutations linked to disease. Now a genetic map crafted by deCODE genetics, a company based in Reykjavik, Iceland, reveals surprising variations in how DNA passes from mothers to their children.
Until now, the largest genetic map for humans was created by the Marshfield Medical Research Foundation in Marshfield, Wisconsin. That map was constructed before the human genome was sequenced, which made it hard to place genetic markers in the right order. The deCODE scientists, in contrast, had the benefit of evolving drafts from the human genome sequence.
To assemble the map, the researchers, led by the company's CEO Kari Stefansson, collected samples from 869 people from 146 families in Iceland. Whenever possible, they included samples from both parents and their children, tracing how various pieces of chromosomes recombined upon fertilization to produce the offspring's new chromosomes. Stefansson's team identified 5136 spots on the genome where changes occurred. Scientists already knew that recombination shuffles the contents of maternal chromosomes 60% more than the contents of paternal chromosomes. Surprisingly, the team also found that some mothers' chromosomes get mixed up in their offspring more than those of other mothers'. Such differences in recombination may help explain why their families differ in disease susceptibility, says Stefansson.
The deCODE map "is about a fivefold improvement" in the resolution of the Marshfield map, says James Weber, director of the Center for Medical Genetics at Marshfield. "They've confirmed recent findings that individual women differ from each other," he adds. However, why that is "we can only speculate at this point." DeCODE published the map online 10 June in Nature Genetics and will make it freely available on its Web site.