Each gene has a characteristic mutation rate. Mutation rates are often described as an average per gene basis. For instance, Drosophila has one detectable mutation per 10,000 loci, that is, 0.01% per locus. In humans spontaneous mutation rates have been calculated for some of the dominant lethi1 genes such as retinablastoma, chondrodystrophy and Huntington's chorea. These genes have rates varying from 0.01% to 0.001% per locus which means that one out of 10,000 or 100,000 loci mutates. This is true of most of eukaryotic mutation rates which range from to per locus. Jn prokaryotes the rates may be much lower, ranging from 10-4 to 10-6 per locus. Mutations can also occur in reverse direction by which it is ineant that a mutant gene can mutate back to its wild type form. However, reverse or back mutation rates are much lower than the forward mutation rates.