In the case of an allele which is going to become fixed in a population, the average number of generations for which the population assumes particular gene frequencies is investigated, using the diffusion approximation. Explicit formulas were obtained and they were checked by computer simulations. As a particular case, it is shown that if a new mutant that is selectively neutral is eventually fixed in a population of size N, it spends two generations on average at each of the intermediate frequencies (1/2N, 2/2N, …, (2N−1)/2N), and the variance at each frequency is four generations.

Mutations giving rise to sexual sterility were induced in Neurospora crassa macroconidia by ultraviolet-light irradiation. Thirty mutants were isolated on the basis of their male sterility in crosses with a wild-type strain. When used as the male parent these mutants exhibited a wide spectrum of sexual behaviour patterns ranging from the production of only small brown protoperithecia (complete male sterility) to the production of large and normally pigmented perithecia but with an undeveloped ostiole and very few if any spores. For many of the mutants the behaviour pattern is different when the strain is used as the female parent. Segregation data reveal that none of these mutants represent mutations of the mating-type locus. These findings suggest that the sexual development cycle is blocked at various stages in the different mutant strains. All attempts to restore fertility by supplying various additives to the medium or by varying the incubation time and temperature were unsuccessful. Conidial viability tests carried out on many of the strains revealed no abnormality in this respect. The aberrant segregation patterns exhibited by many of the mutants are discussed.

After liquid-holding treatment in saline, cultures of yeast exposed to u.v. light showed an increased resistance to a second exposure to u.v. irradiation. This increase in resistance during a second series of u.v. exposures was correlated with an increase in the induction of intragenic recombinants. In contrast, no increase in the frequency of the intergenic recombinants or mutations to prototrophy could be detected during the second u.v. dose range. The results obtained could not be explained by the induction of meiosis during liquid holding or by changes in the timing of cell division after u.v. exposure.

A model of u.v. repair in yeast is postulated in which liquid-holding treatment results in changes in the proportions of lesions repaired by excision repair and recombination repair (respectively).

The effect of optimizing selection, mutation and drift on a metric character determined by a large number of loci with equal effects without dominance was investigated theoretically. Conditions for a stable equilibrium under selection and mutation, in the absence of drift, have been obtained, and hence the amount of genetic variability which can be maintained by mutation has been determined. An approximate expression for the average amount of genetic variability to be expected in the presence of drift in a population of finite size has also been obtained and evaluated.

Eighteen male sterile mutants of N. crassa which block various stages of the sexual development cycle have been functionally categorized on the basis of complementation tests. Nine of these (those which block early stages of the sexual cycle) have also been analysed through recombination studies. On the basis of these data, a minimum of four genes appear to be involved in early stages of sexual development (i.e. probably prior to karyogamy) and several genes are indicated which control later stages of the cycle. Complementation phases with respect to sexual development in heterothallic Ascomycetes are discussed as well as a comparison of results obtained with expectations and with those obtained in related Ascomycetes.

Through a series of backcrosses the centromere region of linkage group I of Neurospora crassa was transferred to the N. sitophila genome, and through another series of backcrosses the centromere region of linkage group I of N. sitophila was transferred to N. crassa. Strains thus synthesized showed, in further crosses, that the N. sitophila centromere region acts as a dominant enhancer of cross-over frequencies across linkage group I of the two species.

In Saccharomyces lactis, a class of mutants isolated for low β-glucosidase activity are reduced in activity for β-galactosidase as well. Genetic studies indicate that their properties are the result of a single mutation in a nuclear gene. In diploide containing a wild-type and mutant β-galactosidase allele, the mutant phenotype is partially dominant. The two enzymes can be separated physically and under appropriate conditions are induced independently in wild-type strains.

An inverted segment in one of the chromosomes of Sordaria brevicollis has been found to have a marked effect on recombination within a gene in another chromosome. There are two effects on the intragenic recombination pattern, namely, a change in the polarity of recombination and a change in the frequency of parental and recombinant outside markers. The results support the idea that a mechanism exists for cancelling crossovers shortly after they have been formed as a means of controlling their number.

The coats of Ta–Ta+ and c–c+ chimaeric mice have been compared with those of Ta heterozygotes and animals heterozygous for an X-autosome translocation which causes c-variegation. With one exception all those features typical of the heterozygotes were found in their chimaeric counterparts, thus showing that the creation of two cell populations by X-inactivation can ultimately lead to the observed heterozygous phenotypes. The one exceptional feature not found in c-variegated chimaeric mice was the progessive increase in the proportion of pigmented hairs in c areas with age which occurs in X-autosome translocation heterozygotes.

The coat of a Modp / + – Ta chimaeric animal was also investigated. Ta hairs showing only the colour effect of Modp were numerous, suggesting that Modp and Ta affect different cell populations. However, Ta hairs showing the structural effect of Modp were very rare. The two observations suggest Modp can operate independently upon two different cell types – melanocytes and hair follicle cells.

The probability of fixation of a mutant gene in a finite population was investigated by taking into account the effect of random fluctuation of selection intensity. It was shown that not only the product of the effective population number and average selection coefficient (Nes) is important, but also the ratio of the mean and the variance of selection coefficient (s/ Vs) has an important influence on fixation probability. In particular, when this ratio is small, a mutant gene, even if selected against, becomes fixed in the population like a selectively neutral mutant. In general, when random fluctuation of selection intensity is not negligible it may be convenient to use ‘effective selection coefficient’. The bearing of the present findings on the neutral mutation-random drift theory of evolution and variation at the molecular level was discussed.

The effects of heterozygous inversions and of the C(3)G mutation on recombination between alleles of the rudimentary locus of Drosophila melanogaster were analysed. It is shown that the presence of either of these genetic alterations significantly changes the frequency and type of intragenic recombinants which are produced.

