Sex determination in Drosphila Ive been taught that its based on genic balancing. But I recently saw this question that has caused me some doubt.!enter image description here I thought it would be the 1st option but unfortunately they said it to be the 3rd. But in the answer key, they mention the same line of that of the 1st option. Ive only been taught genic balance in a general sense .I solved by finding the ratio of no. of X chrom. to no. of autosomes. It would help if someone could give me some clarity.

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I think the first possible answer is wrong because maybe by "set" they mean the total number of autosomes. Where as in the figure below it states that the autosome is only responsible for a repressor(Dpn), and that in normal conditions (in pairs i assume) they product equal amounts for females and males.

If the chromosome 2 was not in a proper pair, then I assume the dosage of the Dpn repressor would be off.

The gene dosage is with the activators produced also, sis-a, and sis-b. So I guess in females they produce twice as much activator for the Sxl protein, which later on allows for the repression of male genes and activation of female genes.