Molecular karyotypes of clones from 1738 RGFP cross. A. Ethidium bromide stained pulsed field gel. Size marker: chromosomal DNAs from Hansenula wingei. Cz = compression zone, a region of the gel where several large chromosomal bands are trapped. Mc = minichromosomes of approx. 100 kb in size. B - D. Autoradiographs of Southern blot of same gel showing results following hybridization with the probes indicated. All blots were washed to 0.1 × SSC at 65°C. E, F. Southern blot of second gel (not shown) hybridised with probes indicated. Chromosome III homologues are identical in size in Fly 7 clones 1, 2 and 6, while clone 2 has two copies of the mRFP gene.