The distribution of visits to a particular gene frequency in a finite population of size N with non-overlapping generations is derived. It is shown, by using well-known results from the theory of finite Markov chains, that all such distributions are geometric, with parameters dependent only on the set of bij's, where bij is the mean number of visits to frequency j/2N, given initial frequency i/2N. The variance of such a distribution does not agree with the value suggested by the diffusion method. An improved approximation is derived.

Wild-type strains of Coprinus lagopus are sensitive to para-fluoro-phenylalanine and ethionine. Resistant mutants to these two analogues are known but all these mutants are recessive in a heterozygous dikaryon except for F7 (pfpr-3) which is semi-dominant. Resistance to two other analogues, however – canavanine sulphate and azetidine-2-carboxylic acid – were found to be wild-type features. One strain of C. lagopus sensitive to canavanine was identified. Selection for canavanine resistance in monokaryons always yielded only dominant resistance, while selection for para-fluorophenylalanine resistance in monokaryons gave only recessive resistance. Canavanine-resistant mutants were due to a single gene mutation which, like the wild-type resistance, were dominant in heterozygous dikaryons. The wild-type resistance was also dominant in a diploid but the mutant resistance was recessive. Selection for resistance to para-fluorophenylalanine in auxotrophically balanced dikaryons resulted in the identification of two new loci (pfpr-10 and pfpr-ll), and two specific dominance modifiers (mod+-10 and mod+-ll). In the absence of the specific modifier, pfpr-10 and pfpr-ll were recessive while, in the presence of even one dose of the specific modifier, resistance was dominant in the dikaryon. The pfpr-10 and pfpr-ll even in the presence of two doses of modifier were fully recessive in the diploid. The action of the modifier genes and the reversal of dominance in dikaryon and diploid is discussed in terms of negative complementation in an oligomeric product of the pfpr gene and localized translation of the relevant mRNA in the cell with the modifier acting as a reinforcer of localization.

exrB derivatives of H/r30 and WP2 are sensitive to ultraviolet radiation (UV), methylmethane sulphonate (MMS) and will not form colonies at 42 °C. Revertants to wild type were selected by plating on either nutrient plates containing MMS or nutrient plates incubated at 42 °C. Revertants selected for MMS resistance were temperature- and UV-resistant, and all revertants selected for temperature resistance were MMS and UV resistant. The frequency with which revertants appeared on MMS plates was about eight times as high as it was at 42 °C, and the variance in frequency of revertants among cultures started with small inocula was higher on selection at 42 °C than on MMS. The spontaneous frequency was estimated to be 10−6 to 10−7, sufficiently low to suggest a single mutational event. Among wild-type revertants selected at 42 °C there was a stable and an unstable class. exrB+ could be co-transduced with malB+ from stable revertants to an exrB strain, but could not from unstable revertants. None of the revertants could support the growth of amber or ochre mutants of phage T4, nor were amber and ochre suppressed exrB strains wild type with regards MMS and temperature resistance. The presence of a polarity suppressor in an exrB strain did not modify resistance to UV or temperature. Neither UV or MMS was able to induce mutations in an exrB strain.

Wild-type strains of Aspergillus nidulans grow poorly on L-histidine as a sole nitrogen source. The synthesis of the enzyme histidase (EC. 4.3.1.3) appears to be a limiting factor in the growth of the wild type, as strains carrying the mutant areA102 allele have elevated histidase levels and grow strongly on histidine as a sole nitrogen source. L-Histidine is an extremely weak sole carbon source for all strains.

Ammonium repression has an important role in the regulation of histidase synthesis and the relief of ammonium repression is dependent on the availability of a good carbon source. The level of histidase synthesis does not respond to the addition of exogenous substrate.

Mutants carrying lesions in the sarA or sarB loci (suppressor of areA102) have been isolated. The growth properties of these mutants on histidine as a sole nitrogen source correlate with the levels of histidase synthesized. Mutation at the sarA and sarB loci also reduces the utilization of a number of other nitrogen sources. The data suggest that these two genes may code for regulatory products involved in nitrogen catabolism. No histidase structural gene mutants were identified and possible explanations of this are discussed.

