A haploid strain of Saccharomyces cerevisiae has been described which, on glucose medium, segregates vegetatively a high frequency of mutants with different degrees of respiratory impairment. The range of mutants seemingly encompasses both non-revertible ρ- petites and revertible point mutations resembling leaky mit- mutations. The segregants have aberrant cytochrome contents and reduced growth capabilities on fermentable sugars other than glucose; these defects apparently correlate with the degree of respiratory impairment. Genetic analysis of this mutator phenomenon has implicated a nuclear gene which appears to show specificity of interaction with the mitochondrial genome as well as a requirement for glucose repression. The mutator effect seems to extend also to the loci in mitochondrial DNA for resistance to the antibiotics erythromycin and oligomycin.

Systematic crosses between various strains of Drosophila melanogaster lead in some cases to partly sterile F1 females (SF females). Two main classes of strain, inducer and reactive, have been denned on the basis of this sterility, which shows very specific physiological features. SF females arise only when reactive females are crossed with inducer males. In contrast, F1 females (RSF) produced by the reciprocal cross between inducer females and reactive males have normal fertility. All wild populations tested are of the inducer category, laboratory strains are either inducer or reactive. Sterility is the result of interaction between two genetic factors denoted I and R, respectively responsible for the inducer and reactive conditions and whose unusual genetic behaviour has been described in other papers. The present paper reports experiments showing that the I–R interaction is also responsible for high levels of X nondisjunction and of mutation in the SF female germ-line. The analogy with the P-M system of Kidwell, Kidwell & Sved (1977b), is discussed as are also the implications of the existence of the I-R system for spontaneous mutation research in D. melanogaster.

Ten glycosidases were measured in suspensions of spermatozoa from the vasa deferentia of two inbred mouse strains and their intercrosses. Eight of these glycosidases were associated with the sperm cells and all of these showed genetical variation between the strains except α-l-fucosidase with optimal activity at pH 5·4. In contrast liver enzyme activities showed no significant variation except α-l-fucosidase. Genetic studies indicated that the variation of spermatozoal β-d-hexosaminidase, α-d-mannosidase, α-l-fucosidase and β-d-galactosidase are inherited at autosomal loci and α-d-galactosidase variation shows X-linked inheritance. We propose a new provisional gene symbol (Afuc-2) for a spermatozoal variant of α-l-fucosidase.

A new type of genetic control of gene conversion is described from the Pasadena strains of the fungus Ascobolus immersus. It is characterized by cis/trans position effects and incomplete dominance. The P, K and 91 factors segregated from each other like Mendelian alleles and controlled the conversion frequencies and patterns of four nearby, closely-linked white ascospore colour mutations, although they did not usually coconvert with these w sites. Mutations of different origin responded similarly to the same control factors.

These control factors greatly affected the total conversion frequencies, the relative frequencies of the different detected conversion classes and various other conversion parameters. The detailed results are consistent either with P, K and 91 affecting both the frequency of hybrid DNA formation and the correction processes for removing mispaired bases, or if they do not affect the correction processes directly, then they must have large effects on the frequency of asymmetrical hybrid DNA formation, which must usually be much more common than symmetric hybrid DNA, and there must be both an inequality in the frequency with which the two homologous chromosomes (in these crosses, + bearing and w bearing) invade each other, and in the frequency with which the two strands of each chromatid invade the homologue in asymmetric hybrid DNA formation.

The gene (dw) causing hypopituitary dwarfism was transferred by repeated backcrosses into strains of mice differing genetically in growth-rate through previous selection. The dwarf mice lack growth hormone, and the purpose was to find out if the differences in growth-rate between the strains were in any part due to differences in their growth hormone status – amount of hormone or tissue sensitivity. In the absence of growth hormone, i.e. in the dwarfs, the strains still grew at different rates, proving that growth hormone status was not the only cause of their differences. The effect of substituting the dw gene was, however, greater in the large strain than in the unselected control, and less in the small strain than in the control. The growth differences between the strains were therefore in part due to growth hormone. Tissue sensitivity in the three strains was compared by their responses to graded doses of exogenous growth hormone. The large and control strains did not differ, but the small strain had a lower sensitivity. The results suggest that the increased growth-rate of the large strain is partly due to an increased amount, or activity, of its circulating growth hormone, while the reduced growth-rate of the small strain is in part due to a reduced sensitivity of its target organs to growth hormone.

