The DcY(H1) chromosome and BARTL assay.

Chromosome breakage and telomere loss in the BARTL assay. The DcY(H1) chromosome (drawn as sister chromatids in G2) is a Y chromosome that carries y⁺ on the short arm and dominant B^S on the long arm. Inverted FRTs are inserted proximal to B^S. The centromere is represented as a solid black circle, the telomeres as green rectangles, B^S gene as a red rectangle and inverted FRTs as half-arrows. When FLP mediates unequal sister chromatid exchange between inverted FRTs a dicentric and an acentric chromosome are produced. In the subsequent mitotic anaphase the dicentric chromosome is pulled to opposite poles and usually breaks. Each daughter cell receives a chromosome with a single broken end and one or both daughter cells lose the B^S-containing acentric fragment.