Bridges, 1926.

One of a group of Y chromosomes that carry insufficient numbers of ribosomal genes to produce wild-type bristle morphology. They are characterized by having 125 or fewer ribosomal cistrons compared to 225 in wild-type Y chromosomes. Since numbers of ribosomal cistrons are unstable and subject to change by magnification or reduction, the estimates of cistron number presented here represent historical observations and need not reflect current compositions. The distinction between chromosomes designated Ybb^- and those designated Ybb not always based on objective criteria; however, Ybb chromosomes are generally hemizygous viable whereas Ybb^- chromosomes are generally hemizygous lethal <up>e.g., in In(1)sc^4Lsc^8R/Ybb^-</up> and therefore have few if any ribosomal cistrons. The original Ybb^- was shown to have a foreshortened short arm; despite this cytological deficiency, however, derivative chromosomes with increased numbers of ribosomal genes have been recovered.