The glands of dipterans (flies) have a useful characteristic for analysis of gene location on chromosomes. During their mitotic division, the normal division of the chromosomes is aborted and the replicated chromosomes remain as an integral unit. The chromosome content thus increases geometrically and produces "giant" polytene chromosomes. The chromosomes remain attached at a point where the centromeres fuse, at the chromocenter.1 This is clearly observed in the chromosomes of the fruit fly salivary gland tissue. The fruit fly chromosomes are ideal specimens since they are in a near constant state of prophase and are incapable of further division. Because they have been extensively analyzed for their genetic composition, co-linear maps of genes within genetic linkage groups have been produced and correlated with the physical location of a band on the chromosomes.
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