In this study, we show that at least one lethal mutation at the 3F–4A region of the X chromosome can generate an array of chromosome rearrangements, all with one chromosome break in the 3F–4A region. The mutation at 3F–4A (secondary mutation) was detected in an X chromosome carrying a reverse mutation of an unstable lethal mutation, which was mapped in the 6F1–2 doublet (primary mutation). The primary lethal mutation at 6F1–2 had occurred in an unstable chromosome (Uc) described previously (Lim 1979). Prior to reversion, the 6F1–2 mutation had generated an array of chromosome rearrangements, all having one break in the 6F1–2 doublet (Lim 1979, 1980). In the X chromosomes carrying the 3F–4A secondary lethal mutation the 6F1–2 doublet was normal and stable, as was the 3F–4A region in the X chromosome carrying the primary lethal mutation. The disappearance of the instability having a set of genetic properties at one region (6F1–2) accompanied by its appearance elsewhere in the chromosome (3F–4A) implies that a transposition of the destabilizing element took place. The mutant at 3F–4A and other secondary mutants exhibited all but one reinversion of an inversion to the normal sequence) of the eight properties of the primary lethal mutations. These observations support the view that a transposable destabilizing element is responsible for the hypermutability observed in the unstable chromosome and its derivaties.
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