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Tandem duplication replacing the wild type 6.5kb EcoRI fragment with an 11.5kb EcoRI fragment. The duplication results in two copies of the RanGAP gene (and the Hs2st gene nested within it) being present in Dp(2;2)RanGAPSd. The organisation of the RanGAP and Hs2st genes in the proximal half of the duplication is essentially the same as in RanGAP[+]. The Hs2st gene in the distal half of the duplication encodes a wild-type protein. The remaining distal transcript encodes a mutant version of RanGAP, whose C-terminal portion differs from wild type beginning at the duplication junction. This results in a truncated RanGAP protein that lacks the C-terminal 234 amino acids.
A 5kb tandem duplication is uniquely associated with all RanGapSD chromosomes.
Analysis of different SD chromosomes containing the Dp(2;2)RanGapSD duplication suggests that the duplication arose only once; all SD chromosomes (9 independently isolated chromosomes have been tested) have the same constellation of polymorphic restriction sites in a 50kb region that extends 20kb distal and 28kb proximal to the duplication, and have one of two conserved haplotypes in the region that is 28-43kb proximal to the duplication. Two different SD chromosomes, SD-Roma and SD-VO17, both lack the pericentric and/or 2R inversions that are typically associated with SD chromosomes and were isolated from Mediterranean populations. If the duplication arose only once, it may have originated in the Mediterranean region on a chromosome that was free of inversions. The specific inversions associated with the different SD chromosomes would then have arisen subsequently after the geographic spread of these chromosomes.