, in humans, the Y chromosome carries the decisive factor for male sex determination. The presence of a Y chromosome, even in the presence of extra X chromosomes (as seen in Klinefelter's syndrome, 47,XXY), results in a male phenotype (masculine development) . In contrast, the question states that XXY is female in Drosophila, indicating that the Y chromosome does not determine maleness in fruit flies (where sex is determined by the ratio of X chromosomes to autosomes). This comparison proves that the Y chromosome is the active sex-determining element in humans.