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Obese mouse and a normal mouse. The obese mouse has a mutation on chromosome 7, which has altered the pattern of fat deposition in its body. The 'obesity' gene is thought to act in conjunction with other genes that regulate the body's energy balance. Mice with the mutation grow 35 to 50 percent fatter by middle age than a normal mouse, despite eating a low-fat diet. Scientists are able to determine the function of genes by observing the effects of gene mutations. Such research may have applications to human health. Photographed at Oak Ridge National Laboratory in Tennessee, USA. (KEYSTONE/SCIENCE PHOTO LIBRARY/OAK RIDGE NATIONAL LABORATORY/US DEPARTMENT OF ENERGY) === ===