Dominant mutation that reverses the color-choice behavior of 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.