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The behavior of an unstable allele of the singed-bristle locus on the X chromosome was studied in connection with the occurrence of lethal mutations on that same chromosome. The unstable allele, weak singed (snw), is under the control of the P-M system of hybrid dysgenesis and, in the M cytotype, mutates secondarily to extreme singed (sne) and to wild type (sn  +) at high rates. Chromosomes whose snw allele had mutated in this fashion sustained lethal mutations at a rate of 3%; whereas, those whose snw allele had apparently remained unchanged, acquired lethals at a lower rate, 1.3%. The significant difference between these values indicates a statistical coincidence between the phenomena of snw instability and X-linked lethal mutation induction. This coincidence can be explained by postulating that mutations at the singed locus sometimes release a genetic element capable of reinserting elsewhere in the chromosome. Alternately, snw instability and lethal induction might be associated because they are the effects of a common cause, perhaps some mutation-inducing substance present in various amounts in the germ cells of dysgenic flies.—The lethals that occurred on chromosomes whose snw allele had mutated to sne mapped preferentially close to singed. The lethals on the snw and sn  + chromosomes did not show this concentration on the map. Cytological analysis of samples of all three types of lethal chromosomes indicated that, with one exception, there was no detectable breakage at the singed locus itself. The single instance of breakage at singed was not associated with any change in the singed phenotype. Thus, the instability of snw apparently does not involve detectable breakage of the singed locus, or if it does, this breakage is not a common event.

AN INCREASE IN THE X-LINKED LETHAL MUTATION RATE ASSOCIATED WITH AN UNSTABLE LOCUS IN DROSOPHILA MELANOGASTER