The direct and correlated response to selection of scutellar microchaetae and scutellar bristles has been analysed by determining the contribution of the three major chromosomes, alone and in combination with each other, to the overall response. The results of the analysis confirm a previous finding, based on a formal statistical approach, that response to selection for microchaetae had highly pleiotropic effects on scutellar bristles. In lines selected, each for high and low microchaetae, genetic changes in the 2nd and 3rd chromosomes are pre-eminent and essentially equal. Inter-chromosomal interactions are of relatively minor importance in interpreting the response to selection for microchaetae but assume greater importance with respect to the correlated character. The results are discussed in terms of the genetic correlation between fitness and the character measured.

The results of a previous paper on the effect of optimizing selection, mutation and drift on a metric character determined by a large number of loci have been extended to include the possibility that, in addition to selection for an optimal value, there may be independent selection in favour of heterozygotes; it is assumed for simplicity that at each locus the heterozygote has the same advantage, s, over each of the homozygotes. Under selection alone there is a stable equilibrium if s > ca2, where c is a measure of the intensity of the optimizing selection and a is the effect of a gene substitution. Under the additional forces exerted by mutation and by drift due to finite population size each locus behaves independently of the other loci as if it had a heterozygous advantage equal to (s − ca2).

Two new types of mutants of the FP2 sex factor have been isolated in males of P. aeruginosa strain PAT. Males harbouring FPd mutants are unable to mediate either sex factor or host chromosome transfer, although they retain the exclusion and precipitation characteristics of wild-type males. Males harbouring the FPs mutant apparently have an altered cell surface as indicated by their loss of precipitability, and although their donor properties are similar to those of wild-type males they show a slightly reduced conjugal fertility.

A previously described sex factor mutant FP* (Stanisich & Holloway, 1972) can be transferred to males carrying either the FPs or FPd factors, and the heterozygous strains produced show restoration of wild-type properties, i.e. wild-type precipitation characteristics of the FPs mutants, and transferability of FPd mutants respectively. This suggests that the mutations of the FPs and FPd factors are recessive to the alleles carried by the FP* factor. The ability to produce such heterozygous strains supports the view that at least two copies of the FP2 factor occur in strain PAT males.

In two-way selection for growth rate in the fungus Schizophyllum commune in two environments (20 °C and 30 °C) response for low growth rate was always greater than for high. This asymmetry was due to inbreeding depression combined with a greater selection differential associated with the low lines in the high temperature environment. Despite the high level of inbreeding applied, two low selection lines derived from the same source (parental isolate 2) showed no decrease in genetic variance during selection. This maintenance of genetic variance was associated with a decrease in stability of development relative to the corresponding high selection lines.

The mutant phenotype Abnormal abdomen is under the control of a major gene, A53g located distally on the X chromosome. The phenotypic abnormalities are a result of the developmental interaction between this major gene and a modifier system associated with the residual genotype. The primary developmental effect of this mutant genotype is an interference with adult histoblast differentiation resulting in a raggedness or loss of tergite material due to the changes in the formation of the adult abdominal hypoderm. A secondary effect is an interference with middorsal fusion of the histoblasts.

The developmental effect of the genotype is influenced by a number of factors which include crowding, humidity, temperature, and the age of the culture. The mutant phenotype is due to an interaction of the major gene and the modifier system in association with these environmental factors. Gene action in relation to the final phenotype is postulated to be a two-step affair. The first is an increase in protein synthesis under the influence of the modifier system; and the second is the reaction of the histoblast, under the influence of the major gene, to this increase in protein synthesis during its differentiation into adult hypoderm. The function of the environment in this sequence of developmental reactions is postulated to be in its control of the utilization of the increased protein associated with the action of the enhancer genes. This hypothesis is discussed in terms of (1) the production and utilization of gene products and of (2) the regulation of development in terms of this production and utilization in a balanced developmental system.

An experiment was carried out to test whether two laboratory cage populations of Drosophila melanogaster from different origins (Kaduna and Pacific) differed in the genes for sternopleural bristle number. The means, variances and heritabilities of the two populations and the synthetic formed from crosses between them were very similar.

Selection for low bristle number was practised in small replicate lines, six of each pure population and nine of the synthetic. On average, Pacific responded to selection rather more rapidly than either Kaduna or the synthetic, but there was little difference in the limit achieved.

Crosses between replicates within populations were made and selection continued, and these lines subsequently crossed between populations and reselected. Additional response was obtained by this procedure but the crosses between the replicates of the pure and synthetic populations attained similar selection limits.

An analysis of effects of individual chromosomes from the selected lines on bristle number indicated that the contribution of each chromosome to total response was about the same in Pacific, Kaduna and the synthetic.

It is concluded that differences in gene frequency, rather than the presence or absence of particular alleles, are mainly responsible for the differences observed between the populations.

