1. Gunson's salivary chromosome preparations of Drosophila subobscura from widely separated sites in Scotland have been re-examined and inversions recorded according to the Mainx nomenclature.

2. Sixty-four diploid sets only were available. Of these, thirty-seven sets were found to be structurally homozygous on all chromosomes.

3. From Drumnadrochit in the north-central area of Scotland, the inversion found on the E-chromosome, so far as is known, has not previously been described. Its break-points have been noted, and the inversion is named E14.

4. A strain of D. subobscura from the small western island of Iona was the only one found to be completely homozygous in the five long arms of the chromosome set.

5. Samples of D. subobscura from two closely related localities in Midlothian, Scotland, also have been examined. Results are based on the analysis of 120 haploid sets in hybrids between the local race and the standard Küsnacht stock.

6. A slight difference in type and frequency of inversions has been noted between the two populations. The inversion E1+2 was recorded from Dalkeith, but was absent at Heriot, while U1, present at Heriot, was replaced by UST at Dalkeith.

7. The A-chromosome was structurally homozygous throughout.

8. Scottish samples of D. subobscura are characterized by their qualitative simplicity of polymorphism, the variety of inversion types being small. Chromosome orders analysed have been compared with those occurring in Western Europe and Israel.

The feasibility of using heterozygous tumours induced in hybrids between Snell's isogenic resistant (IR) strains of mice for mutation studies was examined. The system has been described by Mitchison (1956) and Klein, Klein & Révész (1957).

Fourteen hybrid sarcomas (A × A.SW F1; H-2 genotype a/s) were tested in mice of the parental strains. Seven were specific for the original F1 hybrid genotype and failed to give any variants. Four tumours gave variants that were compatible with one of the parental strains. The variants grew progressively in all mice of the parental strain, even in pre-immunized mice, and failed to grow in mice of foreign genotypes. Of these, one tumour, as. 14 gave true variants towards both the parental strains. One tumour, as.7, gave variants regularly in 35·5% of the A.SW mice but only in 7·7% of pre-immunized A.SW mice. It failed to give variants towards the other parental strain.

The effect of triethylenemelamine (TEM) and X-ray treatment on the rate of variant production in hybrid tumours was studied. The effect of the treatment on the viability of tumour cells was first examined by titrating tumour cell suspensions on groups of mice or on the CAMs of groups of developing chick embryos. The TD50 values (50% end-points) were calculated. Comparable results were obtained by titrating on mice and on the CAM, although titration on mice gave more consistent results. Approximately 102 viable, untreated cells were needed to produce a tumour in mice or on the CAM.

Tumour as.7 was tested in the parental strains after treatment with TEM or X-rays. Using comparable cell doses in the untreated controls and treated series, the percentage of variants produced in one of the parental strains (A.SW) was significantly increased after treatment with TEM or X-rays. With the exception of one experiment, no variants were produced in the other parental strain. In one experiment, four variants were produced towards the A strain after treatment with TEM, three of which were tested: only one was found to be specific for the A strain. Three of the A.SW variants, originating after treatment with TEM, were tested: two were specific while one showed incomplete specificity for the A.SW strain. Treatment of specific tumours with TEM or X-rays did not give variants towards either of the parental strains.

Two of the hybrid tumours and their derived variants were examined cytologically. Variants originating spontaneously and after treatment with TEM were examined. While most of the original tumours had chromosome numbers with an exactly diploid modal number, the spontaneous and TEM-treated variants had modal numbers which were hyperdiploid. The TEM-treated variants also had a larger proportion of cells which were triploid or hypotetraploid. This would explain the incomplete specificity of the variants isolated after treatment with TEM.

While it appears possible to use heterozygous tumours for isolating genetical variants specifically compatible with one of the parental strains, the actual nature of the mechanism of variant production remains dubious.

1. Several lines of Drosophila melanogaster have been selected for increase of sternopleural chaeta-number.

2. Three lines of the same origin, dp 1, dp 2 and dp 6, showed remarkably similar patterns of response involving an accelerated response from 24 to 28 chaetae and a plateau at about 30 chaetae. A line formed by crossing two of these was not responsive to selection, suggesting that the two parent lines were genetically similar.

3. A fourth line, vg 4, related to these others, showed a similar accelerated response but continued to respond beyond 30 chaetae, reaching a plateau at about 37 chaetae.

