Obesity is a disease of excess, but a new study suggests that a few obese patients are actually lacking something--a piece of one of their chromosomes. The loss might remove a gene that helps the body manage blood sugar and appetite. Obesity runs in families, and researchers have identified several genetic variants that seem to boost the odds of becoming obese. However, these variants only explain a minority of cases. In the last decade, researchers have discovered that genetic differences among people can stem from lost or duplicated sections of chromosomes, called copy number variants (CNVs). Because of CNVs, for...