Genetics, whether of the dog or of Homo sapiens, is never a simple subject that can be simply explained. Not only is genetics a science the determining units of which - the genes - are intangible even to the most powerful microscopes, it is not a science of individual cases. The laws of genetics rest upon statistical probabilities. Herein lies a trap for the novice breeder of dogs who, armed with a little genetic knowledge, sets forth to breed a dog that will go best in show at Westminster, paws down.