Endow S A, Komma D J
Genetics. 1986 Oct;114(2):511-23. doi: 10.1093/genetics/114.2.511.
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.
截毛致死（bbl）染色体携带的核糖体基因过少，以至于纯合果蝇无法存活。在放大条件下，bbl染色体在单代中以低频率回复为bb或近乎bb+。这些回复发生得太快，无法用单个不等姐妹染色单体交换来解释，而且似乎不太可能是由于给定细胞谱系内的多个姐妹染色单体交换所致。对几个一步回复体的分析表明，它们是X-Y重组染色体，可能源于bb处的X-Y重组。从Y染色体向bbl染色体添加核糖体基因，解释了bbl染色体比单个不等姐妹染色单体交换事件所允许的更快回复。对在4至9代放大过程中选择的逐步放大的bbl染色体进行分析，显示核糖体基因模式彼此非常相似。同一亲本染色体的逐步放大产物之间rDNA模式的相似性与通过不等姐妹染色单体交换机制的回复一致。
