Model for the embryonic and eyespot effects of the BE / Fr / Spr locus. For each mutant phenotype, possible genotypes are shown with lines representing the locus, dots corresponding to different sites therein, mutations indicated with stars (the order and the distance between sites is arbitrary), and labels +, a, B and C representing the wild-type and three mutant alleles. Mutations at these three sites, isolated or in combination, define different alleles which can explain all our data. A single copy of the BFSa allele has no obvious effect on eyespot morphology (the WT phenotype), but two copies produce enlarged eyespots which have 'normal' colour composition (the BE3 phenotype; phenotypically indistinguishable from BE/BE2). Mutations at sites 1 and 2 together make up the BFSB allele; it has a dominant effect on eyespot size (the BE/BE2 phenotype), and, in combination with the BFSa allele, affects colour composition (the Spr phenotype). This explains the recessive colour composition aspect of Spr inheritance and the presence of BE3 individuals (BFSa homozygotes) in crosses between two Spr individuals (Table 1), and is consistent with Spr having been isolated from the BE stock (see Materials and Methods). A mutation at a third site in this locus (corresponding to the BFSC allele) affects eyespot ring boundaries (the Fr phenotype). The alleles BFSB and BFSC are embryonic recessive lethal and display a segment polarity phenotype.