X-linked modification of the heterozygous phenotypes of X-linked genes has been detected in the X chromosomes of several inbred strains of mice. The effect is similar to that of the alternative ‘states’ or alleles, of the X chromosome controlling element, Xce, identified in T(1; X)Ct X chromosomes. Tests on two such differing X chromosomes have indicated that the phenotypic modification results either from non-random inactivation of the two X chromosomes or from selection operating on the two cell populations differentiated by X-inactivation. The data provide evidence of non-random X chromosome activity in the somatic cells of the female mouse.

Pseudomonas aeruginosa carries polar pili which act as receptors for RNA-containing bacteriophages. In order to confirm, that these pili were not involved in the transfer of the sex factor FP 2, eleven bacterial strains, both FP 2+ and FP 2−, were examined in the electron microscope for the presence of pili and tested for sensitivity to the RNA phage PP7. Pili were found on all strains save one which was resistant to phage PP7. It was also found by electron microscopy that about 25 times more pili per cell were present after PP7 adsorption than before it. This result is discussed with reference to the pilus retraction theory, providing further evidence that some kinds of pili retract instead of acting as simple tubes for the transfer of genetic material. The strong supporting evidence provided by the infective processes of male-specific coliphages is discussed and compared to current knowledge of P. aeruginosa RNA phages.

It was also found that pili were present on the host strain for the P. aeruginosa filamentous phage Pf. Although similar in appearance to RNA phage pili, these differed in that they did not adsorb phage PP7. However, it seemed likely that they were receptors for Pf. A structural comparison is made between P. aeruginosa pili and Escherichia coli F-pili. It is possible that P. aeruginosa pili could be coded for by a plasmid other than FP 2.

The ocular cataracts produced by the dominant CatFr gene in the mouse, Mus musculus, are associated with quantitative changes in the lens proteins (crystallins). The three classes of crystallin are affected differentially in homozygotes. Heterozygotes show a smaller effect. The quantitative levels of crystallin subunits are also affected and these changes are different for each subunit. The overall loss in protein is not readily explicable hi terms of a generalised leakage or a general tendency to insolubilisation. Possible mechanisms for the action of the gene are suggested.

Young mice of the strains A/Y and B10.D2 were repeatedly treated with diethylsulphate (DES) in different doses (36 to 1100 mg/kg). At the age of 9 weeks they were mated to females of the strains A. CA (‘A’ group) and C57BL/10Eg (‘D’ group) respectively. 2101 progeny of these matings were tested for histocompatibility by skin grafting. The spontaneous H-mutation rates were 6·96 × 10−4 per gamete in the A group and 9·6 × 10−4 per gamete in D group. In progeny of treated males the H-mutation rates were 0 in A group and 5·79 × 10−3 per gamete in D group, showing apparent effect of paternal DES treatment on mutation frequency in the last group. Two mutations of the H-2 locus were found, which together with the other three H-2 mutations published so far yielded a mutation rate of 5·18 × 10−4 per gamete. The mutation rate of the H-2 locus is higher than the expected rate per H-locus, indicating a great genetic complexity of H-2.

A more extensive linkage map of Pseudomonas aeruginosa strain PAO has been compiled from data obtained by both, conjugation and transduction procedures. All the markers examined are located on one linkage group and the available evidence suggests that the sex factor FP2 promotes transfer of the chromosome in a polarized manner from only one site on this linkage group.

Two new sublines of the C57BL/Gr strain of mice have been studied which were derived from earlier sublines in which no genetic variance could be demonstrated. The incidence of some 31 minor skeletal variants was examined which could thus go up or down. As new subline differences have arisen with about the same frequency and mean magnitude of effect as in the past, there is no doubt that, at least in the C57BL strain, subline differentiation is a continuing process. Its high frequency (about 0·01 changes per character at risk per generation) is difficult to reconcile with spontaneous mutation rates of single genes in the mouse. The possibility must thus be considered that some other self-perpetuating processes or entities of some degree of stability are handed down in the lines of descent in which they have arisen, or perhaps become unmasked. There is no reason to suppose that this type of event is confined to inbred strains (in which it can be demonstrated fairly easily); it presumably occurs similarly in mixed stocks in which it would scarcely be detectable.

tsd1–1 strains of Ustilago maydis are blocked in DNA synthesis under restrictive conditions. On recovery from the block, both intergenic and allelic recombination are enhanced, and aneuploids are generated. UV is synergistic in the induction of recombination. Direct assays of allelic recombination have shown that gene conversion occurs during the block in DNA synthesis.

A multienzyme complex for five of the enzymes in the prechorismate portion of the aromatic biosynthetic pathway has been demonstrated in Ustilago violacea, and has previously been reported in U. maydis (Ahmed & Giles, 1969). This complex is similar to that found in Neurospora crassa and other fungi. In U. violacea polyaromatic-requiring mutants show pleiotropic deficiencies for all five of these enzymes, similar to the extreme pleiotropic polar mutants of the arom gene cluster in Neurospora (Giles, Case, Partridge & Ahmed, 1967a; Case & Giles, 1971). This result is interpreted as mutational evidence for an arom gene cluster in U. violacea comparable to that in N. crassa. A second low molecular weight, heat-stable isozyme of dehydroquinase is shown to be present at high (constitutive) levels in U. maydis, as previously indicated by Ahmed & Giles (1969), but this activity is increased to extraordinarily high levels in cells grown in the presence of quinate. In contrast, U. violacea strains do not grow on quinate, have a single, heat-labile dehydroquinase species, and lack activities for other enzymes in the quinate catabolic pathway.

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.