The behavioral defects, but not the anatomical aberrations of vam^KS74, have full penetrance. In heterozygotes, vacuoles make their first appearance in the distal medulla about six days after eclosion and the heterozygous flies show much less lamina degeneration than the homozygotes, the anatomical defects being semidominant. Mosaic analysis showed the vacuolization to be independent of eye genotype and the degeneration to be sometimes unilateral. Fate mapping leads to a ventral (blastoderm) focus.

Mutants show many vacuoles in the distal part of the medulla. Appearance of vacuoles age-dependent, first appearing in homo- and hemizygotes half an hour after eclosion and occurring in 100% of these mutants after one hour. In older flies, vacuoles are often visible in the lamina and lobula and occasionally in the central brain. Lamina monopolar neurons L1 and L2 start to degenerate at eclosion and soon afterwards electroretinogram transients disappear. All hemi- or homozygous mutant flies appear nearly blind in tests of movement detection (optomotor response to vertical or horizontal movement and landing response lost). Fixation to a broad stripe has higher light intensity threshold in vam than in wild type. Semidominant.