4. A further line, vg 6, set up by crossing two ‘improved’ lines, one with 30, the other with 22 chaetae, reached 46 chaetae and suggests there may sometimes be merit in selecting from the hybrids of ‘improved’ strains.

5. It is argued that the event making possible the accelerated response is probably a recombinational event.

6. The line vg 4 clearly demonstrates that some of the genes that may be exploited in a selection experiment have to be exploited in a particular order because of gene interactions.

1. Serial cytological observations were made on one abnormal seedling from the thornless segregants of the F2 progeny from crossing two tetraploid (2n = 28) Rubus species. The number of root-tip chromosomes was found to vary between 9 and 46 per cell, with the mode always at 35. The wide variation in chromosome number decreased with time, until there was almost stability at 2n = 35. A vegetative (stipule) cell was 2n = 39. The numbers lower than 35 were not due to terminal fusion.

2. Evidence from the number of satellites per cell suggests that not always were the same chromosomes being eliminated, although there is a general increase in satellite number with increased numbers of chromosomes per cell.

3. There was a correlation between the chromosome numbers and corresponding cell sizes within the aneuploid series. This may be associated with the incremental change of DNA following the addition or subtraction of individual chromosomes, which are highly similar in Rubus.

4. The cause of the instability was not definitely established. Its initiation is probably attributable to the egg mother cell, as Rubus pollen reacts sensitively to chromosome unbalance. No abnormalities were observed in the mitoses, and virus infection was not responsible for the instability.

5. Chromosome number instability of this plant is discussed in relation to other examples from the same and different genera: it is suggested that the changes in chromosome number known in various Rubi may have arisen somatically.

A theory of mass selection in a small population was developed, and the mean change in gene frequencies, the variance of gene frequency changes and the expected gain in the mean phenotypic value of an offspring population were formulated in terms of a generalized selection differential and the additive and dominance effects of genes.

The magnitude of the variance of changes in gene frequency was compared with the magnitude of the variance expected from the genetic random drift in a population with the same gene frequency and of the same size in absence of selection. The former was found to be usually smaller than the latter when the gene frequency ranged from intermediate to high and when selection was directed for a high performance.

The usual prediction equation for gain from selection in an infinite population was compared with the expected gain formula derived for a small population. The size of the population did not cause a serious difference between the two expected gains when there was no dominance effect of genes. Dominance alone could cause the usual prediction to be slightly more biased. The joint effects of the finite size of population and dominance gene action could amount to a considerable bias in the usual prediction equation. Such a bias can be, in the main, accounted for by the inbreeding depression.

A variant strain of Escherichia coli K12 (♂3) was obtained after ultra-violet irradiation. This strain differs from wild type in the possession of a preferred chromosomal site for the F factor.

When wild-type females are infected with the F factor they are converted to F + males which carry the F factor in the infectious state. These F + males (standard) change with low frequency to Hfr males which carry the F factor on the chromosome. When the variant female (♀3) is infected with the F factor it is converted to an F + male (state II) which changes with high frequency to a specific Hfr male (state I) which carries the F factor at the preferred site (Hfr3).

Male strains carrying the preferred site alternate between the specific Hfr state and the F + state. The mechanism of this alternation is the back-and-forth shift of the F factor between its integrated state of multiplication at the Hfr3 locus and its autonomous state of multiplication. This process is termed episomic alternation.

Episomic alternation offers a physical model for regular changes in state of the gene.

The Hfr3 locus has two allelic forms, high rate of change and standard rate of change. The allele for high rate of change may be considered as a genetic system which occurs in two possible states, state I in which the F factor is present at Hfr3, and state II in which it is not.

The isolation of single inheritance units affecting body size in mice has been attempted. By using Large and Small mice as the parental strains, a breeding scheme has been carried out by repeated backcrossing to each strain with selection for both large and small body sizes in each backcross. The selections were started from the second backcross generations. The Small backcross was carried to the seventh generation and the Large to the fifth. Based on analysis of the means and variances for the parental strains and the backcross generations, it is tentatively concluded that a small number of, if not single, inheritance units may have been introduced from the Large to the Small mice. The ‘large’ genes appear to be dominant over the ‘small’ genes.

In a population under artificial selection, the effective population size may be less than the actual number of parents selected because there will be variation between families in the character under selection and consequently in the probability of selection. Expressions are developed for the magnitude of the effect, which will be greater the more intense the selection and the higher the heritability of the selected character. The inbreeding due to outstanding individuals may rise for several generations after their use.

