Schematic diagram of the three-primer PCR assay for molecular karyotyping of 2R b. The white box with an asterisk in Figure 3 is represented here. Areas common to both arrangements are connected by dotted lines. The grey box represents an insertion exclusive to the 2Rb arrangement, to which primer bRev anneals. Primer bFor is a universal primer that anneals to a region common to both arrangements. Although +bRev can anneal to both arrangements at different distances from bFor, size limitations on successful PCR amplification restrict the product to a 2R+b-specific fragment of 630 bp. In combination with bFor,primer bRev amplifies a 429 bp sequence diagnostic of the 2Rb arrangement.