To test whether large changes in the enzyme levels of segmentally aneuploid Drosophila melanogaster can be ascribed to changes in kinetic properties of the enzymes affected, comparisons have been made with regard to the heat stability, substrate concentration dependency, and the presence of heat-stable inhibitors or activators within the extracts of aneuploid and control flies. By these criteria, no differences were found between controls and the α-GPDH activity of flies trisomic for chromosome II segments 27D–31E, 35A–40, 41–45F, and 57B–60F and no differences were evident between the IDH properties of 70CD–71B aneuploids and their controls. The enzyme changes observed in these aneuploids are more likely associated with changes in the rates of accumulation of the enzyme molecules. The IDH of flies trisomic for the 27D–31E region was more heat-stable than that of controls while the α-GPDH of flies trisomic for the 21A–25CD region displayed an apparent Michaelis constant for α-glycerophosphate lower than that of controls. The possible bases for these latter qualitative distinctions are discussed.

Four surface exclusion systems have been identified amongst a group of F-like plasmids in E. coli: SfxI(F), SfxII (ColV2 and R538-1fin−), SfxIII (ColVBtrp and R1-19) and SfxIV (R100-1 and R136fin−). None of these systems was expressed in stationary phase cells or, except for ColVBtrp, during fin+ transfer inhibition, showing that the surface exclusion gene(s) is usually co-controlled with the transfer genes.

Recipient cells carrying two plasmids specifying different surface exclusion systems did not always express both of these: the overall pattern suggested that the four systems and/or their sites of action are related. There was no surface exclusion between donor cells carrying two plasmids determining different surface exclusion systems and recipient cells carrying a plasmid determining either one of these. Our hypothesis to explain this and other results is that surface exclusion prevents interaction between the tip of the pilus on the donor cell and a receptor site on the recipient cell surface. Pili (probably mixed) with two types of tips would be present on cells carrying two different plasmids, the one unresponsive to the single surface exclusion system of the recipient cells allowing transfer of both plasmids.

The hairpin-tail allele (Thp) of brachyury (T) on chromosome 17 of the mouse is unique in that the phenotype of heterozygous offspring depends on which parent contributed the Thp.The present paper offers formal proof of this fact using the th2 allele of T as a marker, and discusses possible modes of action of the hairpin allele.

The tetracycline-resistance determinants of R-factors from different compatibility groups have been tested in Escherichia coli K12 and phenotypically classified into two major classes. Class I determinants confer high-level resistance to tetracycline (> 100 μg/ml) and moderate resistance to minocycline (5–25 μg/ml) while those of Class II gave moderate resistance to tetracycline (50–70 μg/ml) and low resistance to minocycline. Each class was subdivided because of variation in resistance profiles and in the abilities of tetracycline and minocycline to induce increased resistance. Strains carrying two compatible Tetr R-factors of the same or different phenotypic groups did not show increased tetracycline resistance.

The melanotic tumour gene tu bw of Drosophila melanogaster has a specific suppressor su-tu. The genotypes tu bw; +su-tu and tu bw; su-tu show opposite responses, as measured by tumour penetrance, to increasing choline levels in the defined axenic medium. The three major metabolic functions of choline have been examined using additions to the axenic medium to determine which biochemical pathway(s) are different in the two genotypes. It is concluded that the opposite strain responses are due to changes in the pattern of phospholipid synthesis, and that the gene product of the su-tu gene probably functions in this area of metabolism.

A genetic analysis following the initial detection of a female sterile variant resulted in the finding of seven or eight different female sterile mutants, most or all of which are on linkage groups I and II. They were present in heterokaryotic condition in already existing strains, except one which originated spontaneously during the study. All mutants fail to produce functional protoperithecia. Most of them, however, are able to function as female parents in heterokaryons. All mutants differ morphologically from the wild type, most being subtly different, but two being appreciably different. The apparently high frequency of occurrence of female sterile mutants suggests that protoperithecial development is under an elaborate genetic control. Differences in vegetative morphology appear to be a common property of mutants affecting the early stages of the sexual development.

The distribution of heterochromatic regions in the chromosomes of diploid, tetraploid and hexaploid wheat shows that the B genome possesses characteristic large blocks. Though analyses of probable B genome donors indicate that Aegilops speltoides has a pattern of distribution of heterochromatin nearest to the B genome chromosomes, a polyphyletic origin of tetraploid wheat seems more plausible.

This paper provides direct evidence for the occurrence of mitotic crossing-over in leaf tissues and in vitro cell cultures of Nicotiana tabacum. The geno-typic composition of both segregating regions of twin spots was analysed by in vitro culture of the segregating tissue and by regenerating plants from this tissue. The frequency of mitotic crossing-over is influenced by several physical and chemical agents.