Sometimes the nature of the gene itself becomes changed and this process is called gene mutation. The mutated gene then reproduces itself in its new form and the new and the old are then allelic. Mutation happens very rarely although it seems that some genes have a higher mutation rate than others. Most, but not all, mutations are recessive and they may differ from from the original gene in various ways. Mutants may produce striking alterations in the phenotype--as seen in the recessive mutant rex coat; the recessive mutant longhairs and the more recent dominant mutant Folded ear. Others may have only small effects. The process of mutation is generally reversible and back mutation occurs when a mutant reverts to its original form. Many, if not all, of the allelic genes which occur in cat populations have arisen by mutation. With many genes it is impossible to know which is the mutant allele. Usually the dominant is referred to as the normal or wild type and the recessive as the mutant. The term mutant is thus used loosely and it should be realized that the word normal is used only to describe the original condition from which the mutant made alteration. It does not necessarily infer that the mutant itself is in any way abnormal.