Addition to the food of Drosophila larvae of DNA from calf thymus induces a considerable number of recessive lethal mutations in the 2nd chromosome. An analysis of sixty-six such lethals showed that most of them affect the same segment containing probably some thirty to thirty-five sub-units capable of mutating together or separately.

Calf-thymus DNA was mixed with the larval food of Drosophila melanogaster and complete and mosaic lethals were scored in both the X- and 2nd chromosomes. There was a decided mutagenic effect on the 2nd chromosome in both sexes. No complete sex-linked lethals were produced in males, but the production of a moderate number of complete lethals in females and of mosaic lethals in both sexes suggests that, in the absence of germinal selection, the X-chromosome is not completely refractory to the treatment.

The extension of the 2nd chromosome test to later generations showed that mosaics often gave rise to further mosaic progeny. Tests for allelism showed that lethals in the same mosaic line formed series of overlapping deficiencies, indicating that the primary effect in these cases was a transmitted instability of a narrow chromosomal region. Cases in which several mosaics arose out of one mosaic parent show that this instability must have replicated as instability.

1. Hairless (H), a dominant mutant of Drosophila melanogaster, affects the chaetae.

2. Various ‘intensities’ of effect are interpreted as representing different times in chaetal development at which deficiency of ‘+H substance’ is encountered. ‘Temperature effective period’ studies confirm this view.

3. The chaetae at different sites are liable to characteristically different ‘intensities’ of effect. In other words a particular site tends to be affected at a limited time in its development. If it is supposed that a single period of deficiency of +H substance accounts for effects on all chaetae, then it follows that development of the various chaetae is asynchronous. Some histological observations tend to support this prediction.

The implications of asynchrony with respect to pattern formation are discussed.

4. It is argued that the source of +H substance may be an endocrine organ, the ring-gland.

5. Two modifiers Su-H and E-H, whose expression and linkage relations are described in detail for the first time, are, it is suggested, respectively ‘constitutive’ and ‘super-repressed’ mutations of a ‘regulator’ locus controlling negative feed-back on the +H locus.

Diploid strains of Coprinus lagopus have been synthesized from common A heterokaryons either as oidial colonies or sectors. The criteria of growth rate and colony morphology on selective medium were used to distinguish between diploid and heterokaryon colonies. The average oidial size and hyphal width of diploid strains was significantly greater than that of the haploid parental strains.

Diploid monokaryons were very stable and only rarely produced haploid segregants. However, aneuploid intermediates in haploidization have been identified and these segregated further to give haploid monokaryons with recombinant genomes.

Dikaryons formed from diploid and haploid strains produced fruiting bodies. Meiosis and basidiospore production were irregular owing to the formation of triploid or partially triploid fusion nuclei in the basidia. In contrast to their stability in monokaryons, diploid nuclei tended to be unstable when combined in a dikaryon with a haploid nucleus, and often underwent partial haploidization before fruiting. Segregation of genes in the basidiospore progeny reflected whether haploidization had occurred before or after the formation of the fruiting body. If the haploid nucleus had a B mating-type allele common to the diploid nucleus, haploidization effected loss of the common allele.

The serum transferrin and post-albumin phenotype distributions of 228 Sahiwal, 138 Nganda, 265 Boran, 114 Tanganyika Shorthorned Zebu, 72 Teso and 267 Ankole cattle from East Africa were determined.

Five transferrin alleles, TfA, TfB, TfD, TfE, and TfF were present in all the breeds examined, and a sixth allele, TfG, was present in three of the Borans. TfE and TfF were the most frequent alleles, except in the Nganda cattle where TfA was the most frequent. TfB had a frequency of less than 0·1 in each breed. Two post-albumin alleles, PaF and Pas, were present in each breed. In each breed except Sahiwal PaF was two to three times more frequent than Pas. In the Sahiwal PaF and Pas had about the same frequency. It was concluded that both transferrin and post-albumin gene frequencies in East African cattle differ significantly from the corresponding frequencies in European cattle.

There was no evidence of an excess of heterozygotes in the post-albumin system other than that expected from the use of relatively small numbers of bulls in these herds. However, allowing for the same factor in the transferrin system, there appeared to be an excess of transferrin heterozygotes in the cattle populations sampled although the extent of this excess could not be calculated accurately.

Mutant genes in linkage groups I (mating-type chromosome), VI and VII have been transferred from Neurospora crassa to N. sitophila by hybridization and repeated backcrossing. Recombination between these genes has been studied from five-point crosses involving linkage group I and three-point crosses involving linkage groups VI and VII of the two species.

The results show significant differences in the amount of recombination between some of the genes in the proximal regions of the mating-type chromosomes of the two species. They indicate proximal localization of crossovers in the mating-type chromosome of N. sitophila. The results also show significant differences in recombination frequency between the genes in linkage group VI and a close similarity in linkage group VII. They further show that the centromere in the two species may not be interfering with crossing-over in its vicinity to such an extent as to be of any evolutionary significance.

Tetrad data from five-point crosses involving linkage group I (mating-type chromosome) and three-point crosses involving linkage groups VI and VII of Neurospora crassa and N. sitophila have been analysed in order to detect the phenomena of chromatid and chiasma interference on a comparative basis. Marker genes were transferred from N. crassa to N. sitophila by hybridization and repeated back crossing. The details of the methods of transfer and of making crosses have been described in a previous paper (Scott-Emuakpor, 1965).

