Heterozygotes are WT. Segregates Dpy. Segregates sterile Unc--Unc cannot back. Balanced well.

Heterozygotes are WT and segregate WT, Stu and DpyUncs. Molecular lesion: amber mutation terminating translation at amino acid 159. DNA replication continues in the absence of mitosis. Mutants enter mitotis at the normal time and form bipolar spindles, but fail chromosome alignment at the metaphase plate, sister chromatid separation and cytokinesis.