Interesting from an atrcile in the Economist regarding genetics and disease:

http://www.economist.com/science/displaystory.cfm?story_id=6795348

"ApoE4 is relatively common in most racial groups. About 9% of Japanese Americans have it, 14% of white Americans and 19% of black Americans. But the impact of ApoE4 differs significantly between these groups. Individuals of Japanese descent who have two copies of ApoE4 (one from each parent) have a 33-fold increased risk of contracting Alzheimer's disease compared with those who have other versions of ApoE. Those of European descent have a 15-fold increased risk. Blacks, by contrast, have a mere six-fold increase in their risk. “Whether it is due to differences in genetic background or environment, who knows? But if you ignore race, you would never know,” says Dr Risch. Moreover, if the American population was sampled for ApoE4 and Alzheimer's risk without regard for ethnicity, the risk would resemble that of whites, because their numbers would overwhelm the data from the other racial groups. As Dr Risch says, “I think these categories are useful as long as they are predictive. That doesn't prove that the difference is genetic.”