It is assumed that a character under stabilizing selection is determined genetically by n linked, mutable loci with additive effects and a range of many possible allelic effects at each locus. A general qualitative feature of such systems is that the genetic variance for the character is independent of the linkage map of the loci, provided linkage is not very tight. A particular detailed model shows that certain aspects of the genetic system are moulded by stabilizing selection while others are selectively neutral. With reference to experimental data on characters of Drosophila flies, maize, and mice, it is concluded that large amounts of genetic variation can be maintained by mutation in polygenic characters even when there is strong stabilizing selection. The properties of the model are compared with those of heterotic models with linked loci.

Forty-five temperature-sensitive mutants of Aspergillus nidulans which are defective in nuclear division, septation or distribution of nuclei along the mycelium have been isolated, and most have been subjected to complementation analysis and mapped to chromosome. Thirty-five of the mutants were unable to complete nuclear division at the restrictive temperature. Twenty-six of these mutants exhibited a co-ordinate drop in both spindle and chromosome mitotic indices at 42 °C, indicating that they fail to enter mitosis. These mutants have been assigned to the gene symbol nim. Nine mutants exhibited a co-ordinate rise in spindle and chromosome mitotic indices at 42 °C, indicating that they are arrested in mitosis. These mutants were assigned the gene symbol bim. Five mutants failed to form septa and were given the gene symbol sep; and five mutants had an abnormal nuclear distribution and were given the gene symbol nud. All of the mutations were recessive. Most of the mutants were in different complementation groups. Mutants in the same complementation groups were phenotypically similar, but phenotypically similar mutants were not necessarily or usually in the same complementation group. There was no evidence for genetic clustering of phenotypically similar mutants. The mutants were located on all eight chromosomes.

Using a series of Hfr and F prime deletion strains, the F transfer inhibition gene finP has been mapped between ori and traJ. Marker rescue experiments with the Hfr deletion strains further showed that traO, the site of action of the transfer inhibitor, is located immediately to the left of, or possibly within, traJ, and that the polar mutation tra-4 lies in traK.

This paper describes selective differences imposed by environmental ethanol on six genotypes at the alcohol dehydrogenase (Adh) locus in Drosophila melanogaster. Probit analyses were used to relate differences between the percentage survival of adults of different Adh genotypes to ethanol concentration. Regression analyses were used to relate differences between the pre-adult developmental times of different Adh genotypes to ethanol concentration. The directions of differences between some of the genotypes were found to differ in these two components of fitness. The differences in developmental time are linearly related to the differences in the in vitro alcohol dehydrogenase activity expressed by these genotypes. Percentage survival differences amongst adults are not linearly related to these differences in enzymic activity. The development of AdhF AdhF pre-adults is retarded the least on ethanol impregnated media but AdhFAdhS adults are most likely to survive on such media.

During meiosis the two parts of a tandem duplication are able to pair in a double loop instead of pairing with the corresponding region of the homologous chromosome. The frequency of intrachromosomal exchange within this double loop was measured in heterozygous females of tandem duplication Dp(1; 1)Gr by the phenotypes of the exceptional F1 males. The intrachromosomal exchange frequency is increased significantly by both the ‘interchromosomal effect’ of heterozygous inversions in the autosomes and by a double inversion in the homologous X chromosome. Distribution of the exchange events depends on the pairing situation and its frequency within the double loop. The analysis of clusters of intrachromosomal recombinants observed favours the assumption that this exchange type is exclusively a meiotic event.

A high frequency of chromosomal non-disjunction occurs spontaneously in mice heterozygous for some Robertsonian translocations. If animals heterozygous for the translocation and homozygous for different alleles of a marker gene are mated together a few young homozygous for the marker arise through non-disjunction, and their frequency can be used as a measure. This method has been used with the Robertsonian translocation Rb(9.19)163H and the marker ruby ru (chr. 19); Rb(4.6)-2Bnr with brown (b) and misty (m) (chr. 4); and Rb(9.14)6Bnr with hairless (hr) and piebald (s) (chr. 14) respectively. The frequencies of marked young were: Rb163 0/5260 ruru; Rb2 21/1997 mm bb; and Rb6 19/1702 hrhr ss, and the corresponding calculated non-disjunction frequencies in each arm of the translocation were Rb163, <5 %; Rb2, 15%; Rb6, 15%. These figures show reasonably good agreement with values obtained by other methods. A search for genetic or environmental factors affecting the frequency of marked young in Rb2 and Rb6 revealed that in Rb2 the frequency increased with maternal age, whereas in Rb6 the maternal age of the marked young was non-significantly below that of the total progeny. The reasons for this discrepancy are not clear.

Partially dominant mutations to carboxin resistance occur in three, freely recombining, nuclear genes in Aspergillus nidulans. Mutations at all three loci reduce carboxin inhibition of succinate dehydrogenase (EC 1.3.99.1), succinate-cytochrome c reductase (EC 1.3.99.1) and succinate oxidase (EC 1.3.99.1) in mitochondrial preparations. It is therefore probable that the ability of carboxin to prevent growth of A. nidulans is a direct consequence of its ability to prevent succinate oxidation.

The pattern of inheritance of the autosomal recessive adrenal lipid depletion (ald) allele among the AKXL recombinant inbred mouse lines, derived from a cross between AKR/J and C57L/J, revealed that the ald locus is located on chromosome 1 near the dipeptidase-1 (Dip-1) locus. The linkage was confirmed in a backcross generation and the gene order ln-Dip-1-ald was established.

In view of the exceptional usefulness of Drosophila hydei for the analysis of Y-chromosome activity, a technique has been developed which permits the production of large numbers of males lacking a Y chromosome. It is based on the synthesis of new compound(1) chromosomes which carry sufficient rDNA genes to secure survival in the absence of any Y-chromosomal rDNA. Through non-disjunction in males of suitable stocks, fertile Compound(l) females which lack the Y chromosome, phenotypically distinguishable from their normal sibs, are produced in sufficient number to allow the subsequent breeding of X/O males in gram quantities for biochemical studies.

The relative frequency of retention of two mitochondrial loci, determining resistance to oligomycin (oli1) and erythromycin (ery1), has been analysed in petite (rho(−)) mutants derived from a number of unrelated strains of Saccharomyces cerevisiae. The frequency of retention of one marker relative to the other in spontaneous petites showed marked variation dependent on the strain of origin. The differences between strains in this characteristic were shown to be mitochondrially determined. Further, for individual strains, the relative retention of the markers in petites derived after UV-irradiation varied considerably in several cases from that observed with spontaneous petites. The observations on relative marker retention and the varied effects of UV-irradiation are discussed in terms of possible structural differences in the mitochondrial genomes of the various strains.