The accumulation of deleterious mutations in a finite diploid selfing population is investigated. It is shown that the conditions for accumulation are very similar to those for the accumulation of mutations in an asexual population by ‘Muller's ratchet’. The ratchet is likely to operate in both types of population if there is a large class of slightly deleterious mutations.

Sperm from mice heterozygous (t/+) for many t-lethal haplotypes show segregation distortion. More offspring inherit the t-haplotype than the normal wild haplotype. In contrast males heterozygous (tx/tv) for two t-lethal haplotypes are sterile although they produce normal-looking sperm. The experiments were undertaken to show whether sperm from tx/tv mice are the same as t-fertile sperm from t/+. Female albino mice were persuaded to mate first with coloured sterile tx/tv heterozygotes and then with normal white +/+ males. With the exception of one litter (from a t-heterozygous male which proved to have been misidentified) all the progeny were derived from the mating with the normal father. We conclude that t-bearing sperm from t-lethal (tx/tv) heterozygotes are inviable, and different from t-sperm in t/+ heterozygotes.

The herbicides Bentazone and Phenmedipharm kill the leaves of intact tobacco plants but do not affect callus cultures. Tolerant mutants were isolated by treating leaves of previously γ-irradiated haploid plants with herbicide then excising and culturing the green herbicide-resistant cell clones on the otherwise yellowed leaves. Among plants subsequently regenerated were a total of ten stable independently isolated mutants. Sexual crosses show these ten represent four Bentazone and two Phenmedipharm loci; all mutants were recessive to wild type.

Fertilized eggs were transferred reciprocally between large and small mothers, to study maternal effects on body weight in mice selected for large and small size, respectively. Prenatal maternal effects were not important in our material, but postnatal maternal effects were detectable. The postnatal effects accrued mostly from the inadequacy of small mothers for large offspring; small offspring were largely unaffected by the type of mother. Genetic maternal effects were only of limited importance; maternal influences of environmental origin arose from variation in litter size.

A new locus for choline-requirement has been identified in Aspergillus nidulans and designated choC3. It has been located to linkage group VIII at a position 31·9 ± 2·2 map units to the left of the sD locus.

It is proposed that this locus codes either for the enzyme that catalyses the transmethylation of phosphatidyl monomethylaminoethanol to phosphatidyl dimethylaminoethanol or for a molecule essential for the synthesis or function of this enzyme.

The proportions of the two isozyme bands of the X-linked form of phosphoglycerate kinase (PGK-1) were compared in 16 tissues from four groups of adult heterozygous females. Little evidence was found for differences in expression of the two isozymes among tissues but there was a marked difference among the four groups of mice. The proportion of the PGK-1B enzyme was consistently lower in PGK-1AB heterozygous daughters of C3H/HeHa females than in corresponding heterozygotes with a C57BL/6Ha, DBA/2Ha or JBT/Jd mother. This difference was also observed in foetuses on the fourteenth day of gestation irrespective of whether the C3H/HeHa X chromosome was derived from the mother or the father. Sequential sampling of blood from the same heterozygous females provided no evidence for genetically determined cell selection in the adult erythropoietic tissue. The observed differences probably reflect variation at an X-chromosome controlling element locus among inbred strains of mice, similar to that described by Cattanach & Williams (1972) using X-linked morphological markers, although this has yet to be tested.

Non-nodulating mutants of Rhizobium trifolii were found in cultures incubated at 35 °C for 7 days. From among ten strains incubated at the elevated temperature six became non-nodulating. The frequency of Nod− mutants in R. trifolii cultures varied from 1 to 75% depending on the strain tested. It is suggested that gene(s) controlling nodulation are located on a large plasmid.