Figure 30.15: A Single-Gene Mutation Leads to Obesity in Mice In mice the Ob gene codes for the protein leptin, a satiety (feeling of fullness) factor that signals the brain when enough food has been consumed. The recessive ob allele is a loss-of-function allele. Accordingly, ob/ob mice do not produce leptin; they do not experience satiety and thus become obese. The receptor protein in the brain that responds to leptin is coded by the Db gene. Mice homozygous for the recessive loss-of-function allele db cannot respond to leptin, even if they produce it, and they become obese.a