Eight newly isolated 9 + 0 mutants each mapped at one of the four previously known loci. Short flagellum mutants were at three loci, two of which (pf7 and pf8) were closely linked; the third, pf21, was unlinked to these two and mapped on linkage group II. The long flagellum mutants lf1 and lf2 were on linkage groups II and XII respectively. Mutants pf8A and lf1 were both recessive to wild-type. There was no evidence for non-Mendelian flagellum mutants.

A class of rII mutants revertible by both base analogues and acridines is described. The members of this anomalous class are base-substitution mutants suppressed by a phase-shift mutation in a phage gene mapping outside the rII region.

The ‘neutral mutation theory’ holds that most amino acid substitutions in evolution are selectively neutral. The known pattern of variation in human haemoglobins can only be made consistent with this theory if the human species has passed through a bottleneck of numbers in the recent past. If this theory is true, estimates of the necessary size and duration of this bottleneck can be made. A theory is developed which leads to an estimate of Yg, n, the number of alleles present in a population which arise between g and n generations ago, and hence to the estimate

where u is the neutral mutation rate and Ne the effective population size, for the probability that a population contains no such alleles. Using data on haemoglobins, this gives an approximate upper limit to the time elapsed since the bottleneck in human numbers. Either such a bottleneck occurred, or the neutral mutation theory is false; data on other proteins will enable a choice between these possibilities to be made.

In order to investigate the importance of the nuclear genotype during morphogenesis of the cortex in Paramecium aurelia, a search was made for conditional and unconditional mutants affecting cell shape. Ten different mutants were readily recovered following mutagenesis with N-methyl-N-nitro-N-nitrosoguanidine on stock 51s of syngen 4. The detailed pattern of surface units remained unchanged in these mutants but each one can be described as resembling a normal cell twisted about its long axis. Nine of the mutants complement each other and show no linkage. The remaining mutant, a ‘chain former’, has conditional expression, and is only recognizable at 35 °C. The penetrance of this mutant is affected by the food organism used. The conditional mutant also interacts with seven of the other mutants in pairwise homozygous combinations so that in these genotypes it can be classified after growth at 25 °C. The mutants are discussed in relation to the search for genie effects on the cortical pattern.

A mutant with the properties of a recipient has been isolated from the P. aeruginosa donor strain PAT (FP 2 + ) following treatment with the acridine-mustard ICR-191. While this strain displays the properties expected of a female or recipient in a number of genetic tests, the FP 2 determined property of mercury resistance is retained by the strain, suggesting that it may carry the FP2 factor in a mutated form. Treatment of the donor strain PAT (FP2 + ) with acridine-mustard has produced mutant male strains with the ability to form recombinants with other male strains at frequencies similar to that obtained in FP2+ × FP2 − matings. This characteristic has been shown to be due to a mutation in the FP2 factor which is dominant to the wild-type function. The isolation of stable male strains carrying both the mutant and wild type forms of the sex factor suggests that more than one copy of the FP 2 factor occurs in P. aeruginosa strain PAT donors.

The frequencies of genetically determined electrophoretic variants of two enzyme systems in the parthenogenetic crustacean D. magna have been followed in two isolated populations. In both populations a marked excess of hetero-zygotes was found in the later samples. It is concluded that the observed changes in gene and genotypic frequencies are due to natural selection as both migration and genetic drift can be excluded.

Mutants in the ftr cistron of the Basidiomycete Coprinus lagopus have a lesion in sugar transport. Fifty-two alleles are placed in an allele map using recombination frequencies obtained from over 400 heteroallelic crosses. The mutant sites in the allele map are distinctly clustered into three approximately equally spaced regions. It is demonstrated that the clustering is not due to any mutational specificity. Evidence is presented which indicates that the clusters are functionally differentiated both within themselves and from one another. Additivity of recombination frequencies was good over the whole of the allele map and there was no overall map expansion. However, specific marker effect sites could be recognized. The data indicate that marker effect may act to enhance or reduce recombination frequency and that enhancement is equal and opposite to reduction. It is also shown that marker effect enhancement is only observed when the interval being mapped exceeds a certain minimum value, but that there was no upper limit to the size of the interval. Expression of marker effect was influenced both by the background genome and by the nature of the mutation at the second site in the heteroallelic cross. It is suggested that current models of recombination do not adequately explain these data and, more importantly, that then-reliance on initial breakage of DNA strands in the formation of hybrid DNA creates grave mechanical difficulties. A model for genetic recombination is proposed in which the sequence of events is: (i) separation of intact double helices into single strands; (ii) illegitimate pairing between single strands from non-sister chromatids; (iii) strand breakage and exchange of covalent links so as to legitimize the illegitimately paired regions. The model allows for the formation of hybrid DNA either with or without coincident chiasmata. It is envisaged that the error recognized by any excision-repair system involved in recombination is the tangled strands at each end of the illegitimately paired region rather than base mismatching; and that the exchange of covalent links in stage (iii) involves the excision and replacement of segments of DNA.

The gene for resistance to phage BF23 and colicins E1, E2 and E3, bfe, was mapped by a combination of conjugation and transduction crosses. Co-transduction of bfe was found with markers in the region between 76 and 79 min on the Escherichia coli genetic map. The highest frequency of co-transduction was found with argH (47%). Three-factor transductional crosses showed unambiguously that bfe lies between argH and supM, at about 77·5 min on the map.

The translocation break T(2;9)138Ca is located about three map units from the H-2 region on the non-centromeric side of the IXth linkage group. The recombination frequency in the T-H-2 interval is increased in the presence of the T138 translocation to about twice its value in the absence of the translocation.

The highly selective technique of integrative suppression has been used to determine the ability of autonomous transmissible plasmids to integrate into the E. coli chromosome.

All the F-like transmissible plasmids appear to be able to integrate, suppress the initiation defect and form Hfr-type donor strains. However, there is no evidence of integration with the I-like transmissible plasmids examined.

Double cross-overs were detected in a few crosses involving segment ‘29’ mutants of locus ‘14’ in Podospora anserina. The occurrence of such recombination events made it necessary to reconsider the localization of three mutants. Use of an outside marker and the analysis of many other crosses allowed these mutants to be mapped unambiguously. The change in the localization of one of these mutants together with complementation tests on a new mutant leads to the conclusion that the polycistronic unit of transcription contains only four cistrons instead of the five previously thought to exist.