The Ac - Ds system in maize (Zea): genetics of "jumping genes"

    In the presence of an Activator (Ac) element, a Dissociator (Ds) element can cause chromosome breaks. The genetic marker for such breaks in this example is expression at the W locus: the dominant W allele produces a mutant White phenotype in the individual seeds (kernels), and the recessive W⁺ allele produces a wild-type dark phenotype.

    (a) In the absence of an Ac element, Ds is not transposable and chromosome breaks do not occur.  In a W / W⁺ heterozygote, the W allele is expressed as expected and the plant is white.

    (b) In the presence of an Ac element, the Ds element can be transposed ("jump") elsewhere in the chromosome. This transposition can cause a chromosome break at the point of insertion, so that the Ds element and any downstream loci are lost. In a W / W⁺heterozygote, loss of the W allele allows expression of the alternative W⁺ allele on the other chromosome, producing an unexpected dark phenotype. [The second chromosome set is the same as in (a), shown reduced].

    (c) Ds transposition can also cause the element to jump into the middle of the W allele, disrupting its normal function and again allowing expression of the alternative W⁺ allele. The seeds are then a variegated, mosaic pattern of cell patches, with a background due to  W and a superimposed pattern of or W⁺spots.

         However, the disruption is unstable (W^u), as Ds may subsequently jump back out of W^u, restoring function of W.  [The second chromosome set is again the same as in (a), shown reduced].