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A multiple allelic series governing incompatibility reactions in certain plant species. Plants possessing the same two S alleles are incompatible. Plants sharing one S allele are semicom-patible in that about half the pollen grains will be able to germinate on the style and subsequently achieve pollination. Such plants are usually indistinguishable from fully compatible plants (plants possessing two different pairs of S alleles) since pollen is usually produced in sufficient quantity to mask the fact that 50% is inviable. S alleles have been found in many fruit crops (e.g. Primus avium), in several grasses, in clover, and in brassicas. There may be more than 40 such alleles in a species and complex dominance relationships are found.
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