In order to investigate the importance of the nuclear genotype during morphogenesis of the cortex in Paramecium aurelia, a search was made for conditional and unconditional mutants affecting cell shape. Ten different mutants were readily recovered following mutagenesis with N-methyl-N-nitro-N-nitrosoguanidine on stock 51s of syngen 4. The detailed pattern of surface units remained unchanged in these mutants but each one can be described as resembling a normal cell twisted about its long axis. Nine of the mutants complement each other and show no linkage. The remaining mutant, a ‘chain former’, has conditional expression, and is only recognizable at 35 °C. The penetrance of this mutant is affected by the food organism used. The conditional mutant also interacts with seven of the other mutants in pairwise homozygous combinations so that in these genotypes it can be classified after growth at 25 °C. The mutants are discussed in relation to the search for genie effects on the cortical pattern.

In zygotes from matings between Hfr, F′ or F+ donors of E. coli K and F− recipients of E. coli B the restriction of λ. K and λ.C is 100 times less than in unmated B cells. This decrease in the capacity of zygotes to restrict phage λ is transient and is not related to the transfer of genes controlling host-induced modification during the cross.

The first two steps towards increasing ampicillin resistance in Escherichia coli concern the genes ampA and ampB which are located at least 20 min from each other (Eriksson-Grennberg et al. 1965 and the two preceding papers). This paper describes a third class of ampA-containing mutants (designated ampAB) which are resistant to D, L-ampicillin concentrations of 100 μg/ml. When such as F-strain (D31) was crossed to different Hfr strains analysis of the trp+ and proB+ recombinants indicated that resistance genes were located between trp and proB. Altogether five classes of recombinants were produced but only the two genes ampA and ampB were recovered. One interpretation suggested is that the resistance of D31 is due to the presence of ampA and ampB and a chromosomal aberration by which ampA was moved to a position near ampB. It was possible to transduce both intermediate levels of resistance as well as the full resistance of an ampAB donor strain, but the strains produced were unstable. In crosses the presence of ampAB in the recipient reduced the number of recombinants by reducing the number of stable pairs. In an Hfr strain ampAB was shown to give rise to additional difficulties in establishing the cell contact during mating. Some ampicillin resistant mutants also showed decreased ability to propagate the RNA phage MS2.

The origin of a number of R-factors used for genetic studies in Pseudomonas aeruginosa is described.

Segregations at five loci on the left arm of chromosome VII and other data from published sources were used to test a new mathematical model of meiotic segregation in trisomics of the yeast Saccharomyces cerevisiae. Results support this model which predicts that non-centromere-linked genes in trivalent complexes segregate at random. The data indicate that bivalent–univalent complexes are infrequent in trisomics and that recombination between all three homologues is frequent in regions close to the centromere. A test of homologue interference produced no evidence that a crossover between two homologues has any influence on the probability that the same two homologues will be involved in an adjacent crossover.

The gene ‘sex-linked anaemia’ (sla) appeared first in the progeny of a daughter of a male that had been irradiated with 500 r. of X-rays. The daughter was born one month after the irradiation of her father. Mated to an unrelated wild-type male she produced sixteen sons of which six were anaemic. Her fertility was normal. Experience in subsequent matings showed the anaemia to be rather difficult to classify. In litters examined on the day of birth the anaemic young were usually clearly distinguishable by their pale colour. But in older litters, and even in a few at birth, the classification could not be made with confidence. The haematology of the anaemics is described by Grewal (1962).

Expected changes in the gene frequency and the population mean for a quantitative trait are described for selection in a population of size N at a single locus where the favoured allele has initial frequency q0 and selective value s. Models of additive and completely dominant gene action are compared. Results are generally expressed as the half-life of the total change relative to N.

If the favoured allele is additive or recessive the half-life of the gene frequency and mean of the trait are usually reduced when q0 or Ns is increased. However, if the dominant allele is favoured the half-life of gene frequency is still generally reduced as Ns is increased, but has a minimum at low or intermediate values of q0. Since inbreeding depression and selection oppose each other when the dominant allele is favoured the response in the mean of the quantitative trait may change in direction during selection.

The inheritance of resistance to the rodenticide, warfarin, in the Norway rat, Rattus norvegicus, derived from a wild rat population in Scotland was studied in the backcross, intercross and testcross. The resistance was found to be due to a major gene with about the same map position in Linkage Group I as the warfarin-resistance gene, Rw2, which occurs in the wild rat population in Wales. In heterozygotes, the Scottish resistance gene, unlike the Welsh gene, is incompletely penetrant in expression, though the penetrance was found to increase markedly in response to selection. Differences between the Scottish and Welsh types of resistance suggest that the two resistance genes are allelic.

The t-complex on mouse chromosome 17 results in transmission ratio distortion in males heterozygous for complete haplotypes, and sterility in those homozygous for semi-lethal or doubly heterozygous for complementing lethal haplotypes. This sterility is due to inability of spermatozoa to fertilize. The haplotype th7 is an unusual laboratory-derived haplotype, postulated to carry a small duplication of t chromatin. Males heterozygous for th7 show a new form of sterility, apparently due to failure to form copulation plugs during mating. This is accompanied by a strong propensity to acute urinary obstruction. It is suggested that both the failure to form copulation plugs and the urinary obstruction are due to some abnormality in function of the accessory sex glands, and are the result of incorrect dosage of a gene in the postulated duplication. The symbol Msu for male sterility and urinary obstruction is suggested for the locus concerned. Previously a recessive form of abnormal behaviour had also been attributed to this duplication.

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.

The frequencies of non-disjunction at first and second meiotic division, in both male and female of Drosophila melanogaster, are estimated. Numbers of exceptional individuals scored in several broods of increasing parental age are also presented. These results fail to show any evidence of an age-effect. Work claiming to demonstrate the existence of an age effect in Drosophila is reinvestigated. The importance of these results and their relevance to human studies are discussed.

The claim that CB 1506 produces an unusually high proportion of visible to lethal mutations could not be confirmed. The ratio of visible to lethal sex-linked mutations was less than 1:10 in both Muller-5 and attached-X tests. In one brood it rose to 1:5 when unconfirmed visible mutants were included. The much higher ratio obtained by Fahmy and Fahmy is interpreted as a methodological artifact, due to the inclusion in their calculations of mosaic visibles but not of mosaic lethals.

Two visible mutations—twisted abdomen and wsatsuma—occurred more than once. This suggests the possibility of a locus or allele specific effect of CB 1506.