Redundancy due to additional duplications. The same conversion event may be detected using several paralog pairs in the same primary species. For instance in panel (A), A3 can be detected as the target of conversion from both A1 and A2. If the direction of the conversion was reversed, then both A1 and A2 could be detected as targets of conversion from A3. In (B) and (C), the new paralogs A1 and A2 may still be similar enough for A3 to be detected as the target of conversion from both, though only one was the actual source. But here if the direction of conversion was reversed there would be no confusion or redundancy, since the converted A2 would be easily distinguishable from the unconverted A1 (or vice-versa in (C)). In summary, the single conversions in (A)-(C) could be confused with case (D), which has two separate conversion events.
Song et al. BMC Evolutionary Biology 2011 11:226 doi:10.1186/1471-2148-11-226