Balance Theory of Sex Determination X/A ratio in Drosophila

Results obtained from a cross of a triploid (3A + XXX) ♀ fly, and a diploid ♂ (2A + XY) fly in Drosophila
Fig. 17.3. Results obtained from a cross of a triploid (3A + XXX) ♀ fly, and a diploid ♂ (2A + XY) fly in Drosophila.
The experiment of Bridges mentioned above, though demonstrated that Y-chromosome is not important for determination of sex, it did not indicate whether X-chromosome alone determines the sex or if autosomes also play any role in the mechanism. Individuals could, however, be obtained, which had two X-chromosomes as in the normal female, but were intersexes. Each of these intersexes had an extra set of autosomes (A) indicating that autosomes play a definite role in determination of sex. In this connection, Bridges' experiments on intersexes and supersexes are of special importance. Bridges, as early as 1922, came across certain Drosophila individuals which were triploids and thus had three sets of chromosomes (3A + 3X).
These triploid individuals were normal females and were crossed with diploid males (2A + XY). The results obtained from such a cross are shown in Figure 17.3. As is obvious, from such a cross, normal diploid males, triploid females, intersexes, supermales and superfemales were obtained. Chromosome constitution of these phenotypically different sexes had a definite bearing on the mechanism of sex determination, as explained in the next section.
Results obtained from a cross of a triploid (3A + XXX) ♀ fly, and a diploid ♂ (2A + XY) fly in Drosophila
Fig. 17.3. Results obtained from a cross of a triploid (3A + XXX) ♀ fly, and a diploid ♂ (2A + XY) fly in Drosophila.