Transformation studies in pneumococcus had shown that loci determining the resistance to erythromycin and streptomycin were unlinked when strain Cl3 was recipient but linked when strain SIII-I was recipient. This phenomenon also applies to other pairs of markers studied in these two recipients, no matter whether the transforming DNA was derived from strain Cl3 or strain SIII-I. Other differences between the two recipient strains were also revealed. Whereas competent cultures of strain Cl3 were composed of all competent cells, which was in agreement with previous reports of pneumococcal cultures, strain SIII-I normally gave a maximum average of 28% competent cells. Strain SIII-I was unstable, since on repeated sub-culturing the competence peak profile changed and the value of ‘fcq’ increased. These properties were reflected in the two hex− strains 401 and R6x which were found to be similar to the ‘altered SIII-I’ strain. The results from the linkage studies have been applied to the chromosome map and have placed the opt-r2 gene in the str-r41-containing chromosome arm.

From the F2 ratios of crosses between Drosophila melanogaster strains homozygous for the AdhF(F) and the Adhs(S) allele it has been concluded that the developmental time of FF homozygotes is shorter than that of SS homozygotes. This difference is found to be reinforced by increasing levels of crowding. A further analysis of developmental times has been performed by the transfer of larvae to agar medium after they have stayed for periods of variable length on regular food. From the percentage of emerging adults it can be concluded that FF larvae and, to a lesser extent FS larvae, either reach their critical weights for pupation earlier than SS larvae or possess a lower critical weight. These differences in developmental time influence the course of allele frequencies. Between populations kept on a 2-week transfer schedule and on a 3-week schedule a divergence of allele frequencies is observed, in the former a decrease in S frequency occurs. The relevance of the observed differences in developmental time for the maintenance of the Adh polymorphism is discussed for laboratory populations kept on regular food and at varying densities.

The effects of finite population size, occurring either as a bottleneck in a single generation followed by a large expansion or in all generations, are considered for models of two linked heterotic loci. Linkage is assumed to be tight because it is required if there is to be stable linkage disequilibrium, D ǂ 0, in infinitely large populations. (D is the difference between gamete frequencies and the product of the gene frequencies.)

If a substantial perturbation of frequencies occurs as a result of a bottleneck but the population is subsequently very large, D may take hundreds of generations to return to its stable point. In finite populations, the distribution of D can be ⋃-shaped, unimodal or bimodal. The correlation of D in successive generations is higher with tight linkage and is little affected by selection or the size of the population.

The utility of infinite population studies of linkage disequilibrium and its stable points is questioned, and considerable pessimism is expressed about the possibilities of distinguishing selection and sampling effects at linked loci.

Escherichia coli K-12 mutants resistant to glutamyl-γ-methyl ester were isolated. A mutation leading to resistance of up to 1·4 mg/ml of the methionine analogue maps at min 63 and is 13% cotransducible with serA indicating an alteration in the metK gene. Another mutation leading to resistance to 3 mg/ml of the analogue and cross-resistance to other amino acid analogues maps at min 87. This mutation, which has the phenotype of MetJ−, is shown to be situated between the glpK and metB genes and thus indicates a different gene order from the published one.

A new autosomal recessive gene (cab, ‘cardiac abnormality’), which affects heart differentiation appeared spontaneously in our mouse colony. The affected homozygotes die approximately half a day prior to birth or at term, with the myocardium exhibiting vacuolation and large areas of mesenchyme-like cells. A significant finding in mutant foetuses is the lack of histochemically detectable glycogen.

We calculated the rates of segregation due to plasmid incompatibility under several simple models. A common feature of all the models that we considered is that incompatibility is caused by the inability of the segregation mechanism to distinguish between two incompatible plasmids.

We measured the rate of segregation due to incompatibility of a pair of ColE1 derivatives under two conditions: (1) One plasmid was introduced into cells carrying the other by conjugation. (2) Cells carrying both plasmids were maintained by selection and then selection was released.

Interpretation of the results was made more difficult by effects of the Plasmids on the host cell's growth rate. These experiments gave results in agreement with the predictions of a random pool replication model. Published results were also in reasonable agreement with this model.

A method is introduced that allows the simplification of the calculation of equilibrium solutions in multiple locus genetic models of a single infinite population. The method can be applied when the number of different fitnesses is equal to or less than one more than the number of independent allelic frequencies. The results are in terms of relationships – the symmetry constraints – between the gametic frequencies that must be satisfied at any boundary or internal equilibrium. The symmetry constraints are independent of the fitness values and of the recombination fractions. This can lead to some understanding of the equilibrium structure of a model when the full equilibrium solution is not obtained and reduces the number of independent variables in the calculations of the full equilibrium solutions. Examples of two locus models with two alleles at each locus and with two alleles at one locus and three at the other are discussed.

For one-, two- and n-locus models of a quantitative trait assumed to be determined by overdominant loci, it is shown that the mean of an inbred line will be equal to or larger than the mean of the base population if the original gene frequencies satisfy a condition which is generally a function of the degree of overdominance and of the genotypic values.