Mutant mice. Hand holding a grey and a white mouse. The grey mouse has one copy of a normal gene and one copy of a mutant gene called 'Steel', which has diluted its coat pigmentation from black to grey. The white mouse has two copies of the mutant gene, and so has no fur pigment at all. As well as affecting pigment-producing cells, the Steel gene affects blood cells and germ cells (gamete-producing cells). The white mouse is anaemic and sterile. Scientists are able to determine the function of genes by observing the effects of gene mutations. Photographed at Oak Ridge National Laboratory in Tennessee, USA.