(1) Two lines have been selected for small wing cell size from the cage Pacific population. Body size was reduced by about 10% and 15% in the two lines which did not regress when selection was relaxed.

(2) The effects of crossing each line to the unselected population has been determined in a number of repeated tests on the live yeast medium and also on various sub-optimal synthetic media.

(3) The size of the F1, relative to the size of the parents, is greatly influenced by the composition of the larval diet. The F1 may coincide with the mid-parent value but generally significantly exceeds it and is often the same size as the unselected parent population.

(4) In crosses to an unselected population on alternative media the F1 was either the same size as the unselected population or exceeded it.

(5) Crosses between the selected lines produced an F1 which exceeded the larger parent but remained well below the level of the unselected population.

(6) To test for interaction between genes at different loci, chromosomes from the unselected population were substituted in the genetic background of each of the selected lines to provide an array of genotypes in which one, two or three pairs of major chromosomes had homologues derived from different strains. Leastsquares analysis indicated differences between the lines in the distribution of effects among the chromosomes together with the presence of interaction between chromosomes and this was greater for the substitutions in the line which showed the greater consistency of recessive behaviour in crosses to the unselected population.

(7) At the end of the selection experiment two lines were selected for large body size from the F2 of the cross between the two selected lines. Both responded to selection for three to four generations and then fluctuated at a level slightly below that of the unselected population.

(8) The physiological changes which involve correlated changes in body and cell size differ from those which result from selection for smaller body size, at least in the early stages of such selection, and are associated with differences in genetic behaviour. The apparently recessive property, which involves extensive non-allelic interaction, is progressively established during the course of selection. Apparently selection for smaller cell size is particularly effective in disturbing the normal homeostasis of growth and is accompanied by relatively greater loss of heterozygosis than is likely with equivalent reduction in size due to selection for smaller body as opposed to cell size.

All 153 crosses between 18 tomato varieties were grown in F1, F2, F3 and F4. The F2, F3 and F4 were derived by selfing one plant of the previous generation. The F2 plant chosen to give the F3 was selected (1 in 20) for early yield; and the F3 plant chosen to give the F4 was similarly selected. Flowering date was an unsatisfactory character. In crosses segregating for the Mendelian gene ‘uniform’, a significant excess of heterozygotes was selected. The parents transmitted variability of yield (as well as average yield) to their offspring. The division of yield into its components fruit number and fruit size is useful because (1) much of the heterosis in yield can be viewed as a combination effect between these components, (2) the components responded differently to selection, (3) different components showed phenotypic dominance in different directions. The average yield and fruit number responded as expected to selection; fruit size did not. The F1 generation means for all three characters exceeded the parental means; but the crosses were grouped more compactly about their generation mean than were the parents, so that heterosis rarely occurred in crosses involving the best parents. The yields of each cross were analysed into parental main effects (general combining abilities) and interactions (specific combining abilities). No useful prediction of interactions could be made in any generation, either from the same generation in different years or from different generations in the same year. The main effects (general c.a.) were analysed into a part due to regression on parental yield, and a deviation from that regression. No useful prediction of the deviations from parental regression could be made in generations which had responded to selection. The actual advance under selection of different crosses, although not uniform, was unpredictable. During advance under selection, the parental means gave predictions of the (relative) performance of each generation's crosses which were as good as predictions based on the previous generation. This may, of course, be connected with the fact that the parents were inbred and that the amount of heterozygosity decreases in each successive generation. These results indicate, therefore, that in an inbreeding species propagated by seed, the early hybrid generations tell us nothing more than do the parental yields about the relative performance of the inbred lines that can be selected from those hybrids. (This generalization from Lycopersicon esculentum to inbreeders as a whole may, of course, be false.) The relative performance of F1 hybrids, on the other hand, is better predicted from other F1 crosses involving the parents concerned than from the yields of those parents. There was phenotypic dominance of high yield, large fruit number and low fruit weight. The extent of this dominance was not enough to invalidate the analysis by general combining abilities; and since it varied from year to year and from generation to generation, the emphasis that should be given to the top-parent (in contrast to the bottom-parent) in predicting the yields of crosses after selection, is as yet unpredictable.

The regression of progeny on mid-parent value is often used in progeny tests to estimate the heritability of a quantitative character. The statistical precision of such an estimate can be considerably increased without increasing the size of the test, by using assortative mating or selection of parents (or both together) so as to increase the mid-parent variance; but the danger arises that this may introduce bias into the estimate through correlation between non-additive gene effects.

