Polytene Chromosomes in Drosophila

Salivary gland chromosomes in Drosophila and other Diptera undergo a process called endoreplication, in which the chromatids double mitotically but do not separate. Ten such doublings result in 1,024 chromatids lying side by side. Regional variation of chromatid condensation produces darkly-staining bands (b) separated by lighter interbands (ib). The banding patterns are visible microscopically, and are characterisitic of various species and populations within species. For example, the typical pattern in Drosophila pseudoobscura is called Standard (ST).