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
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: X:A ratio 1.0 indicates female Drosophila.