Breakpoint mapping in der(1;7)(q10;p10) and proposed mechanism generating this translocation. (A) Breakpoints are widely distributed within D1Z7 and D7Z1. The ends to the short arm within both alphoids (*) are free from recombinations. A broad arrow indicates the hypothetical critical point on each alphoid. A recombination that occurs beyond this point might compromise segregation of recombined chromosomes and eventually result in their loss. Thin arrows represent the locations of the breakpoints; yellow stars indicate the hypothetic active centromere. (B) Three possible models for the quadriheadral formation with relation to relative location of active centromeres to the breakpoint. Active centromeres may be either contralaterally (Model 1), ipsilaterally (Model 2), or centrally (Model 3) positioned. For the sake of stable maintenance of sister chromatids, the contralateral model might be favored, and only this seems to be compatible with the real distribution of the breakpoints and with allelic distribution in this unbalanced translocation. (C) Proposed mechanism of generation of 46, XY (or XX), +1, der(1;7)(q10;p10). Active centromere sequences on chr1 and chr7 are indicated as yellow and blue circles, respectively. For simplicity, some features of chromosomes are not always as they really are. For example, sister chromatids are depicted separately from before recombination, which should be tightly paired and stuck to each other through cohesion molecules.