It was demonstrated that the number of generations until fixation or loss of an overdominant alleles is influenced by random fluctuation of selection coefficients. When 2 < Vs, where is the mean selection coefficient against either homozygote and Vs is the between -generation variance of the selection coefficient, overdominance generally accelerates rather than retards fixation of segregating alleles. This finding should have important bearing on our consideration of the behaviour of polymorphic variants which are nearly neutral but have very slight overdominance. When the population size (Ne) is extremely large, not only Ne but also /Vs have to be considered in discussing the effectiveness of overdominance.

The Neurospora crassa photoreactivating enzyme has been assayed for by the Hemophilus influenzae and Bacillus subtilis transformation systems. In contrast to the H. influenzae system, u.v.-treated transforming DNA from B. subtilis did not give evidence of reactivation of u.v. lesions by crude enzyme extracts from N. crassa when exposed to photoreactivating light. The u.v. dose required to inactivate B. subtilis transforming DNA is about ten times that required to inactivate H. influenzae DNA to the same level of survival. This difference in dose required to inactivate DNA's of about the same base composition probably reflects the greater u.v. resistance of the B. subtilis recipient strains used. Hypotheses are considered which suggest that N. crassa crude enzyme extracts contain either nucleases which degrade B. subtilis transforming DNA excessively or an inhibitory factor which affects the transformation process itself.

We have studied genetic variation at 27 loci in 42 samples from natural populations of a neotropical species, Drosophila equinoxialis, using standard techniques of starch-gel electrophoresis to detect allelic variation in genes coding for enzymes. There is considerarle genetic variability in D. equinoxialis. We have found allelic variation in each of the 27 loci, although not in every population. On the average, 71% of the loci are polymorphic – that is, the most common allele has a frequency no greater than 0·95 – in a given population. An individual is heterozygous on the average at 21·8% of its loci.

The amount of genetic variation fluctuates widely from locus to locus. At the Mdh-2 locus arout 1% of the individuals are heterozygotes; at the other extreme more than 56% of the individuals are heterozygous at the Est-3. At any given locus the configuration of allelic frequencies is strikingly similar from locality to locality. At each and every locus the same allele is generally the most common throughout the distribution of the species. Yet differences in gene frequencies occur between localities. The pattern of genetic variation is incompatible with the hypothesis that the variation is adaptively neutral. Genetic variation in D. equinoxialis is maintained by balancing natural selection.

The amount and pattern of genetic variation is similar in D. equinoxialis and its sibling species, D. willistoni. Yet the two species are genetically very different. Different sets of alleles occur at nearly 40% of the loci.

Extrachromosomal streptomycin and tetracycline resistance in Staphylococcus aureus E169 has been transduced to three different recipients. An analysis of the transductants demonstrates that: (1) in most cases the behaviour of the markers in the transductants is similar to that in the donor; (2) the markers are co-transduced at frequencies of 2–44% and the frequency is highest when the selected marker is streptomycin resistance; (3) in the co-transductants the markers segregate independently, suggesting that they are unlinked.

The activity of several enzymes has been determined in the livers of homozygous obese and adipose mice, their normal litter-mates, and phenocopies induced in normal mice by aurothioglucose (ATG) injections.

Obese, adipose and ATG mice had higher activities of ATP citrate lyase, malic enzyme (NADP malate dehydrogenase) and pyruvate kinase than normal mice. Heterozygote activities are indistinguishable from wild-type. There was no difference between normal and fat litter-mates in the activity of malate dehydrogenase (NAD-linked), lactate dehydrogenase, isocitrate dehydrogenase and fumarase.

Crosses between mice doubly heterozygous for roth the ad and ob genes produced offspring that were only ‘fat’ or ‘normal’ and no offspring could be phenotypically recognized as the double mutant, either physically or in terms of ATP citrate lyase activity.

Gas–liquid chromatography of the fatty acids of the depot fat showed no differences between any of the types of litter-mate.

The alterations found in enzyme activity in obese and adipose mice are compared to several other enzyme activity differences reported in the literature for obese mice. These are discussed in relation to genetical criteria that may be estarlished to assess, from quantitative data, whether an enzyme is the site of the primary lesion in a mutant pheno-type. Some general observations are made on genetics and the control of metabolism.

