Heterozygotes have an indentation across the dorsal mesothorax giving the appearance of a shrunk thorax. A groove typically runs across the thorax in a V shape. There is some variability in the expressivity; a small fraction of flies have only a marginal phenotype, but the penetrance is nearly complete. Newly emerged flies often do not show the phenotype or have only a faint line on the thorax; the groove becomes visible as the cuticle hardens. Heterozygotes have good viability and fertility.