The question of what is meant by random fluctuations in selection intensities in a finite population is re-examined. The model presented describes the change in the frequency of a gene in a haploid population of size M. It is assumed that in any generation the adaptive values of A and a are equally likely to be 1 + s: 1 or 1: 1 + s. If s is the selective advantage and x the frequency of gene A, then the first two moments of the change in frequency are found to be m(Δx) = x(1 − x)(1 − 2x) θ/2M and

where E(s2) = θ/M. The ultimate probability of fixation is computed, showing that variability in selection increases the chance of fixation of a rare gene. A more general form for m(Δx) also is obtained. This form is compared with the equation currently used in describing random fluctuations in selection intensities.

Interspecific aneuploids were made in the genus Cucurbita by pollinating the autoallotroploid (diploid C. moschata – haploid C. palmata) with pollen from diploid C. moschata. Seventy-eight, or 0·36%, of the potential ovules showed some degree of continued development after pollination. The high frequency of continued ovule and embryo development indicated a preferential distribution or loss of C. palmata univalents during meiosis in the autoallotriploid and that megagametes aneuploid for more than one C. palmata chromosome were capable of initiating continued ovule development after fertilization. Lethal effects of interspecific aneuploidy were expressed by complete embryo abortion to sterility of mature plants. Nine plants developed to maturity and seven were cytologically identified. Four were trisomic and two were monosomic for single C. palmata chromosomes, i.e. they were 2n + 1 and 2n + 19 (n = 20) respectively. One plant was 2n and phenotypically identical to the C. moschata parent. One 2n + 1 plant was fertile and the single C. palmata chromosome was transferred in the succeeding generation.

Penetrance and expressivity of the Drosophila mutant Abnormal-abdomen (A53g) may be reduced by adding inhibitors of protein synthesis, of RNA synthesis, and of oxidative phosphorylation to the defined, sterile medium on which larvae are developing. When grown on regular diet, the mutant flies contain a higher concentration of total protein than do wild-type flies. The metabolic inhibitors which reduce the expressivity of the mutation also lower protein concentration in these mutant flies. The reduction of protein synthesis is directly correlated with the reduction of expression of the mutant genotype. The regulation of the morphological abdominal abnormalities by the complex A53g genotype is discussed in relation to the control, by this same genetic system, of the mechanisms of protein synthesis.

The stimulation of in vitro tRNA aminoacylation by post-microsomal supernatant enzymes isolated from Abnormal Abdomen (A53g) adult flies is a function of the residual genome of these flies rather than of the major mutant gene, A53g. Genes controlling this stimulation are located on the X chromosome as well as on the autosomes. These observations are discussed in terms of the phenotypic effect of the major mutant gene in response to changes in protein synthesis which are under the control of modifier genes responsible for aberrant genetic translation mechanisms.

An infinite population practising a constant amount of selfing and random mating is studied. The effects of the mating system on two linked loci with an arbitrary number of neutral alleles are determined. Expressions are obtained for the two-locus descent measure, and hence genotypic frequencies and disequilibria functions, in any generation. The nature of the equilibrium population is deduced. The special cases of pure selfing or pure random mating and completely linked or completely unlinked loci are considered separately.

Of 150 wild-type strains of Pseudomonas aeruginosa examined, 48 formed recombinants when mated to P. aeruginosa strain PAO FP− and hence presumably possess sex factors. Three different types of sex factor were distinguished by the pattern of transfer of particular markers in different regions of the chromosome and by the ability to confer resistance to mercury in strain PAO. One new sex factor, FP39, was studied in detail, and while similar to the previously studied FP2 in terms of transfer kinetics, natural stability and resistance to curing by acridines, it differed from FP2 in promoting chromosome transfer from a site 10 min to the left of the FP2 origin and in showing apparently aberrant entry kinetics for a leucine marker situated 48 min from the FP2 origin. This was due to FP39 having a genetic determinant either for a structural gene of leucine biosynthesis or a specific suppressor gene for this locus. PAO strains carrying both FP2 and FP39 were unstable for both sex factors, suggesting a relationship between them.

Klebsiella aerogenes V9A possesses at least three galactoside permeases, the lactose permease (LacP), the melibiose permease (MelP) and a third permease (GPIII), and also a galactose permease. It is shown that the first three of these permeases all accumulate both lactose and melibiose efficiently, while the galactose permease takes up neither sugar. Lactose only induces LacP and melibiose only induces MelP. d-fucose, isopropyl-β-d-thiogalactoside (IPTG) and methyl-β-d-thiogalactoside (TMG) all induce GPIII, but galactose does not. GPIII takes up both lactose and melibiose but cannot accumulate lactose in the presence of IPTG. Thus substantial differences are found between the galactoside permeases of Klebsiella and Escherichia coli.

A temperate phage, M6, has been isolated from a lysogenic strain of Pseudomonas maltophilia. An extended range mutant, M6a, plates on P. aeruginosa and is capable of general transduction in this species. This phage may be of value for comparing the genetic maps of different species of Pseudomonas.

Proposals are made for symbols consisting of a primary gene symbol of lower case italic letters referring to the mutant phenotype (three letters for new symbols, old symbols unchanged), an italic capital locus specific letter (or a hyphen in mutants not yet tested for allelism) and an italic mutant number. Optional superscripts are recommended to convey additional information. Further proposals relate to symbols for chromosome aberrations and to strain numbers.

Measurement of ColEl transfer by ColV2, in E. coli K12, showed that ColV2 specifies a surface exclusion system. This system is different from those specified by Flac, and by ColVBtrp and Rl-19. Like these other two systems, the ColV2 surface exclusion system is subject to inhibition by the appropriate transfer inhibitor, and is not expressed in stationary phase cells.