Sex determination in the medaka. Although many details for the molecular model of sex determination in the medaka are still missing, the mechanism is known to be based on the presence of XX/XY sex chromosomes. Following a stage of undifferentiated gonads, males exclusively express DMY, a gene bearing a DM domain, which is a genetic feature that considered central in sex determination pathways of various species. Among the genes induced downstream is DMRT1, which participates in gonadal development and differentiation in fish, birds and mammals. In XX females, the exact genetic cascade triggered in the absence of DMY is unclear, but it supposed to involve sex-specific gene expression, such as FIGa and sex steroid/aromatase regulation.