sev¹, Rh5², Rh6¹ triple mutant flies do not have R7 cells and do not express function R8 opsins, therefore colour vision should be completely abolished. However, the blue/green intensity-response curve for these flies appears indistinguishable from wild-type.

Double mutants of sl¹ or sl² with either sev¹ or sev¹⁴ show a mutant phenotype intermediate to that of either single mutant: 35-49% of ommatidia have one R7 cell, 6-12% contain two R7 cells and the rest have no R7 cells.

Sos^JC2 does not suppress the mutant phenotype, R7 cells are absent from ommatidia.

sev¹ mutants - i.e. sev¹; Ppb¹/+ flies, like wild-type flies, prefer UV over green light, while sev¹;+/+ flies prefer green over UV. When tested in a T-maze for preference between darkness and green light (550 nm), both wild type and sev¹ flies show positive phototaxis, while sev¹;Ppb¹/+ and sev¹;Ppb¹/Ppb¹ flies show negative phototaxis. Similarly, tests in a countercurrent apparatus for movement toward and movement away from white light indicate marked preference of wild-type flies for light and of sev¹;Ppb¹/+ flies for darkness. Both the positive phototactic response of sev¹ and the negative phototactic response of sev¹;Ppb¹/+ flies increase with age, reaching a maximum at 4-6 days. The interaction between Ppb and sev is allele-specific, as is indicated in the sev allele entries. Photoreceptor cell R7, missing in all of the sev alleles, was not restored in any mutant combination of Ppb and sev.