1. The DNA contents of twenty-eight different species and forms of Chrysanthemum have been measured by photometry. It is shown that there are large differences in DNA content between some species with identical chromosome numbers.

2. The DNA contents of natural polyploids are frequently not those expected when comparison is made with diploid forms of the same species. The DNA contents of induced polyploids are those expected.

3. Chromosome length and volume are positively correlated with DNA content.

4. The relationship between chromosome number, chromosome size, DNA content and gene number is considered, and it is suggested that the differences in DNA content may result from the presence of differing amounts of genetically inactive DNA in the chromosomes.

A method is described for quantifying the phenotypic expression of eyeless using gauged estimates of the mean and standard deviation of the distribution of facet numbers.

Gene-environment interactions of four eyeless alleles ey, ey2ey4 and eyK are compared in different genetic backgrounds and on a standardized Pacific background. The original strains differ in mean phenotypic value and in direction of response to variations in the ribonucleic acid content of the larval culture medium, whereas the four Pacific strains are more concordant. Consequently these differences are in part attributable to modifier genes present in the different genetic backgrounds of the original strains. Modifier genes also influence the periods in development at which eyeless is sensitive to nutritional treatments.

On the standardized Pacific background deficiencies of cholesterol cause an increase, whereas deficiencies of RNA or thiamine cause a decrease in the expressvity of all four eyeless alleles, but non-identity of their interaction profiles over an array of different suboptimal environments suggest that they may form a group of heteroalleles.

It is already well established in Escherichia coli that certain genes, or groups of genes, can behave as episomes—that is, they can undergo a free transition between the fully extrachromosomal state and one in which they are integrated in the bacterial chromosome (Jacob & Wollman, 1961; Broda, Beckwith & Scaife, 1964). Although this type of behaviour has been detected commonly among members of the enteric bacteria, it has never hitherto been demonstrated clearly in a Gram-positive species. This communication shows that a reversible transition of certain genes between the extrachromosomal and the integrated state can occur in some strains of Staphylococcus aureus at least.

Brachyphalangy (Xt/bph) is an allele of extra-toes (Xt) in linkage group XIV of the mouse. The phenotypes of both heterozygote and homozygote are distinguishable from, but similar to, those of extra-toes.

A small fraction of mutagen treated X chromosomes which would appear nonlethal in the Muller-5 test carried out on a yeast-containing medium act as lethals when tested on one or other of the defined Drosophila media. Such mutants have been termed ‘nutritional conditionals’. The three mutants obtained in this way have been characterized genetically, but have not yet been shown to act as simple auxotrophs.

An attempt is described to produce chromosome translocations in Glossina austeni which, it is hoped, might ultimately be used for tsetse fly control. The method consisted of selecting among the progeny of irradiated males for cases of inherited partial sterility. Thirty-four per cent of the testable sons of males which received doses of 5–7 krads showed these properties. In two cases the inheritance was patrilineal which suggests that the Y chromosome was involved. In most of the other cases a segregation among both male and female progeny of the partially sterile and normal types occurred and in these cases it appears that autosomes only were involved. Among males in these stocks the segregation ratio was close to 1:1, but in females there was a deficit of the partially sterile type. This may be partly associated with the fact that a large proportion of totally sterile and in-viable females were produced by these stocks. The proportion of zygotes which died at the embryonic, larval and pupal stages as a result of the action of the partial sterility factors varied between factors of different mutational origin. In view of the pattern of inheritance of these factors and their high frequency of induction it is argued that in all probability they are translocations in the heterozygous state.

A large number of mutants that affect the morphology of the ascus of Neurospora crassa has been isolated by means of a screening technique that utilizes the filtration principle to separate colonial type mutants from wild type. In relationships with each other and with wild-type alleles, the mutant genes exhibit zygote dominance and recessiveness. A zygote complementation test for placing the recessive mutants into functional groupings is described. Results obtained by use of the test place the mutants into seven functional groups. Dominance relations in the developing ascus of Neurospora are discussed.

A comparison of the gene-environment interactions of the eyg mutant in two different genetic backgrounds clearly demonstrates that the properties of the genetic background play a major role in the control of the gene-environment interactions of this mutant. Similarly, modifier background is important in the determination of the sensitive stages in eye development to a cholesterol-deficient diet.

The phenotypic identity of the eyeless and eye-gone mutants suggests a close underlying metabolic and developmental relationship. Possible inter-relations of these two mutant genes are discussed in the light of their gene-environment interactions in a standardized genotype.

Colicin factors Ib-P9 and Ela-16 both convert their hosts to chromosomal donors. However, negative fluctuation tests suggest that chromosomal transfer does not result from the formation of stable Hfr variants.

Attached-X stocks appear to be deficient in a small region, probably heterochromatic, which is normally covered by an homologous region on the short arm of the Y chromosome. There is an homologous deficiency in the ec dx stock Y chromosome. When this Ys deficiency occurs in crosses involving attached-X stocks, the matroclinous females are not recovered.

The extent and phenotype of acriflavin-resistant rII deletions have been examined. The properties of these deletions confirm that acriflavin resistance may result from a loss of function at the ac locus and that the ac locus coincides with the rII distal portion of the dispensable region which is adjacent to the rIIB cistron.