Under suitable experimental conditions, short denatured DNA fragments may be bound by high molecular weight denatured DNA immobilized in agar (‘long DNA’). The proportion of fragments bound depends in part upon the degree of taxonomic relationship between the organisms from which the two DNA components are taken. Before the method can be used as a measure of genetic relationship between closely related species or between intra-specific groups, its resolving power requires to be increased. The present study explores some modifications of the method from this point of view, using the degree of binding between DNA from laboratory mice and rats as a model.

Long rat DNA was less effective than long mouse DNA in binding short DNA fragments from either source. It was therefore necessary to test reciprocally, using both short mouse and short rat DNA on long DNA from each species. Combining reciprocals gives a measure of the proportion of fragments bound in within-species and in between-species combinations. With rats and mice, between-species binding was at about 70% of the within-species level. The discrepancy could be increased, and hence the discriminating power of the method improved, if the binding reaction was partially or totally ‘blocked’ with an excess of unlabelled DNA fragments in an appropriate species combination.

Even when short and long DNA came from the same source, only about a third of the fragments were bound. The bound and unbound fractions were recovered, and each reincubated with a fresh sample of long DNA. The two fractions then behaved very differently, suggesting that the original DNA preparation is heterogeneous. The initially unbound DNA showed a very low binding ability in subsequent incubations and may be considered virtually ‘unbindable’.

When the first incubation is a between-species one, the unbound fraction should include not only this ‘unbindable’ DNA, but also any DNA characteristic of that species alone, and not held in common by both species. If it could be isolated, this DNA fraction might be expected to discriminate maximally between the two species. By using three successive cycles of incubation, a fraction was obtained which showed considerably improved discriminating power, in that it showed only about 30% as much binding in between-species as in within-species combinations.

Linkage data, obtained by a combination of selective analysis of haploid recombinants and analysis of segregating heterozygotes, are given for twenty-eight loci in Streptomyces coelicolor A3(2). This brings the total known loci for the organism to thirty-nine. The two linkage groups previously recognized remain separate, and their lengths have been increased to about 60 and 70 recombination units respectively. Whether the two linkage groups correspond to two chromosomes remains an open question.

Three further examples of close linkage of pairs of functionally related loci have been found (a trio of such loci was already known), and three other pairs provide possible examples of the same phenomenon. Some loci which are clustered in Salmonella are unlinked in Streptomyces.

Congenital defects due to genes are the static end results of abnormal processes in embryonic development. Are these ‘burnt out’ and no longer in operation later in life, or do they continue as active physiological processes? To answer this question, thirteen genes with comparatively trivial morphological effects have been subjected to a screen test. For eight of them (bp, f, Ts, Sd, un, se, Os and Ph) a prima facie case has been established for the existence of continuing processes of one kind or another. For the remaining five genes (dn, Pt, T, vt and Miwh) there is no such evidence though the possibility remains that such processes might be detectable by different means.

Phage F116 transduces sufficiently large fragments of the Pseudomonas aeruginosa chromosome for co-transduction of linked markers to occur. The linkage relationships of 176 auxotrophic mutants have been examined by means of this technique, following characterization of the mutants for their nutritional requirements and by other biochemical means. Thirty-two different loci from fourteen biosynthetic pathways have been differentiated by such combined means. Unlike other bacteria, the clustering of related loci of a biosynthetic pathway appears to be rare. This situation was confirmed for the leucine, methionine, adenine, isoleucine plus valine and arginine pathways. No evidence for linkage of markers within the histidine, proline, cysteine and uracil pathways has been found but further data is needed to completely confirm this conclusion for these groups of markers. The importance of these findings in relation to the genetic control of enzyme biosynthesis in P. aeruginosa is discussed. For markers of the tryptophan pathway, two are possibly linked but the other three are definitely not linked. The only confirmed linkage of related markers was between a homoserine locus and a threonine locus.

Jacob & Wollman (1961) produced interruption of chromosome transfer during conjugation in E. coli K-12 by exposing conjugating pairs to high shear forces created by a propeller-type mixer (a Waring blendor). In our laboratory, efforts to use this method were not completely successful because of the relatively long times required to ‘blend’ each sample and the large variations from sample to sample in the efficiency of separating mating couples (‘blending efficiency’). The variations in blending efficiency were thought to be due to lack of homogeneity of mixing in the blendor chamber. We have obtained much more satisfactory results by using a vibratory mixer which shakes the entire sample chamber and thoroughly agitates all the cells. Furthermore this vibratory blendor is much easier to use and permits taking at least twice as many samples per unit time, as compared to the usual blendor method.

A multi-step Chloramphenicol (CM)-resistant derivative of an RC-stringent strain of Escherichia coli auxotrophic for threonine and leucine was resistant also to Aureomycin (AM) and Puromycin (PM). All three antibiotics released the repression of RNA synthesis due to amino acid starvation in the CM-sensitive parent strain, their relative activities being about 1:10:100 for AM: CM: PM. High doses of AM and CM failed to induce RNA synthesis. The CM-resistant strain required greater concentrations of each antibiotic than the sensitive strain to induce the same level of RNA synthesis, and appeared to be about one hundred times, ten times and five times more resistant to CM, AM and PM, respectively, than the sensitive strain.

Chloramphenicol (CM), Aureomycin (AM) and Puromycin (PM) induce RNA synthesis in RC-stringent Escherichia coli starved of a required amino acid. This fact has been used to develop a method for comparing the levels of resistance of single-step CM-r mutants to the three antibiotics. Three levels of resistance to each antibiotic were found among four mutants selected in a single CM-s strain. The mutant with the highest CM resistance has the lowest AM resistance, and vice versa, while the level of PM resistance was not correlated with that of either CM or AM. The four mutants all differed from each other in their patterns of resistance to the three antibiotics.