Crosses were made between four populations of Drosophila melanogaster – three of which were laboratory populations (Kaduna, Pacific and Canberra) and one recently captured (Stellenbosch) – and a line previously selected for low sternopleural bristle number for many generations from a Kaduna/Pacific source. In each of six replicate lines from each cross selection was practised for low sternopleural bristle number, and subsequently these replicates were intercrossed and reselected.

Initially, similar responses were made in each set of lines, but subsequently more variation between replicates was found in Stellenbosch, which was the primary source of lines which responded to a level below that of the original selected line.

It is concluded that this newly captured population contains genetic variability absent from the laboratory populations. Presumably variability has been lost from the latter populations, leaving essentially the same genes segregating in each.

Platyfish, Xiphophorus maculatus, are polymorphic for the patterns red-dorsal (Dr) and spotted-dorsal (Sd) fins, both controlled by closely linked loci on the X chromosome of Jamapa strain, Jp 163A. The intensity of red pigment looks the same in males and females, but spectrophotometric analysis of dorsal fin extracts showed that heterozygous intact males have significantly more red pigment (drosopterin) than homozygous or heterozygous females or castrated males. The mechanism of Dr expression in Jamapa is, thus, similar to the one present in the Belize stock, where a sex difference is readily apparent that is known to be under androgenic control. The Sd phenotype is identical in both sexes. Sd and Dr are not restricted to the X chromosome, and no evidence for gene dosage compensation has been obtained. Within the Jamapa stock the expression of Sd and Dr are best described in terms of dominance and recessiveness. Dr is strongly augmented by a testicular hormone. Dr and Sd have been separated by crossing-over. In natural populations both genes can occur by themselves, linked to each other or to other pigment genes. The development of the Sd macromelanophores is not contingent upon the presence of pterinophores (Dr) in the dorsal fin or elsewhere in the body.

Bare-patches (Bpa), a semi-dominant, sex-linked, male-lethal gene in the mouse is described. It shows 14·5 ± 3·0% recombination with Blo and 5·4 ± 2·2 % with Ta and may, therefore, be allelic with Str.

Only about half the expected number of heterozygous females is observed; the remainder appear to lose their mutant chromosome and become XO females as there is an excess of non-Bpa females, at least 24% of which proved to be XO. It is not yet certain whether the lost X chromosome is always the mutant one nor is it certain whether the loss is a property of the Bpa gene or due to a closely linked factor: a separate factor seems more likely. Some possible mechanisms for XO production are discussed.

Two strains of S. commune characterized by different levels of high recombination frequency in a particular region of the genome (between the two subunits, α and β, of the B incompatibility factor) were crossed, and their progeny tested for recombination frequency in the same region. The difference between the strains in recombination frequency is found to be due to some factor located within the recombining region itself.

The segregation among the progeny indicates that the factor consists of a number of sites, with additive effects. This and the dominance relationships suggest that these sites may be recognition sites which comprise a part of the fine control of recombination.

‘Wider ratio’ octads (8:0, 0:8, 7:1 and 1:7) regularly occurred in wild-type(+) × white ascospore(w) crosses of the Pasadena strains of Ascobolus. Control crosses showed that phenocopies and false octad clusters were absent or rare; no reversion from w to + occurred, but mutation from + to w was found at a number of loci, with nearly all 0+:8w and many 2+:6w octads in + × w crosses arising from mutation, not conversion. Nearly all 8+:0w, 7+:1w and 6+:2w octads appeared to arise by conversion.

The finding of genuine wider ratio octads implies hybrid-DNA formation at corresponding sites in both pairs of non-sister chromatids in the same bivalent, which conflicts with models of the synaptinemal complex requiring that only two of the four chromatids pair intimately at any point. Octad types arising from hybrid-DNA formation at corresponding sites in both pairs of non-sister chromatids were described and formulae were derived for their frequencies. The lack of genuine wider ratio octads in several other Ascobolus studies was shown to be explicable quantitatively in terms of their conversion frequencies.

‘Corresponding-site interference’ is defined as interference between the two pairs of non-sister chromatids of a bivalent in hybrid-DNA formation at exactly corresponding sites. Formulae based on observed octad frequencies were derived for calculating coincidence values for this kind of interference. Corresponding-site interference was found to be weak, with coincidence values differing between crosses with high and with low conversion frequencies.

Evidence is given for a trifactorial system of incompatibility in an isolate identified as Psathyrella coprobia. The three factors are designated A, B and C. They are thought to be inherited independently. The function of factor A is most probably the same as that of factor A of bifactorial species. Factor B is concerned with the initiation of fruit body primordia, while all three factors must be heterozygous for the occurrence of nucleár migration and the formation of mature fruit bodies.