Illustration of 'joint breaks' in the FBP and FBR schemes. Distinct markers are identified by their shape and size. (a) FBP: Markers (x1 x2 x3 x4) occur in the specified order and consecutively in both of the 'fixed' species. In both 'variable' species, the flanks (x1 x2) and (x3 x4) are consecutive but the region between markers x2 and x3 is disrupted (contains additional markers). (b) FBR: Markers (x1 x2 x3 x4 x5) occur in the specified order and consecutively in both of the 'fixed' species. In the variable species 1 the flanks (x2 x3) and (x4 x5) remain unbroken, while in variable species 2 the flanks (x1 x2) and (x3 x4) remain unbroken, but in both variable species the entire block of 5 markers is disrupted. This situation will be identified as a joint break according to FBR but not according to FBP. FBP is a special case of FBR.