It is shown by a mathematical argument that such bias will be negligible provided that all individual gene substitution effects are small compared with the phenotypic standard deviation of the character. Under this condition, deviations from additive effects either within or between loci will not appreciably affect the expected value of the regression on mid-parent, compared with its expected value in a test using random mating.

Correlation between the non-additive gene effects is likely to cause more serious bias to the correlation between sibs, when non-random mating is used.

The complex anatomy of the adult limb skeleton of Os/+ mice is attributable to a reduction of the preaxial margin of the foot plates. This is detectable, in the fore limbs, in the 11-day stage, i.e. before condensations of mesenchyme have taken place. The involvement of the skeleton is thus secondary to an earlier defect in the foot plates. The cause of the latter has not been discovered.

1. Many of the Ustilaginales, or smut fungi, appear to have the qualities necessary for the application of modern techniques of microbial genetics. Ustilago maydis is considered the most suitable species.

2. Investigations of the mating system confirm reports that the production of diploid brandspores in the host is controlled by alleles at two loci.

3. Genetic markers were obtained by inducing mutations in a wild-type strain with ultra-violet light. Of 100 biochemical mutants which were isolated, the growth requirements of 94 were identified. Thirty of these were used in genetic tests.

4. The compact growth of colonies on artificial media allowed new techniques to be developed by means of which large samples of progeny could be isolated and identified easily. The analysis of brandspore colonies consisting of the products of single meiotic divisions is the quickest method for detecting linkage, but its accurate measurement appears to be achieved by examining the individual members of tetrads.

5. Linkage was detected relatively rarely, but eight markers, including the a mating-type locus, were assigned to one or other of two linkage groups. Although recombination values were not always determined accurately owing to irregular basidiospore germination, the auxotrophic markers in each group could be mapped in a linear order. Since no indication of other linkage groups was obtained, the genetic evidence is so far consistent with cytological reports that the basic haploid chromosome number is two in the smut fungi.

6. Three linked markers were used to investigate chromatid interference by tetrad analysis. None was detected in a total of eighteen double exchanges.

The meiosis in the F1 hybrid Oryza saliva × O. australiensis was studied. Contrary to the observations of Gopalakrishnan (1959), true allosyndetic bivalents were not found at metaphase I. The most frequent associations were non-chiasmatic, end-to-end pseudobivalents. Autosyndetic bivalents were recorded mostly in the complement belonging to O. sativa, which are distinguishable by their smallness and lighter staining. The meiotic cycle exhibits timing imbalance with earlier condensation, and possibly migration, of the univalents belonging to O. australiensis. The data on meiotic pairing in the F1 hybrid and the comparative morphology of O. sativa, O. officinalis and O. australiensis inicate that the last species is the most primitive member, having originated from the pre-Sativa and pre-Officinalis complex.

The available data on heterogeneity in centromere distances for a number of loci in several linkage groups are analysed and interpreted. When the crosses are grouped according to wild-type ancestry, heterogeneity is eliminated in any one group except those which consist of backcrosses or intercrosses. Abbott 4 and 12 are shown to be the source of the heterogeneity while Lindegren and probably Chilton wild strains give consistent, homogeneous distances. In a cross between Abbott 12 and Lindegren wild-types, the centromere distances of mt and asco show heterogeneous values between the spore pairs in an ascus indicating that significantly different distances are genetically determined and that the factors concerned show segregation. The genetic determination differs in the various wild strains; the data suggest that at least three factors are involved. In random spore analyses heterogeneity is present in recombination frequencies between linked markers either proximal or distal to their centromere. The mechanism by which heterogeneity in the data might arise is discussed. To obtain homogeneous data it is suggested that all markers used should be repeatedly backcrossed to the Lindegren wild-type.

1. Two different methods of selection were used to obtain stable prototrophic solopathogenic strains of Ustilago maydis from haploids with different biochemical requirements. The strains were shown to be heterozygous diploids.

2. Two diploids which were examined in detail had four markers in common, but they were in different coupling and repulsion phases. Rare spontaneous segregation was detected in one of the diploids, but a high frequency of segregation was obtained in both after treatment with ultra-violet light. The proportion of segregants amongst the survivors increased with the dose. These auxotrophic segregants were detected by means of the replica plating technique.

