Simultaneous Transmission of Three Stable Genotypes in <Emphasis Type="Italic">Drosophila melanogaster</Emphasis>

REPLICATION of the double stranded DNA genomes of bacteria takes place by a semi-conservative mechanism¹, but although autoradiographic studies have confirmed that eukaryotic DNA is replicated semi-conservatively^2,3, our lack of knowledge about the structure of the eukaryotic chromosome means that this finding does not prove that the conserved unit is a single polynucleotide strand of DNA. Indirect information supporting the hypothesis that the conserved unit consisted of a single polydeoxyribonucleotide strand has accumulated from transmission studies in chemical mutation experiments. Two phenotypic classes of mutants are readily distinguishable as a result of chemical mutagenesis; mosaic (fractional) mutants and complete (whole body) mutants. Mutation studies assume that the treated gametes contain one DNA polymer per chromosome and that the polymer is made up of two complementary nucleotide strands. With chemical mutagens (excluding acridine dyes) it is probable that only one of the two complementary strands would be chemically altered⁴. Following fertilization, DNA replication occurs, fixing both the mutational event and its complementary wild type site. The zygote will possess a mutation which will be phenotypically expressed in the adult fly depending on the early morphogenic movements in the fly's development. Assuming that only one strand is altered, the two cell lines would contain a mutant genotype and a wild type genotype. Two genotypically mutant cell lines would arise if two mutational events occur in the opposing polydeoxyribonucleotide strands within the same genie region. In this case, there would be no genotypically wild type cells in the embryo.