Flies have an outgrowth at the front edge of the eye which often appears to be a mirror-image duplication of part of the orbital region including the orbital and fronto-orbital bristles. Sequential duplications are also seen. The phenotype is almost totally recessive; only about 1% of heterozygotes express the phenotype, while the penetrance in the homozygote varies from 41 to 99%, depending on the cross and sex. The expression of the phenotype is quite variable. It is most evident as an orbital duplication when there is a connection between the bristle-bearing tissue in the medial part of the anterior eye and the region of the orbital and fronto-orbital bristles. Clonal analysis indicates that the duplicated orbital region often originates from a cell population different from that giving rise to the normal orbital region.

Heterozygotes have either a malformed pit in the middle of the eye or, more often, a nick at the front edge of the eye with a bristle- or antenna-like outgrowth. The phenotype is usually seen in only a small percentage of flies, expression is enhanced when the flies are raised on food containing Brewer's yeast. Homozygotes have a larger nick and antennal outgrowth, with 100% expression of the phenotype in a pr¹ Mal¹ stock.

Heterozygotes have an eye phenotype ranging from slight disarrangement of the facets to removal of more than half the facets. Best expression of the phenotype is seen when the flies are raised on food containing Brewer's yeast.

RK3.