Question

. 7. Don’t panic about the following problem! Although the setup is long, you just need to read the problem carefully and use a bit of logic to solve it. In Drosophila, like in humans, females have two X chromosomes, males have X and Y chromosomes, and organisms are diploid. However, unlike in humans, sex determination in Drosophila does not depend on the presence/absence of Y chromosome. Instead, it is determined by the ratio of X chromosomes to autosomes (X to A ratio) and relies on the cascade of alternatively spliced factors, some of which are themselves regulators of alternative splicing.   The splicing repressor Sex-lethal (Sxl) resides at the top of this cascade. Whether the active Sxl protein will be present or not is determined by the X to A ratio, which controls the splicing pattern of the early expressed form of Sxl. If the ratio of X to A = 1, the early expressed Sxl transcript is spliced in a way that leads to the formation of the functional Sxl protein. After that, Sxl maintains its own expression by controlling the splicing of its own pre-mRNA. When the ratio of X to A = 0.5, the Sxl transcript is spliced in a different way and no functional Sxl protein is produced. In addition to controlling its own splicing, Sxl is necessary for formation of the active product of the Transformer (Tra) gene, which serves as another regulator of splicing. Sxl contributes to the formation of active Tra product by binding to its transcript and repressing a particular splicing event.           a) Predict the resulting gender of the XY flies in which the native gene has been deleted and the only Sxl expression comes from a particular transgene. This transgene contains a Sxl cDNA prepared from the transcript normally found in XX flies. Briefly explain your answer.      (3 points)








b) Would the gender change if the same transgene was expressed in the otherwise wild-type XY flies (which also have normal native Sxl gene in their genomes in addition to the transgene)? Briefly explain your answer. (3 points)

c) What about the gender of the XX flies in which the Sxl-binding site in the Tra gene is mutated? Briefly explain your answer.    (3 points)  

Answer 1

The main determining factor role of gender in Drosophilla is played by the Sxl gene product whose presence or absence lead to the development of gender .