and usually two genes for pheromones.

The A locus ensures heterothallism through a specific interaction between HD1 and HD2 proteins. Within each group, a HD1 protein can only form a functional heterodimer with a HD2 protein from a different group, not with the HD2 protein from its own group. Functional heterodimers are necessary for a dikaryon-specific transcription factor, and its lack arrests the development process.