Preponderance of Mosaics

The preponderance of mosaics among induced mutants has been recognized as a characteristic feature of chemical mutagenesis. When mutations were induced by X or 7 rays, it was noticed that the number of induced mosaics increased strikingly with the progress of spermiogenesis after meiosis. Mutations induced by chemicals, however, manifested themselves as mosaics even when the chemicals were administered before meiosis. 

Since mosaics and whole-body mutants can be assumed to be the products of mutational events occurring in one of the double helices of DNA in the former and in both helices in the latter, the finding seemed to indicate that each homologous chromosome has already split into two chromatids by the 4th instar, day 3, and that the mutagenic action of these chemicals is confined mostly to one of the double helices of DNA. Therefore, there must be a total of at least four chromatids or eight DNA helices in the spermatocyte at this stage. 

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