1. Mu particles can be removed from stock 540 Paramecium aurelia by incubation in a solution of 1·0 mg/ml RNAase for 12 h. Only a relatively small percentage of cells become mu-free and it is suggested that this variation is due to the activity of the M genes producing metagons at different times.

2. Paramecia, made mu-free by growth at elevated temperatures, can be re-infected with mu by allowing the cells to swim in a brei of stock 540 paramecia. The re-infection is only permanently successful with cells that were made mu-free 10 days earlier.

The allele kr1, conditioning ready crossability between wheat and rye when wheat is the female parent, is at a locus already known to be on chromosome 5B of wheat. This locus was mapped using a telocentric chromosome consisting of the long arm of chromosome 5B. Mapping was carried out by analyses of F2 and testcross progenies. The two experiments gave a mean recombination frequency, between the locus and the centromere, of 11·45 ± 3·0%. The possibility of different recombination frequencies in male and female meiosis is discussed.

The kinetics of spermatogenesis in the mouse, and the possible genetic controls, have been investigated in different genotypes by means of the method of velocity sedimentation at unit gravity to separate testis cells and nuclei labelled with tritiated thymidine. The progression of the radioactivity through different sedimentation classes of cells provides a measure of their kinetics. The results demonstrate kinetic differences at the stage of spermatocyte differentiation between the C57BL/6 and AKR inbred strains of Mus musculus musculus and in randombred mice of the subspecies Mus m. molossinus. These kinetic differences are controlled by genetic factors which are not linked to the Y chromosome. The relevant autosomal alleles of C57BL/6 are dominant over those of AKR in F1 hybrids.

Mutant colonies of yeast are described which varied in colour from blue to green and in intensity of colour when grown on medium containing methylene blue. Blue mutant colonies contained more dead cells, decolorized redox dyes more slowly, had a higher respiratory quotient on glucose containing medium, and absorbed more methylene blue dye than did wild-type. The blue colony phenotype was induced both spontaneously and at high frequency by ultraviolet light and ethylmethane-sulphonate in both haploid and diploid strains. Many of the light-coloured colonies isolated following mutagenic treatment reverted to wild-type when subcultured but most dark-coloured colonies remained stable. Stable blue mutants were shown to arise by mutation of many separate nuclear genes in haploids and were often caused by recessive lethals in diploids.

Two wild-type strains of Coprinus lagopus isolated from a single basidiocarp differ by a factor of two in their basal level of alkaline phosphatase. The gene responsible for this difference is allelic to reg-2 and unlinked to the pho loci; the allele conferring a lower basal level is dominant both in diploids and dikaryons.

The variability of recombination frequencies between alleles of the ftr cistron is assessed and shown to be normally in the range of ± 25% though occasionally replicates differed by a factor of two and in rare cases even more extremely. It is shown that the variation results from biological differences between separate attempts at the same cross and cannot be ascribed to sampling error or any other controllable cause. In the light of this variability the marker effect alleles already recognized in this cistron are re-examined with a new series of crosses. It is confirmed that marker effect may act to enhance or reduce the recombination frequency but it is shown that enhancement can be expressed in two entirely different ways. There is one type of site which causes a uniform doubling of the recombination frequency in all crosses no matter where the second site is placed in the map. The second type of marker effect enhancement site causes a very great increase in recombination frequency in crosses against relatively closely linked mutants, but its effect diminishes as the distance between the marker effect site and the second site is increased.

The necessary and sufficient conditions for the stability of the equilibrium point with no linkage disequilibrium are obtained for the three locus model with multiplicative fitnesses. It is shown that there are six inequalities that must be satisfied in order for this equilibrium to be stable. Three of the inequalities require that there be heterozygotic superiority at all loci. The other three are exactly those inequalities which are required for each pair of loci to be stable with linkage equilibrium if they are considered to be an isolated two locus system. Thus, all the information needed to determine the stability of this equilibrium with three loci is contained in one and two locus theory.

A new model of mutational production of alleles was proposed which may be appropriate to estimate the number of electrophoretically detectable alleles maintained in a finite population. The model assumes that the entire allelic states are expressed by integers (…, A−1, A0, A1, …) and that if an allele changes state by mutation the change occurs in such a way that it moves either one step in the positive direction or one step in the negative direction (see also Fig. 1). It was shown that for this model the ‘effective’ number of selectively neutral alleles maintained in a population of the effective size Ne under mutation rate υ per generation is given by

When 4Neυ is small, this differs little from the conventional formula by Kimura & Crow, i.e. ne = 1 + 4Neυ, but it gives a much smaller estimate than this when 4Neυ is large.

Mutants resistant to growth inhibition have been obtained by selecting on media containing inhibitory concentrations of either d-glucosamine or l-sorbose. All of the mutants isolated were found to be sugar transport-defective and cross-resistant to growth inhibition by the three analogues 2-deoxy-d-glucose, d-glucosamine and l-sorbose. Like the mutants previously selected for resistance to 2-deoxy-d-glucose, the new mutants described are all shown to be alleles of the ftr cistron.

Strains homozygous for mutants affecting the length of the L4 vein in Drosophila melanogaster were selected for increased or decreased vein length. Substitution of chromosomes selected for their effect on one mutant into the genome of a non-homologous mutant has shown that many modifiers of mutant expression must be considered as acting generally on the character rather than affecting only a specific mutant.

In Klebsiella aerogenes strain V9A melibiose and raffinose fail to induce the lac operon and strongly repress its induction by isopropyl-β-d-thiogalactopyranoside (IPTG). This repression is specific to the lac system and is not released by cAMP, which releases the catabolite repression exerted by glucose and sucrose. It is concluded that melibiose and raffinose bind to the lac repressor molecule in Klebsiella V9A, competitively displacing IPTG, and that the resulting complex is able to repress the lac operon. Other Klebsiella strains tested show the same repression phenomenon.