Several temperature-sensitive mutants of Escherichia coli were isolated which lyse at the restrictive temperature. Some of these possess a biochemically defined lesion in cell-wall mucopeptide synthesis. Three genes, termed murC, E and F, have been localized between the azi and leu markers. From transductional data a fine structure map was constructed of the mur mutations, establishing the order of the genes. The genetic relationship between these cell wall genes and neighbouring genes involved in cell division is discussed.

A simulation study was undertaken of methods of subdividing populations into several small sublines and utilizing the variances generated between lines by selecting among them. Crosses of chosen lines were made, and either selection was continued in a single large population (single cycle) or the population was subdivided again (repeated cycles). As a control for the efficiency of these schemes, a single large population was maintained and selected at the same intensity from the outset. Simple models were used of additive or completely dominant genes, usually of equal effect and equally spaced on a single chromosome.

The single and repeated cycle structures give similar results, but the repeated cycle structure is more extreme.

With additive models intense selection between lines gives short-term advances, but causes a reduction in the limit when compared with a single population. The effect on the limit is greatest with free recombination, very small with complete linkage. If no selection is practised between lines the limit is unaffected, but takes longer to attain.

With complete dominance, and the recessive allele initially at low frequency, greater responses from selection are obtained within sublines than in the large population, large gains are made from selection between sublines, and a higher limit can be reached. If the recessive allele is at high initial frequency the subdivision is not beneficial.

Some simple theory is developed to explain these results. It is concluded that subdivision and crossing schemes are unlikely to be very useful except for elimination of deleterious recessive genes.

An autosomal translocation in the tsetse fly Glossina austeni was studied genetically, cytogenetically and for its effects on viability. Flies homo-zygous for the structural change could be identified by outcrossing to wild-type and demonstrating semi-sterility in all the progeny.

A cytogenetical analysis of male meioses in samples of pupae which were sibs of the semi-sterile progeny showed them to be structurally heterozygous for the translocation. Matings of the translocation heterozygotes and homozygotes gave the expected progeny ratios, with the exception of a deficit of females classified as translocation homozygotes. This was due to their sterility or inviability. Those female homozygotes which did breed showed a subnormal lifetime pupal production. These deleterious recessive effects were probably due to the translocation itself although the influence of linked loci could not be ruled out. These effects would prevent the mass rearing of this particular translocation for a tesetse control project.

Two other stocks which showed semi-sterility were found to carry autosomal translocations and two which currently showed holandric inheritance have Y-autosome translocations. One stock with holandric inheritance of extreme sterility carries a double translocation involving two autosomes and the Y chromosome.

The melanotic tumour gene tu-C4 in Drosophila melanogaster shows incomplete dominance, together with variable penetrance and expressivity. It is tentatively located in the region of locus 52–53 on the third chromosome. Tumour formation in mutant homozygotes involves a precocious haemocyte transformation leading to the appearance of lamellocytes at the beginning of the third larval instar. These aggregate to form tumour-like masses which subsequently melanize. The process of tumour formation is in broad outline similar to that found in other tumour strains. Melanotic tumour formation is treated as a dichotomous threshold character, assuming an underlying normal distribution of liability relative to a fixed threshold. The expression of the tumour gene can be influenced by the levels of protein, phospholipid, nucleic acid and carbohydrate in the larval food medium, and changes in dominance and penetrance induced by sub-optimal environments deficient in these nutrients are positively correlated. Reinforcement by selection of the dominance relations of tu-C4 was accompanied by correlated changes in penetrance. Conversely, selection for increased penetrance was accompanied by correlated changes in dominance. Dominance and penetrance, it is concluded, are fundamentally related aspects of tumour gene expression. Recruitment of dominance modifiers linked to the tumour gene was excluded by the mating scheme employed, and the observed changes in dominance relations in response to selection were due largely to modifiers located on the second chromosome. Changes in dominance relations produced by selection could be significantly reinforced, or reversed, by environmental factors and consequently show a substantial genotype – environment interaction effect. These facts are relevant to current theories of dominance evolution.

Segregation of the prophage of bacteriophage 90 has been observed in reciprocal crosses between lysogenic and non-lysogenic parents of Pseudomonas aeruginosa strain PAO. Linkage of the prophage was shown to three genes determining histidine biosynthesis in that region of the chromosome 7–13 min from the site on the chromosome at which the sex factor FP2 promotes chromosome mobilization.