The genetics of two esterase systems in adults of the olive fruit fly Dacus oleae (Gmel.) have been studied by starch gel electrophoresis. Two unlinked autosomal genes, each with multiple alleles, control the synthesis of a number of isozymes. Gene A controls the synthesis of an acetylcholinesterase, and gene B that of a pseudocholinesterase or lipase. Gene B produces hybrid enzyme in heterozygotes for active alleles. In crosses the segregation of seven active and one silent allele in gene A, and of six active and one silent allele in gene B, have been studied. Six more active alleles for gene A and five active alleles of gene B have not been studied in this paper.

1. A series of experiments have been carried out with Drosophila melanogaster in order to improve further a population, highly selected for a quantitative character, by introducing genetic material from an inferior population. The effects of selecting the latter before crossing to the selected line, of waiting after crossing before restarting selection and of varying the intensity of selection after crossing have been studied. The inferior population used was the large random breeding population from which the selected line had been produced by selecting downwards for sternopleural bristles.

2. There proved to be some incompatibility between the conditions necessary to achieve the two criteria of success—the time to surpass the selected line and the extent by which it was eventually exceeded. The lines which surpassed it soonest had not been selected before crossing and had been selected intensely immediately afterwards. Those which surpassed it most had had the greatest amount of selection before crossing followed by the most intense selection afterwards.

3. Crossing-over did not appear to be a limiting factor after crossing. Neither a period of relaxation after crossing nor a lower intensity of selection after crossing increased the chance of success.

4. Genetic analyses were made of several lines which had exceeded the original selected line. In separate lines, second, third and fourth chromosomes were found which were superior to the chromosomes of the selected line. This and other evidence shows that the limits in such lines with a small number of parents are not in any way absolute but artefacts of the selection programme. The extreme third and fourth chromosomes were both recessive to that of the selected line in the effect on bristle score.

5. A theoretical discussion of such selection programmes is given, assuming that the character is controlled by independently segregating loci. It is shown that, if two alleles at different loci have an equal but small chance of fixation in the initial selection, one because it is rare and the other because, though frequent, it has little effect on the character, such a programme will tend to pick up the former rather than the latter. Of two such rare alleles with an equal initial chance of fixation, the one additive and the other recessive, the chance of fixation of the latter will be greater in a crossing programme.

1. The response to selection for increased number of bristles on one abdominal segment was studied over 12 generations using a factorial design of three population sizes (10, 20 and 40 pairs of parents) and five selection intensities (10, 20, 40, 80% and controls).

2. The responses on the average agreed well with those expected from the estimated base population heritability, but individual replicates diverged considerably.

3. Larger populations tended to give greater response to selection, due mainly to larger realized heritabilities.

4. There was no consistent effect of selection intensity on realized heritability.

5. For populations with the same number of individuals scored, less intense selection gave greater realized heritabilities.

When an R determinant for streptomycin is transferred into a conditionally streptomycin-dependent E. coli B mutant—which requires in minimal medium either histidine or streptomycin—the latter behaves like a histidineless strain. This phenotype modification shows that the repairing action of streptomycin is prevented. The specific requirement of the strain is not now replaced even by streptomycin concentrations up to 10000 µg/ml at which the conditionally streptomycin-dependent mutant could originally grow, and which are well beyond the resistance level characteristic of the R determinant itself. These data seem to suggest that a reduction in permeability of the cell membrane cannot be held responsible for the phenomenon observed.

Following segregation in two unrelated wheat crosses, albinism was associated with chromosome 7B nullisomy which permitted expression of mutant genes on chromosomes other than 7B. The frequencies of albino segregante differed in the two instances, being dependent upon the morphology of the 7B monosome, which was either telocentric 7BL or normal.

A viridis phenotype also segregated from one cross. Its expression was independent of 7B aneuploidy. Failure to obtain lines segregating only for green and viridis phenotypes was due to semi-lethality resulting from the presence of two 7B isochromosomes.

1. An experimental evaluation of Robertson's (1960) theory of limits in artificial selection was attempted. A number of lines from the Canberra base population were selected for abdominal bristle number over 50 generations with population sizes of 10, 20, and 40 pairs of parents and selection intensities of 10, 20 and 40% as well as unselected controls.

2. In general, the total response obtained increased with an increase in Nī (product of population size and standardized selection differential).

