Rob DeSalle, an evolutionary biologist at the American Museum of Natural History in New York, says that could take a while. The health conditions associated with a poor diet mostly affect middle-aged adults, who have probably already had children and passed their genes on. Perhaps, he speculates, if more children and teens get obesity-related ailments, such as heart disease and Type II diabetes, fewer will survive to reproduce, stripping craving-related genes from populations more quickly. Even then, weeding out all 100 genes is unlikely. Also, genes associated with obesity aren't killers. They don't code for sickle-cell anemia or cystic fibrosis. If those bad genes have hung on for a very long time, DeSalle says, marginally bad ones could hang on even longer.