We studied the organization of rDNA spacer sequences within several rDNA loci in D. melanogaster. Every locus showed many discrete length classes of rDNA spacer, ranging from 2.4 kb to about 20 kb. Different loci show characteristic distributions of spacers within the various length classes. Using this molecular characteristic as a genetic marker, segregation, recombination and occurrence of unequal crossing-over were studied. The rDNA loci segregated with a Mendelian pattern; interhomologous recombination events occurred at the expected rate; conversely unequal crossing-over within the rDNA locus appears not to be as frequent as expected.
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