Temperature-sensitive mutants in genes 12 and 18 of phage P22 show recessive DNA-negative phenotypes, but dominant integration-negative phenotypes. In contrast, am mutants in genes 12 and 18 are recessive for both DNA synthesis and lysogeny. The data are interpreted to indicate that the active forms of these two gene products are multimers.

The process leading to gene recombination can be interrupted in the filamentous bacteria Streptomyces coelicolor by growing mixed cultures on cellophane disks lying on complete medium. The mycelium is harvested, broken, diluted and the broken hyphae plated at different time intervals. By this means some markers can be excluded from heteroclones or from recombinant progeny in early samples. The recombinant pattern clearly changes with time, with an increase of markers contributed to the recombinant progeny. In crosses between male (NF) and female (UF) strains, the maleness is the first donor trait to appear in the cells of the recipient parent. The fertility factor does not produce a transfer origin on the donor chromosomes; the donor contribution may extend on either side or on both sides of the factor which appears to be compulsory for zygote formation. The longer the time of contact between parental cells, the longer the segment of the donor chromosome contributing to the recombinant progeny. When spores are formed they contain almost exclusively recombinant nuclei derived from segregation processes.

In their recent paper, Waddington & Robertson (1966) discuss the general importance of disruptive selection and make various points that call for comment.

First, their statement that ‘It is most desirable that Thoday's results … should be repeated, with special attention to the strict virginity of the flies used, …’ appears to some readers as a scarcely veiled imputation that my experiments, especially that of Thoday & Gibson (1962), were executed without technical competence, though Waddington has informed me such imputation was not intended.

1. The antibody response was studied in twenty-five monozygous (MZ) and ten dizygous (DZ) pairs o f cattle twins, which were inoculated with red cells from one cattle donor carrying the M, V and U′ antigens. These red cell antigens are controlled by genes at three different loci. Most o f the recipients lacked two or all of these antigens. All had passed puberty at the onset of the experiment.

2. The antibody response against the three antigens differed markedly. Anti-U′ appeared on the average 11 days after the 1st injection and the maximum titre values were high. The V antibodies were produced at a slow rate, while anti-M was intermediate.

3. There was some influence of age on the rate and intensity of antibody production. Older animals gave a faster response and, with regard to anti-M, also higher titres.

4. Twins belonging to the same MZ pair usually produced v e r y similar antibody curves. However, there was at least one noticeable exception, which is given special consideration in the Discussion. The differences between MZ pairs accounted for a considerable portion of the variance in the different measures of antibody response and in most cases this portion remained significant also after the elimination of the influence o f age and sex. The DZ pairs were more variable.

5. The simultaneous response o f two or three antibodies in the same animal was studied. There was a significant positive association between antibody types which suggests the existence o f individual (probably genetic) differences in the general antibody-producing ability.

X-rays and deficiencies in DNA repair had a synergistic effect on genetic damage associated with P-element mobility in Drosophila melanogaster. These interactions, using sterility and fecundity as endpoints, were tested in dysgenic males deficient in either excision or post-replication DNA repair. Three sublines of the Harwich P strain were used for the construction of hybrid males. These sublines differ in P-induction ability based on gonadal dysgenesis sterility (GD) and snw mutability tests, in P-element insertion site pattern, and in the types of defective P-elements, such as KP elements, they possess. A lower degree of gonadal dysgenesis was correlated with the presence of KP elements. GD sterility and snw mutability were not always correlated. Dysgenic hybrids originating from the standard reference subline, Harwichwhite, were much more sensitive to the post-replication repair than the excision repair defect. In contrast, sterility of hybrids derived from the weak subline was least affected by, and that of hybrids of the strongest subline was most affected by either DNA repair deficiency. The exacerbation by X-rays of the effects of DNA repair deficiencies on genetic damage indicates that both repair mechanisms are required for processing DNA lesions induced by the combined effect of P activity and ionizing radiation.

Genetic and morphometric distances between 12 inbred strains of mice ranging from closely related substrains to a sub-species were estimated using published data on single locus polymorphisms, and on the basis of up to 44 measurements on seven different bones, respectively. Simulation was also used to investigate sampling effects for the single loci. There were strong and statistically highly significant correlations among all measures of genetic distance ranging from 0·58 for the comparison of single loci with the logarithm of the Mahalanobis distance based on 24 measurements on four bones, to 0·72 for estimates of genetic distance based on single loci and the morphology of the mandible. These findings are in sharp contrast with those of Wayne & O'Brien (1986) who claimed that ‘structural gene and morphometric variation of mandible traits are uncoupled between mouse strains’. Their failure to find such a correlation is probably because their sample of inbred strains included only a single pair of closely related substrains, and no substrains separated for less than 40 years, and because they failed to correct for non-linearity between morphometric and single-locus measurement scales. Simulations and regression analysis suggested that genetic distances could be estimated with approximately equal precision using morphological data on bone measurements or about 10 cladistically informative single loci, which would usually involve sampling about 50 loci. Data based on single-gene markers is usually more informative than morphometric data for studying the similarity of independently-derived strains. However, similarities among closely related populations such as sublines of an inbred strain can usually be studied more efficiently using morphometry.

