Three Sex Determination Systems — Visual Reference
| System | Organism | Male karyotype | Female karyotype | Heterogametic sex | Gametes from heterogametic sex |
|---|---|---|---|---|---|
| XX-XY | Humans, Drosophila, most mammals | XY (2n) | XX (2n) | Male | X sperm + Y sperm (1:1) |
| XX-XO | Grasshopper, cockroach | XO (2n-1) | XX (2n) | Male | X sperm + O sperm (1:1) |
| ZW-ZZ | Birds, butterflies, some fish | ZZ (2n) | ZW (2n) | Female | Z egg + W egg (1:1) |
Diagram Labels for XX-XY Human System
- Father gametes: 22+X (produces daughter) and 22+Y (produces son) — in equal proportions
- Mother gametes: 22+X only (all eggs are X-bearing)
- Zygote XX: female (44+XX = 46 chromosomes total)
- Zygote XY: male (44+XY = 46 chromosomes total)
Key Rule Summary
- XX-XY: Male heterogametic → father determines sex
- XX-XO: Male heterogametic → male has one fewer chromosome than female
- ZW-ZZ: Female heterogametic → female determines sex (reverse of humans)