Bobbed lethal (bbl) chromosomes carry too few ribosomal genes for homozygous flies to be viable. Reversion of bbl chromosomes to bb or nearly bb + occurs under magnifying conditions at a low frequency in a single generation. These reversions occur too rapidly to be accounted for by single unequal sister chromatid exchanges and seem unlikely to be due to multiple sister strand exchanges within a given cell lineage. Analysis of several one-step revertants indicates that they are X-Y recombinant chromosomes which probably arise from X-Y recombination at bb. The addition of ribosomal genes from the Y chromosome to the bbl chromosome explains the more rapid reversion of the bbl chromosome than is permitted by single events of unequal sister chromatid exchange. Analysis of stepwise bbl magnified chromosomes, which were selected over a period of 4–9 magnifying generations, shows ribosomal gene patterns that are closely similar to each other. Similarity in rDNA pattern among stepwise magnified products of the same parental chromosome is consistent with reversion by a mechanism of unequal sister strand exchange.
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机译：带刺的致死（bb l ）染色体携带的核糖体基因太少，纯合蝇无法存活。 bb l 染色体在一代人的低频条件下以较低的频率还原为bb或接近bb + 。这些回复发生得太快，无法由单个不相等的姐妹染色单体交换解释，并且似乎不太可能是由于给定细胞谱系中的多个姐妹链交换引起的。几个单步回复子的分析表明它们是X-Y重组染色体，可能是由bb处的X-Y重组引起的。从Y染色体向bb l 染色体添加了核糖体基因，这说明bb l 染色体的恢复比不平等姐妹染色单体交换的单个事件所允许的更快。逐步放大的bb l 染色体的分析显示，这些染色体在4–9倍的放大世代中被选中，显示出彼此非常相似的核糖体基因模式。同一亲本染色体的逐步放大产物中rDNA模式的相似性与不平等姐妹链交换机制的逆转一致。
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