One-to-one correspondence between the two genetic units and the tandemly duplicated transcriptional units of the polyhomeotic locus of Drosophila

polyhomeotic (ph) is a complex locus in Drosophila defined by two genetic u nits. Two mutational events are necessary to obtain the null lethal phenoty pe. Molecular analysis has shown that the ph locus contains two transcripti onal units coding for two very similar proteins. Although a strong argument in favor of a strict correlation between the genetic and molecular units c an be constructed, there is no direct evidence for the hypothesis. Here, we show for all cases with detectable molecular defects that X-ray-induced ge neration of an amorphic allele from a pre-existing X-ray-induced hypomorphi c allele with a lesion limited to one unit invariably involves a rearrangem ent in the other unit. This result proves that each genetic unit correspond s to one transcription unit.