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 genetic relationship between germspores and parental mycelia has been investigated in crosses involving multiply-marked heterokaryons and in simple difactor crosses.

It is concluded that for each zygospore the number of different diploid nuclei undergoing meiosis is often more than one, but generally not more than two. The meiotic products divide to produce the nuclei in the usually uninucleate germspore primordia and then continue dividing to produce germspores. Unmated nuclei do not pass directly from parents to progeny.

Each germsporangium contains differing numbers of viable germspores, with an average of about 7000; different genotypes are represented by varied numbers of germspores and often expected genotypes are missing. When the results of several germsporangia are added together or germspore stocks from many germsporangia are analysed, reciprocal genotypes appear in equal numbers and reliable recombination frequencies may be calculated.

Two strains of poultry selected for high growth-rate were found to have an anomalous lens morphology indicating a failure of the normal process of growth regulation. The implications for lens fibre differentiation are discussed, as are the implications for genetic selection for growth-rate.

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.

Eleven mutants of R-factor R57 have been isolated which show constitutive expression of resistance to tetracycline (Tc). These derepressed (Tdr) mutants all gave a much greater resistance to Tc and to its analogue, minocycline, than could be obtained by optimal induction of cells carrying the wild-type (T+) determinant. Cells carrying each of the Tdr mutants together with T+ of either R6-S or of a plasmid found in Escherichia coli mi19 showed inducible Tc resistance, indicating that the Tdr mutants were all recessive, i.e. of repressor-negative type. Tdr1 was not recessive to the T-determinant of RP1, suggesting that the repressor gene products of the T-determinants in R57 and RP1 have different specificities.

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.

Mouse chimaeras made by aggregating embryos homozygous for nine recessive genes with embryos carrying the corresponding dominants were mated with partners of the recessive strain. Progeny were either of the recessive type for all loci examined, or showed the dominant characters at all loci. Tail length in homozygous vestigial-tail progeny was unaffected by whether the fertilizing spermatozoon had undergone maturation in a chimaera or in a control vestigial-tail male. There was thus no indication that the germ cells had been in any way modified by their intimate association with germ cells and somatic tissue of contrasting genotype in the chimaeras.

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.

A diffusion model was developed to investigate the effect of a mutant substitution by natural selection on heterozygosity at a linked neutral locus. Using this theory, we made extensive numerical analyses to compute the expected total heterozygosity (i.e. the sum of the fraction of heterozygotes over all generations until fixation or loss) at the neutral locus. It was shown that the hitch-hiking effect is generally unimportant as a mechanism for reducing heterozygosity. The effect becomes significant only when the recombination fraction between the selected and the neutral marker loci is smaller than the selection coefficient. In order to check the validity of the mathematical theory, Monte Carlo experiments were performed, and the results were in agreement. It has been suggested that linkage is important only in transient small populations such as at the time of speciation.

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.

The R factor, R144drd3, mobilized chromosomal genes in Klebsiella pneumoniae strain M5a1 at similar frequencies to those observed for this R factor in Escherichia coli K12. In a derivative K. pneumoniae donor, strain HF3, R144 persisted in the autonomous state but now gave rise to polarized transfer of the chromosome in the order O his…arg 2…leu…trp. R144drd3 was unstable in K. pneumoniae M5a1; after successive subculture drug resistance, colicinogeny and transmissibility were lost. In strain HF3, transmissibility was preferentially lost, but determinants for drug resistance, colicinogeny and superinfection immunity were retained; 10–11 copies per chromosome of R144 were present in log phase cultures of both strains.

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.