Plasmids are an example. Common in plants, fungi, animals. Drosophila may have as many as 50 or 100. Alteration of host gene expression insertion into coding region can abolish gene function insertion into control region affects expression of the gene. Depends on the method used to detect mutation using phenotypic expression will underestimate mutation rates.
Frequency is much lower.
Usually is not replacement of the original amino acid, but a different one that restores function. Evolutionary implications of mutation rates Rate of change in frequency of a given allele due to mutation is very low. 10 -5 mutations per gamete per generation humans have about 150,000 functional genes. Even if only a few are advantageous, that is a lot of raw material for natural selection. E.g. antennapedia makes antennae into legs.