Elements of Genetics pp. 246-248 (1950)
Darlington and Mathers

Self-Incompatibility

With suitable breeding experiments the linkage of this gene, S, as it is called, with other genes can be established and its mutations recorded. Of course in a fully operative system the number of allelomorphs of the S gene can never be less then three and is always in fact much more numerous. In the small species Oenothera organensis, with a total wild population of perhaps no more than 500 plants, 35 allelomorphs have been identified. In the larger species of Red Clover, Trifolium pratense, one group of 24 plants had 41 different allelomorphs and another group of 20 plants had 37 allelomorphs. And, although closely related, the 60 cultivated varieties of Sweet Cherry examined must have over 20 allelomorphs.

The S gene tells us many things of importance. Its large number of allelomorphs is without parallel. The specificity of their action in the style is nearly always complete, since Sx pollen fails on a style carrying Sx no matter what the other allelomorph may be. In some cases this specificity extends to the strength of action, for some allelomorphs are stronger than others whatever others are present, though in other cases the allelomorphs can strengthen one another's action. Moreover the properties of the pollen itself are determined by the single allelomorph carried in its own nuclei after segregation: there is no delayed effect from the other allelomorph present in its diploid parent (and in its sister pollen). The rapid and specific action puts one in mind of the relation between gene and antigen in the determination of blood groups. The analogy is still more evident from the effect of a rise of temperature which, so Lewis found, speeds up the growth of compatible pollen, yet slows down the growth of the incompatible. Incompatibility is thus due to a positive blocking reaction.