The charge state model of Ohta & Kimura (1973) for the number of electrophoretically detectable alleles in a finite population, is extended to include mutations of both one and two charge changes. The effective number of alleles (ne) is increased only slightly by this extension. Electrophoretic profiles of neutral variants are shown on average to be leptokurtic and have their odd central moments equal to zero. The expected frequency distribution of pairs of gametes which differ by 1,2,3,… charge units can be obtained as the sum of the appropriate terms from two geometric series.

The model, suggested by Ohta & Kimura, of an infinite number of selectively neutral alleles, in which alleles can only mutate to neighbouring alleles, has been extended to include a migratory pattern. The stepping-stone and island models of migration have been considered. In the latter case, it has been found that as the number of colonies becomes large, the expected number of alleles, ne, becomes approximately equal to

where m is the migration rate, v is the mutation rate and Ne is the effective size of each population. This solution tends to that of Ohta & Kimura as m → 0 but one can see that for any appreciable value of Nem, a large increase in ne is obtained. In order to check the validity of iterative results, models with a finite number of alleles have been considered, and their solutions have been found to converge quickly to those of the infinite case. The results exemplify the great power of migratory structure and neutral alleles to maintain a large amount of heterozygosity. Double step mutation and the finite time solution have also been considered.

Sorting out of cytoplasm determinants can be achieved in Podospora anserina by the use of protoplasts. In this way four cytoplasmic mutations have been isolated. These mutations affect a precise stage of development of the fungus. In crosses with wild-type strains, the mutants show maternal inheritance when no cytoplasmic contact precedes fertilization. However, when cytoplasmic mixing occurs before fertilization, the cytoplasmic wild-type factor shows dominant and/or suppressive properties over the mutant factor.

The isolation and genetic characterization of a new mutator gene, Mutator-forked3N (Mu-f3N), of Drosophila melanogaster are described. This mutator gene is unique in that it seems to increase specifically the reversion frequency of the unstable mutant forked3N (f3N, 1–56.7), since the frequency of spontaneous sex-linked recessive lethals in males and females and the frequency of reverse mutations at eight additional X-linked alleles were unaffected by Mu-f3N. The mutator is a dominant gene that has been mapped to the region between f3N (1–56.7) and Beadex-2 (Bx2, 1–59.4) in the X chromosome, and it seems to function only in the ‘ cis’ configuration. The mode of action of Mu-f3N is compared with that of other mutator genes.

The UV-inducible prophage J51 has been mapped late on the chromosome of P. aeruginosa strain PAO, relative to the entry point of the sex factor FP2. This was determined following the analysis of the segregation of unselected markers in conjugational crosses between appropriately marked donor and recipient strains. A more precise location of about 50 min was obtained from the kinetics of increase in infectious centres due to zygotic induction during interrupted mating experiments.

The Indian pygmy mouse, Mus dunni, exhibits great variation in the number of chromosome arms while its diploid number of chromosomes remains constant. The variation seems to be due to addition or deletion of C-band positive constitutive heterochromatin in the short arms of autosomes.

Four translocations in which one breakpoint was known to be at or closely linked to the LGIV marker Steel, Sl, were studied in an attempt to identify cytologically the as yet unassigned Linkage Group IV of the mouse. The cytological results indicate that chromosome 10, which had previously been matched with LGX, is involved in each of the four translocations, the break being in 10D. Recombination between Sl and the LGX marker grizzled, gr, was determined to be 6–14%. The combined cytological and genetic results indicate that LGIV and LGX are both in chromosome 10, and that the order may be centromere-gr-Sl.

A population cage experiment has been carried out to estimate fitness for a sample of fourteen non-lethal third chromosomes in D. melanogaster. This measurement, which should take into account all aspects of fitness, gives an estimated mean fitness of chromosome homozygotes of approximately ten percent.

Relative DNA contents of samples of nuclei from hyphal tips and from gametangia at different stages of development have been obtained by cytofluorimetry. The occurrence of nuclei with 4C, 2C and 1C DNA complements in gametangia and mainly 2C complements in hyphae indicates that meiosis takes place in the gametangia of Phytophthora drechsleri.