3. Thus, total response increased with increase in the number of individuals scored, or, for a fixed number of parents, increase in selection intensity increased both rates of response per generation and total response.

4. But for the same total number scored, the response increased as selection intensity decreased. However, the proportion selected had only a small effect as compared with that of the total number scored.

5. Sublines in which the population size was reduced after 16 generations of selection but with the selection intensity kept constant, immediately fell behind their parent lines and gave much less response.

6. Agreement between replicate lines was generally poor, particularly for the 10- and 20-pair lines.

7. Patterns of response in individual lines were frequently irregular and ‘waves of response’ were not uncommon.

8. The results are discussed in terms of several theoretical models of selection limits. In general, agreement with these models was poor, as much of the response appeared to be due to a few genes (or gene combinations) with large effects on bristle number.

Within the genus Apodemus, DNA from A. sylvaticus and A. flavicollis could be distinguished on the basis of reciprocal DNA/DNA annealing in agar, even when unfractionated DNA was used. In preliminary experiments, DNA from geographical races of A. sylvaticus could not be distinguished, nor could DNA from two closely related species of Peromyscus.

Within the species Mus musvulus, the fast-renaturing fraction of L-cell DNA, separated on a hydroxyapatite column, was able to discriminate between L-cell (originating from the C3H inbred strain) and NZB (New Zealand Black) inbred strain DNA in agar.

Metaphase trivalent frequencies from a number of new chromosomal constitutions (various combinations of maize chromosome 2-Tripsacum homeologue primary and secondary interchanges) are presented and compared to previous findings from similar material. It is suggested than an underlying direct relationship may exist between extent of gentic map present in duplicate and triplicate and expectation from a sharing of map probability of crossing-over among all homologues. Theoretical considerations are discussed.

A study was made of reproductive performance and embryonic mortality in XO and XX females. In the stock used, the mean litter size of XO females (4·46) was greatly below that of XX ones (8·17). One series of pregnant females of both karyotypes was dissected after 15 days' gestation, and another series after 3½days' gestation. In the former, there was a significantly greater amount of embryonic mortality in XO females both before implantation and at the small and large mole stages. By far the greater amount occurred before implantation. The data from dissections after 3½ days' gestation concerned pre-implantation embryos, since normal embryos at this point are at the late morula or early blastocyst stage. The embryos from XO females contained a large group of obviously and characteristically abnormal ones; they comprised 60/280 of the embryos from XO females, compared with 4/189 of the XX ones. They appeared to have developed abnormally from a very early stage, probably the two-cell stage, and were considered to represent the missing OY class of zygotes. In addition, it was concluded that there was probably an abnormally low segregation of nullo-X gametes from XO females.

A method has been developed which will permit the mapping of the genetic material of Pseudomonas aeruginosa. This has been made possible by the use of potassium nitrate in the mating medium and other modifications which have increased the frequency of recombination. In certain circumstances, the ilvA12 marker may be transferred by more than 1% of the donor cells.

1. In order to determine the nature of the genetic variation causing the response to selection in our lines (Jones et al. 1968), various analyses were performed.

2. There was no consistent change in heritability, estimated from half-sib correlation or from the phenotypic correlation between the bristle numbers of two abdominal segments, after 10 to 20 generations of selection.

3. Realized heritabilities over the 10 generations subsequent to the heritability estimations were less than in the early generations but bore little relationship to the estimated values.

4. Six lines contained recessive lethals with appreciable effects on bristle number as indicated by high variances, large regression on relaxation and large response to reverse selection.

5. Reverse selection lines taken from the main lines at generation 40 indicated that genetic variation was still present in almost all of the lines. Only one line failed to respond to further forward or to reverse selection.

6. The three highest lines were crossed in pairs and reselected. Two of the three possible crosses gave further response, exceeding the higher parent after one and three generations, but the other cross failed to pass the highest parent line.

7. A combination of large gene effects, linkage, and gene interaction effects have been suggested as the cause of irregularities in the response of the lines. It has not been possible to determine the relative importance of these effects.

Xasta flies appear to segregate five types of gametes in unequal numbers, namely two which contain both or neither of the chromosomes affected by the translocation and inversions, two further classes which contain one affected and one unaffected chromosome, and finally the remainder which have an unbalanced chromosomal content. These conditions are necessary to fit the results observed in crosses involving the Xasta stock. Xasta exhibits balanced polymorphism under crowded conditions and this may be due to the production of toxic substances by Xa larvae which delay the development of wild type-larvae.

