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.

A variety of Klebsiella strains examined all show either a strong (ML+) or a weak (ML−/+) lactose-positive phenotype on MacConkey agar. ML+ Klebsiellae have about 10 times the β-galactosidase activity of ML−/+ strains in cultures both induced and not induced for this enzyme. Of 14 ML+ strains of diverse origin tested, at least 13 carry a lac operon on a plasmid which can be transferred to Escherichia coli. The seven plasmids so far studied in detail all belong to the F compatibility group but are unable to promote their own transfer. To explain these results it is suggested that the Klebsiella group derive from a common ancestor with a chromosomal lac operon of low efficiency, which was made good by the acquisition of a lac operon from another bacterial strain, probably E. coli: the new lac genes remained as a plasmid, possibly because they could not be integrated in the new host.

A new model of mutational production of alleles was proposed which may be appropriate to estimate the number of electrophoretically detectable alleles maintained in a finite population. The model assumes that the entire allelic states are expressed by integers (…, A−1, A0, A1, …) and that if an allele changes state by mutation the change occurs in such a way that it moves either one step in the positive direction or one step in the negative direction (see also Fig. 1). It was shown that for this model the ‘effective’ number of selectively neutral alleles maintained in a population of the effective size Ne under mutation rate υ per generation is given by

When 4Neυ is small, this differs little from the conventional formula by Kimura & Crow, i.e. ne = 1 + 4Neυ, but it gives a much smaller estimate than this when 4Neυ is large.

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.

Mutants resistant to growth inhibition have been obtained by selecting on media containing inhibitory concentrations of either d-glucosamine or l-sorbose. All of the mutants isolated were found to be sugar transport-defective and cross-resistant to growth inhibition by the three analogues 2-deoxy-d-glucose, d-glucosamine and l-sorbose. Like the mutants previously selected for resistance to 2-deoxy-d-glucose, the new mutants described are all shown to be alleles of the ftr cistron.

Strains homozygous for mutants affecting the length of the L4 vein in Drosophila melanogaster were selected for increased or decreased vein length. Substitution of chromosomes selected for their effect on one mutant into the genome of a non-homologous mutant has shown that many modifiers of mutant expression must be considered as acting generally on the character rather than affecting only a specific mutant.

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.

In Klebsiella aerogenes strain V9A melibiose and raffinose fail to induce the lac operon and strongly repress its induction by isopropyl-β-d-thiogalactopyranoside (IPTG). This repression is specific to the lac system and is not released by cAMP, which releases the catabolite repression exerted by glucose and sucrose. It is concluded that melibiose and raffinose bind to the lac repressor molecule in Klebsiella V9A, competitively displacing IPTG, and that the resulting complex is able to repress the lac operon. Other Klebsiella strains tested show the same repression phenomenon.

‘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.