This logical model assembles the current knowledge on the regulation of primary sex determination in chicken. Relying on experimental data, a gene network was constructed, leading to a logical model that integrates both the Z-dosage and dominant W hypotheses. The model showed that the sexual fate of chicken gonads results from the resolution of the mutual inhibition between DMRT1 and FOXL2; the initial amount of DMRT1 product determines the development of the gonads. In this respect, the W-factor functions at the initiation step as a second device, by reducing the amount of DMRT1 in ZW gonads when the sexual fate of the gonad is settled; i.e. when SOX9 functional state is determined. Developmental constraints that are instrumental in this resolution were identified. These constraints correspond to qualitative restrictions regarding the relative transcription rates of the genes DMRT1, FOXL2 and HEMGN. The model further clarified the role of oestrogen in maintaining FOXL2 function during ovary development.