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 the missing heritability, which some have jokingly attributed to “dark matter” within the human genome, an analogy with the dark matter invoked by cosmologists to explain the missing mass of the universe.

Some experts believe the missing risk might be carried in a large number of rare DNA variants not included in current scans of patients’ genomes. Because it costs too much to decode all three billion units of DNA in a patient’s genome, biologists use chips that scan just the 500,000 sites where variation is most common. These sites were expected to explain the genetic component of common diseases like cancer or schizophrenia, but mostly do not do so.

Another explanation is that the missing heritability lies in aspects of cell biology that are not yet understood.

Decode scientists have found one such instance. They report in Friday’s Nature that a DNA variant increases a person’s risk of Type 2 diabetes by 30 percent if inherited from the father, but reduces the risk by 10 percent if comes from the mother.