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  4. Sex determination. in Drosophila is determined by the ratio of

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Q. Sex determination. in Drosophila is determined by the ratio of

COMEDKCOMEDK 2005Principles of Inheritance and Variation

Solution:

Sex in Drosophila melanogaster is determined by genie balance theory proposed by Calvin Bridges (1926). According to this theory, instead of XY chromosomes, sex is determined by the ratio between X-chromosomes and autosome genomes in Drosophila. Bridges found that genic ratio (X/ A) of 1.0 produces fertile females, whether the flies have XX + 2A or XXX + 3A chromosome complement. A genie .ratio XI A of 0.5 forms a male fruit fly, This occurs in XY + 2A as well as XO+ 2A. Hence, the expression of maleness is not controlled by Y-chromosome, but is instead localized on autosomes.