Epistatic models are examined for conditions which result in a negative covariance between a sire's purebred and test-cross progenies. It is found that heterozygote superiority is not a necessary condition for its occurrence. The frequencies of the same allele in the selected population and in the tester should be divergent. The implications of using the covariance in prediction are discussed. The important result to note is that selection in the crossbred may not be optimum even with a negative covariance.

Bacteriophage εγ is capable of transduction both by replacement of a genetic segment of the recipient by the homologous genetic material from the donor strain and by the formation of defective transducing particles capable of lysogenizing the recipient strain of S. anatum.

The isolation of strains carrying such prophages, which have incorporated the lactose or arabinose operons, is reported. Lysogenic strains, carrying both normal and defective transducing prophage, form high-frequency transducing lysates. Other strains, carrying only defective prophage, show evidence that the association of prophage genes and transduced materials is stable since the loss of one frequently entails loss of the other.

A bacteriophage was isolated from a strain of Salmonella manilla. This phage, υ15, was found to have adsorption and conversion properties similar to phage ε15. The two phages show similar transduction efficiencies but phage υ15 lysogenizes more efficiently. Phage υ15 is not subject to repression by ε15 type prophage. The densities of the two phages are indistinguishable.

Phage υ15 can ‘cure’ ε15 type prophages and recombination is possible between genomes of the two phages. It seems probable that they are heteroimmune representatives of a single bacteriophage family.

Salmonella anatum lysogenized by a doubly mutant strain of bacteriophage εγ shows a markedly rough colonial phenotype on tetrazolium agar. Mutants that abolish the roughness are reported.

In the fission yeast Schizosaccharomyces pombe, growth responses of mutants strains in the de novo purine synthesis pathway, in the purine interconversion system, and of various double mutants, have been studied upon different purine-supplemented media. The results show that exogenous purine utilization as nucleotide source is based exclusively upon pyrophosphorylation of purine bases, and they make it possible to identify most of the enzymic steps acting, in this organism, upon a purine ring to give another purine ring.

A scheme of the interconversion system in Schizosaccharomyces pombe is given.

A class of abnormal sex-ratio (SR) condition in Drosophila characterized by the elimination of male zygotes is caused by infection with maternally transmitted SR-spirochaetes. A given spirochaete strain, WSR, was shown to have an associated DNA virus, spv-2, in a latent condition. This virus is induced to multiply when another DNA virus, previously designated spv-1, infects WSR spirochaetes; spv-1 is normally associated with another strain of SR-spirochaetes, NSR, and is in a manifest state. Spv-2 infects and lyses NSR, thus eliminating spirochaetes from fly hosts which, in these experiments, were D. melanogaster.

The elimination of WSR through lysis by spv-1 results in the ‘curing’ of the SR-condition in the host flies. However, the elimination of NSR by spv-2 does not lead to the immediate elimination of the SR-condition in the host. A hypothesis is presented in favour of the view that a substance, androcidin, is produced by SR-spirochaetes and that this substance is actually responsible for the death of male zygotes. NSR may produce a more potent androcidin than WSR.

