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Model of the evolution of a B chromosome. 1 Translocation chromosome derived from duplicated A chromosome fragments results in 2 a decay of meiotic A–B pairing and the formation of a proto-B. 3 The accumulation of organellar and A chromosome-derived DNA fragments, amplification of B-specific repeats, erosion and inactivation of A-derived genes (Muller’s ratchet) and the gain of chromosome drive forms a B chromosome
