Drosophila melanogaster rosy

Drosophila melanogaster rosy, usually written ry, is a protein-coding gene encoding xanthine dehydrogenase, an enzyme involved in purine degradation. FlyBase identifies it as FBgn0003308 and CG7642. The named gene, its enzyme product and the visible eye-colour phenotype used in genetic experiments are different levels of description.^[ref]

The locus became an informative marker in early P-element vector experiments. A 1982 study transferred a wild-type rosy-containing DNA fragment into mutant embryos and obtained animals with inherited chromosomal copies and wild-type eye colour. The phenotype made the introduced sequence's function visible; molecular and inheritance measurements established that more than a transient signal had occurred.^[ref]

As a teaching comparison, two insertions could both restore the marker phenotype while carrying different neighbouring DNA or occupying different genomic positions. The shared visible outcome would not establish identity of the complete constructs. A marker therefore helps select or recognize an outcome, while sequencing, mapping and transmission answer additional questions about what was introduced and where it resides.