Isoenzymes of alcohol (ADH), malate (MDH), glutamate (GDH) and isocitrate (IDH) dehydrogenases, and a fast migrating esterase (EST-l) were separated by disk electrophoresis from dry seed extracts of diploid, tetraploid, hexaploid and octaploid species or amphiploids belonging to the subtribe Triticinae. Only ADH and EST-l isoenzymes showed inter-species variation; the other dehydrogenases, which show stringent substrate specificities (‘critical’ enzymes), revealed the same pattern in all diploid and polyploid species. The qualitative zymogram studies showed that (1) the number of variant enzyme bands increased with the level of ploidy, (2) the amphiploid isoenzyme pattern was additive of the parental species, (3) enhancement in the number of bands was due to the presence of not only parental bands, but also hybrid bands formed by association between heteromonomers. Quantitative data were obtained by densitometry of the enzyme bands as well as spectrophotometric measurements of enzyme activity in crude extracts. Increase in the level of enzyme activity was observed with ploidy level. In spite of the evidence that all duplicate/triplicate genes are expressed, increased enzyme activity observed in the polyploid species was not proportional to the level of ploidy or expected gene dosage. On the basis of ADH and EST-l zymograms obtained in 2 × and 4 × wheat, probable zymograms for these enzymes in the B-genome donor to 4 × wheat were extrapolated. Neither Ae. speltoides nor Ae. bicornis showed the extrapolated ADH pattern. Amphiploids involving Ae. speltoides and Triticum monococcum or T. aegilopoides fully reproduced the EST-l zymogram of 4 × wheat, but not the ADH. Ae. bicornis × T. aegilopoides amphiploid showed an ADH zymogram similar to that of 4 × wheat, but the EST-l bands were different.

The contribution of maternal and foetal genotype to the regulation of extra-embryonic (exocoelomic and amniotic) fluid and embryonic fluid in the mouse is discussed. Three of the four inbred strains studied (A, BALB/c and CBA) had similar patterns of fluid accumulation. The fourth (C57BL) differed significantly. In the reciprocal F1 crosses between CBA and C57BL a maternal effect was present. In the F2 no increase in variance of extra-embryonic fluid was noted.

The effects of two mutant genes were studied. Blebs (my) had no effect on the total fluid in the conceptus, but led to a rearrangement of fluid within the embryo. Brachyphalangy (Xtbph) caused an overall increase in fluid within the conceptus. Distribution of the excess depended upon the phenotype (exencephalic versus non-exencephalic) of the embryo.

Considering a single mutant gene in a geographically subdivided finite population, it is shown that the average number of heterozygotes, due to this gene, that appear in the population before fixation or loss of this gene is equal to twice the total population number. This is invariant under the geographical structure of the population.

Allelic recombination frequency and pattern in fungi show strong gene-to-gene variation. At least in two fungal species, Neurospora and Schizophyllum, recombination frequencies in certain chromosomal segments are genetically determined. The possibility that chromosomal proteins and/or synaptinemal complexes may play an important role in regulation of recombination frequency and pattern is discussed.

Twenty-five population cages of D. melanogaster were set up, each containing a different wild-type second chromosome and the marker chromosome Cy. In all but one case where contamination apparently occurred, the Cy chromosome persisted in the population at high frequency, showing a selective advantage of Cy/ + heterozygotes over wild-type homozygotes. Overall, the results indicate that homozygosity of the entire second chromosome causes a depression in fitness of the order of 85%.

The structure and function of flagella are genetically determined and single gene mutants – for example, lacking in motility or of abnormal flagellar length – have previously been investigated. When such mutants are crossed with wild-type, temporary dikaryons – prozygotes – are formed with two nuclei and a common cytoplasm. The properties of the four flagella – two originally abnormal – have been observed as a function of time. In wild-type × non-motile mutant crosses, restoration of motility has been observed in a number of cases. If the dikaryons are deflagellated regeneration occurs, together with restoration of motility or of normal length to the previously abnormal pair. Complementation at the cytoplasmic level has been found in paired mutants.

It has been possible to examine the correlation between the presence of phage-like particles and R bodies by using serial sections of kappas of stock 562. No exception has been found to the rule that 562 kappa particles with R bodies also have phage-like particles; however, 3 out of 78 kappa particles with phage-like particles lacked R bodies.

The E.M.S.-induced auxotrophic mutant 1308 of Drosophila is located between 13·7 and 21·0 on chromosome 1. Survival of larvae is possible if whole RNA is provided in the diet, and this is shown to be a requirement for a source of both purines and pyrimidines. A proportion of larvae will grow on pyrimidine nucleosides alone. Tests for utilization of bases and of precursors suggest that 1308 is a double mutant, and not a single mutation preventing the synthesis of a common precursor of purines and pyrimidines.