The S-f allele, in spite of having a single genotypic form, as shown by an identical amino-acid sequence, has shown to have two different phenotypic expressions, one active (S-fa) producing an RNase, and another inactive (S-fi) not producing an RNase and conferring self-compatibility. The full activity of the S-fa allele has been observed in the cross 'Vivot' (SfaS23) x 'Blanquerna' (SfiS8), where the pistils of 'Vivot', with the active allele S-fa, produce an active RNase recognizing the S-fi allele of 'Blanquerna' pollen. As a consequence, only the S-8 pollen of 'Blanquerna' is able to grow through the pistils of 'Vivot' and reach the ovule in order to achieve fertilization. The analysis of the progeny of this cross has found only two genotypes among the seedlings, SfaS8 and S23S8, in an approximate ratio of 1:1, confirming that the S-fi from 'Blanquerna' has not been transmitted to the offspring.