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.

Detectable allelic variation at the Gpi-1 loci on the short arms of the homoeologous group 1 chromsomes in wheat is not common. However, a variant null allele at the Gpi-D1 locus is present in some stocks of Chinese Spring. This has allowed the locus to be mapped between the ω-gliadin locus carried distally on the short arm of chromosome 1D, Gli-D1 (34·5%) and the high-molecular-weight glutenin subunit locus carried near the centromere on the long arm, Glu-D1 (36·2%). The origin of this isoenzyme polymorphism in Chinese Spring stocks is described and its potential significance is discussed in relation to quantitative analysis of aneuploids, alien chromosome addition and substitution lines and intervarietal chromosome substitution lines involving Chinese Spring.

The gene conversion model reported by Birky & Skavaril (1976) has been analytically studied by using the theory of diffusion models of Kimura (1964) in population genetics. It has been shown that the fate of new mutations in systems with multiple genomes may be satisfactorily treated by the diffusion model.

This paper describes the results of an experiment to measure the effect on mean population fitness of recombination in the second chromosome of Drosophila melanogaster. There was a small and non-significant effect of recombination in lowering egg-to-adult viability of heterozygotes for wild-type chromosomes. A large (7%) and significant effect of recombinant chromosomes on the fecundity of Cy female carriers was detected.

Prior reports from this laboratory have described the experimental basis for our understanding of the rosy locus (ry: 3–52·0) of Drosophila melanogaster as a bipartite genetic entity consisting of a structural element that codes for the xanthine dehydrogenase (XDH) peptide and a contiguous cis-acting control element immediately to the left of the structural element. Although the left end (5′) of the structural element has been well defined, the right boundary (3′) has been given only casual treatment in our prior reports. In our recent studies of rosy locus expression we have been concerned with the production and identification of mutations in the non-structural regions immediately flanking the structural element. An improved definition of the right end of the structural element is essential to this analysis. In addition to producing a better definition of the right boundary of the structural element, this study produced several phenotypically novel mutations. These mutations were classified initially ascontrol element mutations, but upon analysis were found to map within the rosy structural element. No evidence was obtained for the existence of a control element contiguous with the right end of the structural element.

Ten recessive Opaque-2 (O2) alleles of independent origin were characterized at the molecular level. The results revealed a high level of polymorphism at the O2 locus. In addition, our data suggest the possible cause for the recessive character of some of the alleles investigated, and allow us to infer some conclusions concerning the degree of relationship between the o2 mutations. Comparison of genomic sequences spanning the first exon and obtained from a series of wild-type and recessive alleles revealed the presence of a hypervariable region, involving different dipeptides, in the N-terminal part of the O2 protein.

1. The linkage order of four tryA mutants of S. typhimurium, and cysB-12, is:

Attempts to plot the position of tryA-50 were unsuccessful.

2. Some of the reversions of tryA-8, tryA-47, tryA-56 and tryA-50 were analysed genetically; tryA-52 does not revert. All four auxotrophs gave reversions that were phenotypically and genetically indistinguishable from that expected by back-mutation of the original mutant site.

3. Both tryA-8 and tryA-50 produced reversions that grew as wild-type but were due to unlinked suppressor mutations. Some of these were super-suppressors in that they suppressed both tryA-8 and tryB-4; others suppressed many site mutants in the tryA gene but did not suppress tryB-4.

4. All the slow-growing reversions of tryA-8, tryA-50 and tryA-56, and a minority of the semi-fast reversions of tryA-8, were due to unlinked suppressors.

5. All the slow-growing reversions of tryA-47, the semi-fast reversions of tryA-56 and the majority of the semi-fast reversions of tryA-8 were due to genetic changes that were inseparable, in very extensive experiments, from their original mutant site.

6. Slow-growing reversions of tryA-47 produced faster growing mutants. Some of these were due to mutation in unlinked modifying genes and in others the genetic change was within the tryA gene. Nine of the latter had the genetic change just to the left of the 47S site; in one the change was inseparable from the 47S site. None had this further change to the right of the 47S site. These further changes, in the absence of the 47S site, gave prototrophic phenotypes; they are inter-site suppressors.