Self-incompatibility (SI), the recognition and rejection of self-pollen, is useful for studying genetic and phenotypic aspects of cell-cell signaling. Two allele classes (Class A and B), separated by sequence of SRK alleles, exist in 'Arabidopsis lyrata' (Brassicaceae). Alleles were divided into at least three dominance levels; Class A and B alleles were not separated by dominance. I investigated both the dominance levels identified (phenotypic expression) and differences in expression of the classes of SRK alleles (molecular expression). For the phenotypic analysis, compatibility of crosses within full-sib families suggested four dominance groups, three containing only Class A alleles, and a fourth containing only Class B alleles. Molecular expression found that while Class A alleles are not expressed in leaf tissues, Class B alleles are unexpectedly expressed in floral and leaf tissues. Class B alleles may represent alleles recruited to the locus, and gene conversion may increase diversity at the SI locus.