Passage:
In her 1940s cytogenetic investigations of Zea mays, Barbara McClintock challenged the prevailing consensus that genetic loci were immutable by demonstrating that mutable kernel variegation resulted from mobile controlling elements. Conventional geneticists had long attributed variegated kernel patterns to unstable chemical mutations occurring at fixed chromosomal loci. McClintock refuted this orthodoxy by documenting recurrent, non-random physical breakages at a specific site on chromosome 9, which she designated the Dissociation (Ds) locus, occurring whenever a second regulatory element, Activator (Ac), was present.
Statement: In the passage, the author references the physical breakages at the Dissociation (Ds) locus primarily to provide observable cytological evidence that links physical chromosomal movement to phenotypic variation, thereby challenging the model of fixed-locus chemical mutations.
Answer: Answer