Key Findings

• Sex is defined by gamete type
• There are exactly two sexes in anisogamous organisms
• Intersex conditions do not create additional sexes
• The sex binary is universal across taxa

Abstract

In anisogamous organisms, the sexes—male and female—are functional classes defined by the type of gamete an individual has the biological function to produce. Males produce sperm; females produce ova. This definition is universal across all anisogamous taxa.

Summary

This commentary argues that biological sex is a binary defined by gamete type in anisogamous organisms. The paper defends this position against claims that sex exists on a spectrum.

Conclusion

The sex binary is a fundamental biological reality in anisogamous organisms, defined by gamete type. Arguments against the sex binary based on intersex conditions, mating types, or spectrum depictions are based on conceptual confusion.