If you give a rat a running wheel and it decides not to use it, are genes to blame? And if so, what does that tell us about why many people skip exercise? To examine those questions, scientists at the University of Missouri in Columbia recently interbred rats to create two very distinct groups of animals, one of which loves to run. Those in the other group turn up their collective little noses at exercise, slouching idly in their cages instead. Then the scientists closely scrutinized and compared the animals’ bodies, brains and DNA.