The Ac-Ds system of maize is a muatble allele system. It was on the basis of studies on this system that Barbara McClintock proposed the concept of transposable elements. She received the 1984 Nobel Prize in medicine for her pioneering work on transposons.
Transposons, by inserting themselves into genes, cause mutations, and disrupt the trasncription of normal mRNA. This kind of mutation is transposon-insertion mutation and it creates a recessive allele (as functional gene product is not produced). The mutations caused this way are unstable because the mobile transposon can be excised from the gene and it can restore the function of the gene. Recessive transposon-insertion alleles are known as mutable alleles as they revert to dominant alleles at rates higher than for recessive alleles that contain other types of mutations.
There are several Ac and Ds trasnposons cloned and sequenced. These are good examples of how many of the transposons work. Ac stands for activator and Ds for dissociator. Both of them represent transposons and are part of the same system. However, they function in different ways. The Ds element (found within the coding sequence of a gene) is incapable of excising itself from the gene and therefore mutant alleles harboring a Ds element are usually stable. Ac elements are capable of excising themselves from the position where they reside and move elsewhere, causing the allele to revert to the dominant type after excision. If they land up within a new gene after the trasnposition event, Ac elements create a mutation in the gene. This is now detected as a recessive allele.