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Icelandic biologists have discovered that the genetic risk of several common diseases, like Type 2 diabetes and cancer, can depend on which parent a DNA variant is inherited from.

The finding may help explain part of a serious gap in understanding the genetics of common diseases.

Using an extensive genealogy that includes almost all the present population of Iceland and many in previous generations, the Reykjavik company DeCode Genetics managed to distinguish which chromosomes came from the father and which from the mother in some 40,000 people.

The company then ran standard tests, known as genome wide association studies, the tool that scientists have hoped would track down the roots of common diseases and fulfill the promise of the Human Genome Project.

But with most of these common diseases, the tests have so far identified genetic variants that account for only a small percentage of the risk. This is in contrast to simple diseases, most of them rare, where a single gene is the cause and the disease has a clear family pedigree.

The failure has left biologists puzzled about...