3. The types of segregant which were obtained from both diploids were consistent with the view that they arose as a result of mitotic crossing-over. After low doses of radiation the reciprocal products of crossing-over were often detected. There was no evidence from the phenotypes of the segregants that haploidization was occurring. The diploidy of a sample of the segregants was confirmed by mating-type tests, and by the fact that they showed further segregation after another dose of radiation.

4. A slow-growing unstable segregant recovered after a high dose of radiation proved to be a monosomic strain which consistently reverted to a stable diploid homozygous for one chromosome. It was possible to use this auxotrophic diploid together with a haploid with different biochemical requirements, to synthesize a prototrophic triploid strain. The triploid was much less stable than the diploid strains.

5. By means of pathogenicity tests with certain segregants it was possible to distinguish the function of the two loci which control the mating system. The a locus is responsible for the fusion of haploid sporidia and has no effect on the pathogenicity of the heterokaryon which is under the control of the b locus.

6. The effects of ultra-violet light on mitotic crossing-over do not seem to be easily compatible with a copy choice or similar mechanism of recombination.

The recessive mutant pudgy in the mouse has a greatly shortened vertebral column with highly irregular fusions between vertebrae and fragments of vertebrae. Ribs and sternum are also involved, but the rest of the skeleton is quite normal. The deformities arise from a defective segmentation. Although the paraxial mesoderm forms somite tissue with an epithelially arranged outer layer, this material either shows only an abortive segmentation into somites or, in the tail, none at all. A belated segmentation into dermomyotomes ultimately takes place, but the sclerotomic material remains continuous and gives rise to erratically abnormal blastemata which then chondrify and ultimately ossify.

The sensitivity pattern of the Drosophila testis to TEM was analysed by means of a dual-purpose strain that allows the scoring of induced crossovers and sex-linked lethals in the progeny of the same flies. It was found that TEM produces the highest frequency of mutations in spermatocytes or late spermatogonia, while early spermatogonia are even less sensitive than mature spermatozoa. The discrepancy between this conclusion and that obtained by Fahmy & Fahmy (1955) is attributed to a difference in the method of analysis. The sensitivity pattern of the Drosophila testis to TEM resembles that to mustard gas and differs from that to X-rays. The sensitivity pattern of the mouse testis to TEM differs from that of the Drosophila testis.

Starting from a geographically hybrid foundation population of Drosophila subobscura, selection for fast and for slow development has been practised without inbreeding on a diet with an unusually high level of protein. Realized heritabilities in the fast and slow lines were + 0·063 ± 0·029 and + 0·186 ± 0·031 respectively. A half-sib analysis of the foundation population and full-sib analyses of the first two and the last two selected generations were carried out. Hybrids between the two lines were approximately intermediate between their parents.

Two types of genetic explanation of the asymmetrical response are discussed. The first assumes directional dominance of alleles for fast development. Such an assumption can explain the asymmetrical response, but runs into difficulties in explaining the nature of the genetic variance in the selected populations and the intermediacy of the hybrids between the two lines.

A second assumption, which appears to fit the facts better, is that there exists a ‘developmental barrier’ preventing development at a rate appreciably faster than that of the foundation population. In physiological terms this implies that more rapid development requires a more profound modification of the population than could be achieved by a few generations of selection. In genetic terms, it implies epistatic interactions between genes at different loci: gene substitutions at a given locus which increase development rate on a genetic background causing slow development have little or no effect on a genetic background causing rapid development. In other words, there is a law of diminishing returns as more and more alleles for fast development are accumulated in the genotype. It is suggested that genetic situations of this kind may be common in populations which have been exposed to directional selection for a long time in reasonably large populations, either in nature or in domestication.

(1) Fimbriated cells of E. coli K-12 are of two types. In the Fim+ type the expression of fimbriation is susceptible to reversible environmental suppression. This type gives rise to environment-stable mutants termed Fimσ+.

(2) Fimbriated cells can yield two types of non-fimbriated mutant, Fim− and Fimσ−, the latter arising from populations of Fimσ+ cells. Neither type reverts to the fimbriated state.

(3) Both types of fimbriated and non-fimbriated cells can be distinguished by physiological and genetic criteria.

(4) Fimbriation of both types can be transferred to non-fimbriated cells of both types, and segregates among recombinants, in crosses mediated either by transduction or sexual conjugation.

(5) The genetic control of fimbriation involves at least two determinants, for one of which a chromosomal location (fim) has been mapped.

(6) Certain anomalies of fim segregation are interpreted in terms of negative interference over relatively large regions of the bacterial chromosome.