Ten markers have been investigated for their times of entry into auxotrophic mutants of P. aeruginosa strain 1. Using this information and unselected marker analysis two linkage groups have been demonstrated. The results could be accounted for by assuming that there were two chromosomes or that a third linkage group was involved, but we cannot distinguish between these two hypotheses at the present time.

FP transfer to recombinant and non-recombinant recipients of Pseudomonas aeruginosa strain I has been investigated under controlled conditions. When transferred to recombinants, the sex factor appeared to enter first and did not seem to be linked to the early chromosome markers. Five to twenty percent of the recipient population were found to be FP+, depending on the time allowed for pairing. Only 40% of the recombinants were FP+ at 90 min. A possible relationship between FP and chromosome has been discussed.

A pedigree analysis of several cell-colony generations following a mutagenic treatment with nitrous acid has shown that in S. pombe a genetic instability is produced that replicates several times and produces a mutation in independent lines.

It has been shown that the mutants isolated in the progeny of a mosaic colony all contain a genetic alteration that cannot be resolved by genetic analysis and therefore the mutations have occurred at the same genetic site. This finding is confirmed by interallelic complementation and phenotypic analyses.

The first mutation step towards ampicillin resistance in Escherichia coli occurs in the ampA gene, and gives resistance to a D, L-ampicillin concentration of 10 μg/ml. Using interrupted conjugation and transduction experiments ampA was found to be located at 82 min on the time scale of Taylor & Thoman (1964). A number of adjacent markers were studied and the probable gene order of the ampA region was found to be argH-metA-uvrA-ampA-purA-fdp-pyrB. Two independent alleles of ampA were cotransduced with fdp+ and purA+. The phenotypic expression of ampA in a purA strain has been investigated.

A monosome was identified as chromosome 7 of the A genome of the tetraploid cottons by its linkage with the Lc1yg2R2 markers. This makes eight chromosomes of the tetraploids now identified by monosomes. For over a decade the N locus has been considered to be linked with the Lc1yg2R2 loci. Monosomic analysis, however, reveals that N is on a separate chromosome. Studies with a telocentric of chromosome 7 show that the Lc1yg2R2 loci are located in the long arm. It is suggested that the R2 locus rather than the Lc1 locus is in the proximal position. These findings are discussed in relation to data, particularly that involving the linkage group, which supposedly illustrate the compensatory recombination phenomenon in cotton.

We have previously described strains of Escherichia coli K12 resistant to D, L-ampicillin concentrations of 50 μg/ml (Eriksson-Grennberg et al. 1965; Boman et al. 1967). Such strains were assumed to be double mutants carrying the genes ampA and ampB. We here describe genetic steps which produce strains carrying only the ampB gene. Determinations of resistance showed that ampA increased the resistance provided by ampA+ by a factor of 10–15. AmpB increased the resistance of both ampA and ampA+ by a factor of 2.

R-factors were introduced into two sets of strains with all combinations of ampA, ampB and their wild type alleles. AmpA and the R-factors gave additive effects on resistance, while ampB doubled the ampicillin resistance mediated by ampA as well as by the R-factors. AmpB also enhanced the chloramphenicol resistance of R-factor and of the wild type chromosomal genes. It is suggested that ampB resembles the modifying genes previously described and that R-factors can be useful for the identification of such genes.

1. The comparison of the dry weight of thin layer haploid and diploid colonies of A. niger on complete medium and complete medium supplemented with p-fluoro-phenylalanine led to the conclusion that there is a difference in growth rates of hyphae under these different conditions.

2. The growth curves of the same strains on both media were established. On complete medium, haploids and diploid show a growth rate increasing linearly for about 20 h after germination and reaching a maximum which is then maintained. On p-fluorophenylalanine, the haploids show a similar curve, although the maximum growth rate reached and maintained is about half that on complete medium; for the diploid, however, the maximum is less than the corresponding one in the haploid and, once this maximum has been reached, the growth rate goes down linearly to a very low value which is then maintained.

3. The cytological study of the hyphal tip cell showed, in the presence of the amino acid analogue, a reduction of the mean size of the diploid nuclei together with an increase of the number of nuclear fragments. This explains the growth rates observed and is accepted as a confirmation that p-fluorophenylalanine, by its action on the mitosis, favours chromosome losses which lead finally to the production of haploid nuclei.