In Drosophila, X:A ratio of 1.0 indicates:
X:A ratio 1.0 in Drosophila meaning two X chromosomes and two autosome sets provides strong numerator signal activating Sex-lethal establishment promoter. Double dose of transcription factors sis-a, sis-b, scute and runt cooperatively binds SxlPe enhancer driving robust early Sxl transcription around blastoderm. Resulting Sxl protein autoregulates its maintenance splicing avoiding stop-codon exon and triggers female transformer and doublesex isoforms DsxF. Phenotype therefore female with functional ovaries, female pigmentation, dosage compensation off. Ratio 0.5 yields male, intermediate 0.67 intersex, 1.5 metafemale inviable.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: X:A ratio 1.0 indicates female Drosophila.