White-spored mutants of A. immersus were used in experiments carried out to study the relationship between conversions occurring simultaneously in linked genes, and to check whether conversion occurs at random in the two chromatids of a chromosome.

Conversions in two loosely linked loci were shown to be independent of each other and randomly distributed between sister chromatids. On the other hand, conversions in two closely linked loci showed a high correlation of occurrence.

1. Auxotrophs were sought in a slow-growing reversion of the tryA47 strain of S. typhimurium. This reversion differs from tryA47 by a genetic change that is inseparable from the 47 site and has been designated 47S. Out of thirty-nine auxotrophs that derived from independent mutations, twenty-two grew on minimal medium supplemented with anthranilic acid. Eight of these auxotrophs were examined and each was shown to have the 47S site unchanged and to carry a further mutation in the tryA gene. These further mutations were shown to be at different sites in different auxotrophs.

2. Auxotrophs were sought in the wild-type LT-2 strain. None out of thirty-six were mutant in the tryA gene.

3. The 47S site in tryA47S was replaced by 47+ transduced from wild-type LT-2. Auxotrophs were sought in this strain and only one out of nineteen was mutant in the tryA gene.

4. Auxotrophs were sought in a wild-type reversion of tryA47. Out of twenty-six none were mutant in the tryA gene.

5. The 47+ site in wild-type LT-2 was replaced by 47S transduced from tryA47S. Auxotrophs were sought in this strain and twenty-two out of fifty-five were mutant in the tryA gene.

6. We conclude that tryA auxotrophs are only recovered at a high frequency when the 47S site is present in the tryA gene.

7. In strains with the 47S site in the tryA gene the frequency of auxotrophs that will grow on minimal medium supplemented with indole but not on minimal medium supplemented with anthranilic acid is appreciably higher than in those strains without the 47S site in the tryA gene. These auxotrophs are mutant in genes that are in the same operon as tryA.

1. An investigation of the serum esterases of the field-vole, M. agrestis, has been carried out by the method of starch-gel electrophoresis.

2. A total of 254 animals, mainly from Carron Valley, near Stirling, have been examined.

3. Evidence is presented for a system of four enzymes controlled by two genetic loci, which it is proposed to call Es-3C and Es-3R.

4. Each locus specifies one polypeptide sub-unit of the E3 enzyme, which is a dimer.

5. The two loci each have two alleles present at high frequencies in the population examined.

Using a strain of D. melanogaster carrying an X-4 translocation, comparison between groups of individuals cultured at 14°, 19° and 25°C. showed good correlation between heterochromatization, variegation in malpighian tubules and Notch. All these phenomena are enhanced by low temperatures. Correlation was less good within each temperature group, and considerable variability was observed between individuals within the group, and between nuclei from one individual.

Experiments involving a temperature change during the embryonic period indicate that (1) heterochromatization is especially temperature-sensitive during the early embryonic period but may be increased by low temperature later, (2) larval malpighian tubules are sensitive to temperature only during the early embryonic stage, and (3) N is influenced by temperature during early embryonic life and also during the larval period.

Our observations, in conjunction with those of other workers, could be explained as follows: exposure of individuals to low temperature at a time when a specific system is beginning to differentiate will cause the processes concerned to be blocked in some cells. At least during the early embryonic stages this appears to involve heterochromatization of the relevant locus. Once the processes are established which will lead to the formation of a character, further heterochromatization has no effect on the phenotype. Temperature may affect pupal or adult phenotype in this way or by a direct action on the metabolic processes of cells.

Further experiments showed: (1) the greatest temperature-sensitivity of all three phenomena within the first 6 hours of embryonic life; (2) striking fluctuations of the effect of temperature, especially within the early embryonic period; (3) close correspondence between all three phenomena in time of response to temperature. Some alternative interpretations are considered.

Hair follicle initiation and development was examined in tabby mutants and compared with a previous study of this sequence in normal mice. Initiation commenced at about the 17th embryonic day and continued for 1 and possibly 2 days after birth. Although central primary follicle initiation was of shorter than normal duration, it was more rapid, with the result that the central primaries achieved a greater maximum density, and their total numbers were greater in tabby than in normal mice.

In general structural terms, follicle group development proceeded normally except that no secondary follicles appeared, and for this reason, the mature grouping pattern of follicles was comparatively simple. Central primary follicles, the first to be initiated, were evenly spaced in the skin but the later association of lateral primaries with them was unusually close, and the resulting groups of primary follicles were relatively more compact in the mutants than they were in normal mice.

No simple explanation in terms of the altered timing of follicle initiation or the reduced complexity of the follicle group could be given for the abnormalities of the tabby coat.

Selection for high and low numbers of secondary vibrissae in tabby mice produced some correlated changes in the maincoat follicle population, but there was no apparent alteration to the time of onset of initiation. Evidently selection did not simply modify the effects of the Ta gene.

A complicated system of chromosome instability related to gene suppression has been analysed. In addition, unstable genetic events analogous to those previously described in A. nidulans and in other organisms have been detected and one type of unstable variant recovered may be determined by V-type position effect. Furthermore, the selective systems used, clearly offer scope for analysis of genetic instability in general.

Heterokaryons were made between a cytoplasmic variant of Neurospora called SG, which shows a slow growth rate, and normal strains. Fusions were made in such a way that the fate of the protoplasmic mixes in individual hyphae could be followed. A semi-quantitative analysis of nuclear and cytoplasmic migration along established hyphae was possible. Apparently, components of SG and normal cytoplasms are able to associate intimately in a single hyphal segment and later segregate intact. More often, however, an interaction of cytoplasms occurs which results in a new stable cytoplasmic type called slow SG. It is postulated the cytoplasmic determinants of SG are particulate and that the slow SG strains are the result of reassortment of the SG factor with other autonomous cytoplasmic entities. Acriflavine in the proper concentration inhibits and kills SG strains on solid medium without affecting normal strains. The acriflavine is most effective against germinating spores. The SG phenotype is redefined on the basis of this sensitivity to acriflavine.

Two codominant alleles, Mup-a1 and Mup-a2, controlling electrophoretic variation of one of the components of the major urinary protein (MUP) complex of the mouse Mus musculus have been found to be linked to the black–brown coat color alleles (linkage group VIII).

Individual yeast strains show characteristic differences in the amount of chloramphenicol required to inhibit the synthesis of the respiratory system. This varies from 0·05 mg./ml. to 4 mg./ml. A correlation between chloramphenicol and tetracycline resistance appears likely but no correlation between chloramphenicol and erythromycin resistance was observed. These relationships were emphasized by cross-checking spontaneous resistant mutants for each antibiotic. Nearly all mutants showing spontaneous resistance to chloramphenicol had a simultaneous increase in tetracycline resistance but no increase in erythromycin resistance. Spontaneous resistance to erythromycin on the other hand, had no striking effect on tolerance levels to chloramphenicol and tetracycline. Increases in erythromycin resistance were commonly accompanied by an increase in resistance to other macrolide antibiotics. There are similarities between these effects and those described for certain bacterial systems.