Cytological and genetic consequences of Ds dissociation

    Chromosome 9 in maize (Zea mays) is recognizable cytologically by a Knob at the end of the long arm. [Note that in maize, homologous chromosomes remain synapsed at the centromere during meiosis].

    Creighton & McClintock (1931) showed that breakage ("dissociation") of the chromosome can occur at the Ds locus, with loss of an  acentric fragment distal to the breakage point. Dissociation occurs only where a dominant Ds allele is present on the chromosome, in contrast to the wild-type Ds⁺ allele (absence of a Ds element).

    In a multiple heterozygote tester strain, the distal arm of one chromosome includes a Ds element linked (in cis) to a series of dominant wild-type markers C Sh Bz Wx, and the other chromosome includes a Ds⁺ allele [absence of a a Ds element] linked to the corresponding recessive alleles c sh bz wx. If the Ds element causes a chromosome break, loss of the dominant markers on the acentric fragment uncovers expression of the multiple recessive markers on the alternate chromosome, which produce a very distinctive phenotype. [Use of multiple markers guarantees that a single mutant phenotype is not the result of a random mutation]. The breakage may thus be detected by inspection of the phenotype of  kernels on the ear, which will be of two types. Cytological examination shows that kernels with the wild-type phenotype have always have two Knobs, whereas those with the recessive phenotype always have only one, consistent with dissociation. In subsequent crosses, it can be shown that the number of Knobs is inherited in a mendelian fashion.

    This experiment was one of the first that showed definitive correlation of a cytological phenomenon with a phenotypic consequence, and that gene loci occur in a linear order. Subsequently, McClintock showed that dissociation at a Ds element occurs only when another element, Activator (Ac) is present elsewhere in the genome, and further that the dissociation phenomenon is sometimes reversible.

Dissociation can also be demonstrated  with the single locus C.