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A detailed cytogenetic study of male-viable and lethal deficiencies affecting the w-spl interval in Drosophila melanogaster has revealed the existence of genetic duplication such that, for example, the consequences of the loss of salivary chromosome band 3C3 are essentially compensated for by the presence of band 3C5-6, and vice versa. Although each of the duplicate elements possesses rst + and vt + activity, rst and vt phenotypes appear in males when 3C3 and part, but not all, of 3C5-6 are deleted. The degree of rst and vt expression can be correlated with the amount of material lost from 3C5-6. Deletions removing the entire 3C3-6 interval are male lethal. Despite the duplicate elements, at least one EMS-induced, presumptive point mutation expressing only rst is known; two others express both rst and vt. No loci other than rst and vt occur between W and spl. Band 3C2 appears to be associated with the w locus, which probably extends into the interband space between 3C1 and 3C2. The w locus is not involved in the rst-vt duplication in the 3C3-6 region. — The cytogenetic characteristics of the 3C region—a high coefficient of crossing over, frequent induced chromosome breakage, ectopic pairing, constriction, and an extended replication period—can be correlated with the fact that in 3C a relatively long stretch of DNA, nearly 2% of the entire X chromosome, is highly compacted into but few adjacent bands. These characteristics do not necessarily represent special properties of intercalary heterochromatin; they can be interpreted as reflecting the properties of any similarly organized euchromatic region.

Two tandem duplications of the X chromosome associated with the white eye color locus are described. In heterozygous females both revert to a nonduplicated chromosome without detectable meiotic recombination. Clustering of revertants suggests the reversion event occurs in the germ line prior to meiosis. Similarly in males one duplication also reverts with clustering implying a premeiotic event. Revertant X chromosomes derived from males are either nonduplicated or deleted. Intrastrand exchange can account for some but not all revertants recovered.