Klebsiella aerogenes V9A carrying a lac plasmid in addition to its chromosomal operon showed strongly positive fermentation of lactose on MacConkey lactose agar plates, and was found to transport the lactose analogue thiomethyl-β-galactoside (TMG) at a rapid rate. The strain that had been freed of the plasmid showed moderate transport due to the chromosomal lac operon. When a plasmid bearing a mutation in lac Y was inserted into a strain with a normal Y gene, the resulting diploid became lactose-negative in phenotype. The presence of E. coli F′lac factors that carried lac Y mutations, whether deletions or missense or nonsense mutations, also rendered lac Y+Klebsiella lactose-negative. Such diploids, after growth in 1% lactose, transported TMG at a much lower rate than the corresponding plasmid-free lac Y+Klebsiella. However, this interference by lac Y− plasmids with the expression of the chromosomal lac Y gene was not seen when cells were induced with IPTG or when the chromosomal and plasmid lac operons were both constitutive. It was found that this effect of the plasmids was dependent on their possessing an intact lacZ gene.

Populations of linear asci are classified according to the number of spores in an ascus. The resultant five numerical classes are further classified into ten spatial classes according to the arrangement of the spores in an ascus and, by inference, into ten historical classes according to the number and origins of failures during the developmental process. An analysis of the observed frequencies of numerical classes allows derivation of the efficiencies of the first meiotic division, the second meiotic division and sporulation in a fission yeast. The analytical method may be useful in locating the site of action of sporulation inhibitors and in identifying meiosis mutants from sporulation mutants.

Deterministic simulations have been carried out of populations under normalizing and disruptive selection for a trait determined by genes with additive effects at six loci. In some simulations a pair of alleles at a seventh locus determined the rate of recombination between the seven loci. Normalizing selection with a single optimum, fixed or fluctuating, invariably led to genetic homozygosity. If the optimum fluctuates widely, the approach to homozygosity may be accompanied by a large decline in the mean fitness of the population. Disruptive selection was simulated by having two ‘niches’ with separate optima and separate density-dependent regulation, but with the adult population mating randomly. If the optima are widely separated, this leads to stable polymorphism. Selection produced linkage disequilibrium, normalizing selection causing repulsion and disruptive selection coupling between + and − alleles. This linkage disequilibrium accelerates the phenotypic response to selection, but delays changes in gene frequency. Selection always favoured alleles for low recombination at the expense of alleles for high recombination.

Spermatozoa from inbred strains of mice were found to vary significantly for levels of cyclic AMP when extractions were performed in a reproducible manner. The F1 hybrid between high and low spermatozoal cAMP strains showed spermatozoal cAMP levels typical of the low strain. An analysis of spermatozoal cAMP in individual mice from the back-cross of the F1 to the high strain suggested that alleles at more than one locus determine strain differences in spermatozoal cAMP. The major histocompatibility locus of mice, H-2, which had been found to have an effect on liver cAMP levels did not seem to affect spermatozoal cAMP levels. t-Alleles, which appear to alter fertilization rates by effects on motility, had no apparent affects on spermatozoal cAMP.

Male sterility, male recombination, and transmission ratio distortion – all examples of a syndrome known as hybrid dysgenesis in Drosophila melanogaster – were found to involve chromosome–cytoplasm interactions. The latter two have temperature optima near 25° and involve pre-meiotic events. In addition, sex ratio distortion, and induction of certain translocations of the X and Y chromosomes (but not the autosomes) were found to be part of hybrid dysgenesis. Both are caused by chromosome–cytoplasm interactions with pre-meiotic events playing a crucial role. The results agree with previous data on female sterility in hybrid dysgenesis, which also has cytoplasmic components and premeiotic origins.

An electrophoretic variation in a mouse serum protein moving closely to the vitamin D binding protein is described. The variation is determined by two codominant alleles at one locus with the allele in DBA causing fast mobility and that in C57BL causing slow mobility. This locus is located in the proximal part of chromosome 9, 14·3 cM from the d locus and 31·9 cM from the Bgs locus. The protein has not yet been identified but possible candidates among the serum proteins are discussed.

The lethal t-haplotypes of the mouse fall into several distinct complementation groups, and varying degrees of partial complementation occur when haplotypes from different groups are crossed. The haplotype th20 arose as a mutant from the haplotype t6, and is thought to include a small deletion covering loci near the t-lethals. In this paper the complementarity of th20 was compared with that of its parent haplotype t6. th20 showed weaker complementation than t6 of the haplotypes tw5, tw32 and tw1. Hence the loci of these lethals were probably exposed by the deletion in th20. However, th20 showed full complementation of tw18 and showed full viability in compound with the dominant brachyury allele hairpin-tail, Thp. Therefore, the th20 deletion probably does not overlap the presumed deletion in Thp, nor extend to the locus of the tw18 lethal. The significance of the weak complementation of tw5, tw32 and tw1 is discussed.

A sex-linked mutation with a phenotype similar to that of an allele of the Mottled series has been discovered in a mosaic male. This mutation can be normally transmitted through the sperm of the original male but not through the oocytes. Several interpretations of this abnormal transmission are discussed.

The semi-dominant X-linked gene tabby (Ta) in the mouse and its recessive autosomal mimic crinkled (cr) produce the same mutant syndrome involving abnormalities of the hair, teeth and certain exocrine glands. Previous work has provided some indication of interaction between these loci in terms of vibrissa number. The results of the present study demonstrate that, for tooth size, mice doubly heterozygous for tabby and crinkled show a more extreme phenotype than either heterozygotes for tabby or crinkled alone.

Assortative mating is generalized to include social homogamy and phenotypic homogamy as two special cases. This generalization, called mixed homogamy, enables tests of hypotheses on the nature of assortment. Cultural inheritance is also extended to include two components: transmitted from parental environments, and non-transmitted sibship environment. Familial correlations are derived for a variety of relationships under mixed homogamy.