Abstract
SUMMARY: The genes coding for the 18s and 28s ribosomal RNA (rRNA) are present on both theXandYchromosomes ofD. melanogasterat a site known as thebobbedlocus. Exchange was observed in males between a normally orientatedXandYchromosome (Dp(1; 1)scv1andBSY y31d) with a frequency of 0·079%. One-quarter (7 in 27) of these exchange products between two + chromosomes which both carried sufficient rRNA genes for abb+phenotype exhibited abbphenotype. Evidence is presented that one-half, and possibly all, of the exchanges involved the repetitivebbgenes. These results together with those reported by Palumbo, Caizzi & Ritossa (1973) imply that the repeatedbbgenes of either (or both) theXorYchromosome are not arranged with uniform polarity and, further, that spermatogonial exchange between theXandYchromosomes may be restricted to thebbloci.