Pair-rule gene

[3] They used a genetic screen to identify genes required for embryonic development in the fruit fly Drosophila melanogaster.

They found five genes – even-skipped, hairy, odd-skipped, paired and runt – where mutations caused the deletion of a particular region of every alternate segment.

Later work identified more pair-rule genes in the Drosophila early embryo – fushi tarazu, odd-paired and sloppy paired.

[4] Once the pair-rule genes had been identified at the molecular level it was found that each gene is expressed in alternate parasegments – regions in the embryo that are closely related to segments, but are slightly out of register.

The bands of expression of the pair-rule genes correspond to the regions missing in the mutant.