| Comments |
P.1672 right column bottom paragraph to p.1673 left column top paragraph: "Schalet (1960) detected 51 visible mutations in 490,118 X chromosomes at 13 specific loci, yielding a rate of 8.0 × 10^-6 per locus per generation. The fraction of these due to base pair substitutions is unknown, given the evidence that a large fraction of visible mutations in Drosophila are caused by insertions of transposable elements (Finnegan and Fawcett 1986), it is probable that at least half of Schalet's mutations were of this nature. Mukai and Cockerham (1977) enriched the mutation frequency by accumulating isozyme mutations in 1000 chromosomes sheltered by heterozygosity in a balanced-lethal system for almost 175 generations. In 1,658,308 locus-generations they found three electrophoretic-mobility (band-shift) mutations and 17 null (band-loss) mutations. However, these strains exhibited a high rate of chromosome breakage, probably because of an active transposon (Yamaguchi and Mukai 1974) it is therefore probably more realistic to ignore the nulls, a procedure also justified by the high average ratio of base pair substitutions to other mutations in microbes (Drake 1991). Mukai and Cockerham (1977) estimated that about 0.3 of all amino acid changes were detectable as band shifts. In addition, only about 2/3 of base pair substitutions change an amino acid. Thus, the mutation rate per locus per generation is (3/1,658,308)/(0.3)(2/3) = 9.0 × 10^-6. Averaging the two studies, [investigators] take 8.5 × 10^-6 as a representative rate. The proteins studied by Mukai and Cockerham (1977) were encoded by an average of 973 b and some regulatory sequences must also have been present, so dividing by 10^3 gives 8.5 × 10^-9 mutations per b per generation. The number of cell divisions ancestral to a sperm in Drosophila is about 25 for the young males typically used in laboratory experiments (Lindsley and Tokuyasu 1980 Drost and Lee 1995 J. M. Mason, personal communication), so dividing by 25 gives µb = 3.4 × 10^-10. In Drosophila, G ˜ 1.7 × 10^8 b (Ashburner 1989). [Investigators] will take as Ge the amount of DNA in 1.6 × 10^4 genes, each of length 10^3 b (Bird 1995) this gives Ge = 1.6 × 10^7 b. These and derivative values are given in Table 5." For description of parameters see link Derived by multiplying mutation/bp/replication, BNID 100365, by 25, the number of germ cell divisions prior to sperm formation. See BNID 109990 |