The incidence of many minor skeletal variants in the inbred mouse strain C57BL can be influenced by the diet on which the parents live: in many cases, the effect is mediated by a correlation with body size. This also seems to be true in Falconer's (1973) Q-strain in which body size has been increased or decreased by selection. However, there was so much heterogeneity between replicates within selection lines that variants influenced by body size could be detected as a group, but not identified individually.

Equations are presented relating the number of generations elapsed to the fractions of bacteria carrying one or both plasmids in a clone derived from a cell containing two incompatible plasmids.

The survival of the plasmid RP 1–1 in E. coli depends on the presence of a functional polA gene. Unlike other plasmids that have this requirement, the replication of RP 1–1 is inhibited by chloramphenicol. RP 1–1 cannot be inherited by E. coli recA mutants, and inactivation of the recA gene product in a recAts mutant leads to destruction of the plasmid. RP 1–1 cannot be inherited by recB or recC E. coli unless the strains also carry a suppressor of these genes, such as sbcA or sbcB. The pattern of replication of RP 1–1 in E. coli K12 and its mutants suggests that the survival of this plasmid in this species is the outcome of a balance between DNA polymerase I-specified replication and exonuclease destruction.

The genetic variability at 16 enzyme loci in 52 Scandinavian triploid parthenogenetic populations of the beetle Adoxus obscurus has been studied by starch-gel electrophoresis. The overall genotypes of different beetles have been compared with each other and with a sample from a diploid bisexual population from Canada. Eighty per cent of the parthenogenetic beetles have the same overall genotype. The remainder belong to six genotypes, three of which are found in only one population. The variability within and between parthenogenetic populations of A. obscurus is much lower than in other comparable parthenogenetic insects studied by us. This is interpreted to be a consequence of the efficient migration of the genotypes with the highest fitness, leading to a replacement of less-adapted genotypes. A. obscurus is a flying insect, whereas the other parthenogenetic insects studied by us are sluggish flightless forms.

Euploid and aneuploid plants of Triticum aestivum, variety Chinese Spring were pollinated with, pollen of Hordeum bulbosum. Euhaploids and aneuhaploids of Chinese Spring were obtained from the crosses. Meiotic chromosome pairing was analysed in 25 different aneuhaploids and the results were compared with those obtained from euhaploids. The evidence provided by the meiotic studies was used to identify chromosomes whose activities affected the genetic control of chromosome pairing.

Meiosis was abnormal in a 23-chromosome aneuhaploid and in the 22-chromosome sectors of a chimaeral plant. Both plants were thought to have resulted from the incomplete elimination of the genome of H. bulbosum from hybrid embryos. It is suggested that the meiotic abnormalities in the two aneuhaploids were caused by the residual barley chromosomes.

In a previous paper the theory of a model of electrophoretic profiles due to Ohta & Kimura was considered. This model assumes a finite population with a linear series of possible alleles with mutation between nearest types but no selection. In the present paper a model with both mutation and selection is constructed which results in a stable population distribution closely fitting empirically observed features of the Ohta–Kimura model. The problem of discriminating between selective and non-selective models for electrophoretic models is considered.

Meiotic chromosome associations in homothallic strains of S. cerevisiae were investigated by analysis of meiotic spore colonies and crossing over in +/+/− trisomics. The segregants of these aneuploids produce phenotypically distinguishable tetrasomic spore colonies. The data indicate that trivalent associations occur with a high frequency in trisomics of chromosome XVII and that the frequency of second division segregation is markedly increased over that found in the normal dipoid.

The Lac plasmids in two Klebsiella strains, V9A and RE1544, give characteristic Lac-permease defective mutations which produce a lactose-negative (ML−) phenotype in the host strain, in spite of the presence of a lac operon on the chromosome. These ML− clones can revert to the wild-type (ML+) phenotype, and the mechanism of this reversion is examined. In V9A carrying its own Lac plasmid (FKlac), it is shown that reversion of ML− to ML+ is usually the result of mutation to constitutivity of one of two galactoside permeases which are not induced by lactose but accumulate lactose when otherwise induced or made constitutive. However, in one out of the 51 ML+ revertants tested the mechanism of reversion appeared to be a change back to wild type of the Lac plasmid's own permease gene. In V9A carrying the Lac plasmid (pRE6) from RE1544, successive changes of phenotype were obtained of ML+ to ML− to ML+ to ML− to ML+; these were all found to be the result of changes in the plasmid permease gene, and could be simply explained if an IS sequence could insert and excise from this gene.

The yellow mutant (y − 1:0·0) of Drosophila melanogaster shows a reduction in body pigmentation associated with a decrement in locomotor activity and in male competitive mating ability. The effects of the mutant gene are specific to locomotor activity in the adult fly, measures of larval activity being unaffected. In the presence of active females yellow males offer a reduced intensity of sexual stimulation because they are less able to maintain contact during courtship. However, the impaired locomotor performance of yellow males is not the general cause for their reduced competitive mating ability, since the stimuli provided by yellow mutants courting inactive females appear to be both quantitatively and qualitatively indistinguishable from those of their wild-type male sibs. Nor is there any good evidence, as measured by the frequency of rejection responses, that the courtship stimulation offered by the yellow males is less acceptable to the females. The mutant males are nevertheless unsuccessful in achieving copulation with such females. It is suggested that impairment of mating ability in yellow males may be caused by changes in the efficiency for mating of their secondary sexual structures due to the effect of the yellow gene on the properties of the cuticle.

Baker (1971) presented genetic evidence that the inviability of In(1)scL8/O and scS1/O males in Drosophila melanogaster is due to position effect suppression of ribosomal RNA cistrons. Although scL8/O and scS1/O males are inviable, scL8/scL8 and scS1/scS1 females are viable. We therefore asked the following question: Is scL8/O or scS1/O viable when part of a gynandromorph? In other words, is position effect suppression of rRNA genes autonomous or non-autonomous in gynandromorphs? In this paper preliminary evidence is presented which suggests that position effect suppression of rRNA cistrons is non-autonomous. The evidence is that scLS/ or scS1/O (male) parts of gynandromorphs are not only viable but normal in appearance.

An isolation method for N-methyl-N′-nitrosoguanidine-induced catalase negative mutants of P. shermanii based on replica plating is described. In contrast to previous methods, it extends to the early stages of tetrapyrrole biosynthesis which are common in both corrins and porphyrins. It may thus aid in elucidating the mechanism and control of porphyrin and corrin biosynthesis. Some preliminary results are discussed.