This is a tricky one, you gotta note two things:
Insertion of Ds in the C gene creates colorless corn-kernel cells.
Excision of Ds from the C gene through the action of
Ac in cells and their mitotic descendants allows color to be expressed again, producing the spotted phenotype.
According to the diagram I've posted, Ds is shown as a piece of DNA that has inactivated the C gene (as in the example you've given) [the allele is called c-mutable(Ds) or c-m(Ds) for short] by inserting into its coding region.
A strain with c-m(Ds) and no Ac has colorless kernels because Ds cannot move; it is stuck in the C gene. A strain with c-m(Ds) and Ac has spotted kernels because Ac activates Ds, in some cells, to leave (called excise or transpose) the C gene, thereby restoring gene function.
Other strains were isolated in which the Ac element itself had inserted into the C gene [called c-m(Ac)]. Unlike the c-m(Ds) allele, which is unstable only when Ac is in the genome, c-m(Ac) is always unstable. The Ac type can also be transformed into an allele of the Ds type. This transformation was due to the spontaneous generation of a Ds element from the inserted Ac element. In other words, Ds is, in all likelihood, an incomplete, mutated version of Ac itself.
I'm not too sure of the answer since it has been years since I've studies this stuff, but hopefully you can deduce something out of this and share it with me (us).
Good luck spliceofslife.
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