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| Modern Genetic Analysis Chapter 14. Regulation of Gene Transcription Transcription: Gene Regulation in Eukaryotes an Overview Figure 14-29. Tab is due to a gene fusion. (a) The locations of the Abd-B and sr genes on a map of a normal chromosome 3 are depicted. The two genes are (very) approximately 5 million base pairs apart, and there are normally hundreds of genes in between them. (b) The Tab inversion has one chromosomal breakpoint within the Abd-B gene and the other within sr (arrows in part a). As a result of misfusion of these breakpoints, an inversion arises in which pieces of the Abd-B and sr genes are fused at either end of the inversion. (c) A magnified view of the centromere-proximal breakpoint of the inversion. The inversion breakpoints have produced a DNA molecule in which the promoter region of the Abd-B gene is now only 28,000 base pairs away from enhancer elements of the sr gene. This causes the Abd-B transcript to be ectopically expressed in some parts of the fly's thorax, where the wild-type Abd-B gene is not transcribed.

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