The Ac - Ds system in maize (Zea): genetic analysis 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 c locus: the recessive c allele produces a mutant colorless phenotype in the individual seeds (kernels), and the dominant c+ 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 c / c+ heterozygote, the dominant c+ allele is expressed as expected and the plant is colored.
(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, such that all downstream loci are lost. In a c / c+ heterozygote, loss of the dominant c+ allele allows expression of the recessive c allele on the other chromosome, producing an colorless phenotype. The disruption is irreversible, since the acentric fragment is lost.
(c)
Rarely, Ds
transposition will cause the element to transpose into the middle
of the c+ locus, disrupting its normal function
and again allowing expression of the alternative c
allele, as above.
(d) This phenomenon creates an unstable disruption and creates an unstable allele of c+ (cu). If Ds subsequently jumps back out of cu, the c+ allelic function is restored. The seeds will then be a variegated, mosaic of cell patches, with a background due to c and a superimposed pattern of or c+spots.