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Scientific Proceedings (ioi). 



of the character. There are now about 30 known dominants in 

 Drosophila melanogaster, of which fully two-thirds were first found 

 as single heterozygous individuals. This very large proportion of 

 dominants appearing as single individuals means that the actual 

 mutation has occurred very close to the final stage in the forma- 

 tion of the gamete — probably little if any prior to maturation. 



That mutations may occur in the oogonial cells prior to matura- 

 tion is proved by a few cases in which a single female has given 

 rise to more than a single individual of a new sex-linked recessive 

 character. In the first of these cases the mutant called "cut" 

 occurred as six males among the 131 sons of a particular female. 

 The mutation "tiny- wing" occurred as two sons among about 

 150; "sable-duplication" occurred as three males among 133; and 

 "ivory" as about 10 per cent, of the sons. There are one or two other 

 such cases known. In all of these cases the facts are in accord with 

 the hypothesis that the actual mutation occurred in the oogonial 

 cells of particular females, and from one to a few cell-generations 

 prior to maturation. There has been one case in which a female 

 homozygous for an autosomal recessive was outcrossed to a wild- 

 type male, and produced among 61 offspring 6 flies that were 

 heterozygous for a new allelomorph of the recessive. The muta- 

 tion responsible for the new allelomorph occurred in the wild- 

 type male in the spermatogonial stage far enough previous to 

 spermatogenesis so that approximately a sixth of the sperm carried 

 the gene. The proportion of gametes that carry the mutant 

 character corresponds to the time previous to maturation at which 

 the particular mutation occurred. 



That mutation may occur in the zygote immediately after 

 fertilization is proved by the discovery of nearly a dozen mosaic 

 individuals in which a new mutant has appeared as part of a fly. 

 If the new mutant is a dominant it may appear as part of a female, 

 and if a sex-linked recessive as part of a male. Such mosaics 

 arise by mutation in the zygote, and the parts descended from the 

 mutant cell show the new character while the remainder of the 

 animal shows the original type. 



