two gene groups at the A locus function independently but redundantly, so only one group out of the two needs to be heterozygotic for it to work. Similarly for the two gene groups at the B locus. Thus, mating between two individuals succeeds if

Thus there are mating types, each of which can mate with other mating types.

Occurrence

Reproduction by mating types is especially prevalent in fungi. Filamentous ascomycetes usually have two

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