The genetics of two esterase systems in adults of the olive fruit fly Dacus oleae (Gmel.) have been studied by starch gel electrophoresis. Two unlinked autosomal genes, each with multiple alleles, control the synthesis of a number of isozymes. Gene A controls the synthesis of an acetylcholinesterase, and gene B that of a pseudocholinesterase or lipase. Gene B produces hybrid enzyme in heterozygotes for active alleles. In crosses the segregation of seven active and one silent allele in gene A, and of six active and one silent allele in gene B, have been studied. Six more active alleles for gene A and five active alleles of gene B have not been studied in this paper.

Following segregation in two unrelated wheat crosses, albinism was associated with chromosome 7B nullisomy which permitted expression of mutant genes on chromosomes other than 7B. The frequencies of albino segregante differed in the two instances, being dependent upon the morphology of the 7B monosome, which was either telocentric 7BL or normal.

A viridis phenotype also segregated from one cross. Its expression was independent of 7B aneuploidy. Failure to obtain lines segregating only for green and viridis phenotypes was due to semi-lethality resulting from the presence of two 7B isochromosomes.

Metaphase trivalent frequencies from a number of new chromosomal constitutions (various combinations of maize chromosome 2-Tripsacum homeologue primary and secondary interchanges) are presented and compared to previous findings from similar material. It is suggested than an underlying direct relationship may exist between extent of gentic map present in duplicate and triplicate and expectation from a sharing of map probability of crossing-over among all homologues. Theoretical considerations are discussed.

A method has been developed which will permit the mapping of the genetic material of Pseudomonas aeruginosa. This has been made possible by the use of potassium nitrate in the mating medium and other modifications which have increased the frequency of recombination. In certain circumstances, the ilvA12 marker may be transferred by more than 1% of the donor cells.

Ten markers have been investigated for their times of entry into auxotrophic mutants of P. aeruginosa strain 1. Using this information and unselected marker analysis two linkage groups have been demonstrated. The results could be accounted for by assuming that there were two chromosomes or that a third linkage group was involved, but we cannot distinguish between these two hypotheses at the present time.

A pedigree analysis of several cell-colony generations following a mutagenic treatment with nitrous acid has shown that in S. pombe a genetic instability is produced that replicates several times and produces a mutation in independent lines.

It has been shown that the mutants isolated in the progeny of a mosaic colony all contain a genetic alteration that cannot be resolved by genetic analysis and therefore the mutations have occurred at the same genetic site. This finding is confirmed by interallelic complementation and phenotypic analyses.

Strain Bs8 is a u.v.-sensitive derivative of strain B. It is unable to reactivate irradiated phage (HCR) and it cannot be induced to form filaments. The HCR properties are attributable to a gene, uvr8, cotransducible with gal+. When uvr8 was transduced into radiation-resistant strain B/r the resulting phenotype was indistinguishable from Bs8. When transduced into a lon K-12 strain the phenotype was more sensitive than Bs8, filament-inducible and mucoid. When PI-Bs8 was used to transduce proC+ into a proC K-12 strain, 4% of the transductants were lon, i.e. about as u.v.-sensitive as Ion K-12, filament-inducible and mucoid. Radiation resistance could not be transduced from strain Bs8 with proC+ into a proC lon K-12 derivative. Nor did Bs8 show any evidence of being exr. Bs8 is a double mutant of strain B, behaving as though it were HCE, in a B/r background.

An additional Y-chromosome in either male or female did not have an influence on the mutagenic action of formaldehyde–food. It is concluded that the lack of response of female germs cells to this treatment is not due to lack of a Y-chromosome, and that the specific response of spermatocytes among germ cells is probably not due to the striking activity of the Y in these cells.

Kershner's (1949) observation that the sex-linked lethal frequency in X-rayed males was significantly reduced by an additional Y-chromosome could not be confirmed.

1. The occurrence of fusion between plasmodia produced from amoebal clones of P. polycephalum was studied.

2. The occurrence of fusion was found to be strain-dependent and the factors responsible segregated in the progeny of a cross.

3. The segregations found in crosses between several strains led to the conclusion that four alleles (f1–f4) of one gene f were controlling fusion in these strains.

4. Fusion occurs only between plasmodia carrying identical f alleles, except in one class of results.

5. A model accommodating all the results, including the ‘exceptional’ class, is proposed. It requires that the action of the f factors is to inhibit fusion between dissimilar strains rather than to promote fusion between identical strains. Certain physiological deductions from this model are discussed.

6. The locus (mt) determining mating type of the amoebae is not concerned in plasmodial fusion and is unlinked to f.

7. The rate of fusion between some pairs of strains is apparently influenced by modifying genes.

8. It is suggested that, as a result of the operation of the f gene and of the previously described killing reaction, heterokaryons will occur rarely in natural populations of P. polycephalum.

Plaque-forming transducing (pt) particles of bacteriophage ø 80 have been used to transduce genes governing tryptophan synthesis. Main interest was devoted to strains which carry deletions extending for varying distances into the tryptophan (trp) operon, covering trp A and ending in trp B, trp C or further. If the deletion does not over-reach trp C, infection by pt particles carrying the trp (CBA)+ segment gives rise to stable partial diploids which express the complete tryptophan independence. If the deletion is further extended, transductants are not entirely complemented and appear only on medium containing indole. If the recipients do not carry any prophage in the attachment site of ø 80 (att80) the indole-requiring transductants are very unstable; if the recipients do carry such prophage, even defective, all transductants are very stable. Stable indole-requiring diploids can be superinfected by other pt phages carrying the complementary genes of the tryptophan operon, giving rise to supertransduced indole-independent bacteria. Transduction to complete independence of tryptophan or any precursor has also been achieved in three steps. Supertransductants yield after u.v. induction pt phages carrying the initial trp segments and segregate rare cells with incomplete ability to synthesize tryptophan. They are therefore polylysogenic polyploid heterogenotes carrying overlapping segments of the trp operon linked to prophage genomes.

When recipients are carrying a prophage in att80, transductants appear at a low frequency. The reduction is particularly pronounced with ø 80 pt transducing segment trp (CBA)+ to immune bacteria.

Different types of chimerism result from double fertilization. Differences result from fertilization of first and second polar bodies and from early fusion of two zygotes. These three types of chimerism have different phenotypes or different proportions of possible phenotypes, and variation is greater in respect of gene loci close to the centromere. The phenotype frequencies in the different types of chimerism are tabulated and the phenotype frequencies expected in different mating types are shown. Studies of such cases offer the only method of autosomal gene localization relative to the centromere at present available in man.

A strain of Ustilago maydis has been isolated, after treatment with nitrosoguanidine, which lacks an extracellular endonuclease capable of causing two strand breaks in DNA. Genetic analysis suggests that the strain carries two mutations and is probably also deficient in a second nuclease.

Genotypic and phenotypic changes of HCM in P. aeruginosa are due to a variety of causes. In addition to interstrain genetic differences and the semi-permanent effects of growth at 43°C, mutations affecting both restriction and modification have been isolated by direct selection and through the pleiotropic effects of p–fluorophenylalanine resistance mutations. It is suggested that some of these changes affecting HCM may be taking place through alterations of the ribosomes.