We have studied the association of the ten possible B blood-group genotypes with mortality during incubation in the 1966 generation of two relatively noninbred populations of Light Sussex chickens. These were related substrains 6D and 6F derived in the 1964 generation by equal division of strain 6, which we had previously studied in its 1962 generation. The B blood-group genotypes of zygotes but not of dams were associated with marked mortality differences in 6F, while in 6D almost no differences were found. Comparisons of the findings in 6D and 6F with the results previously published for the ancestor strain 6, after adjustment for overall between-population differences, showed marked changes between 6 and 6F in the mortalities associated with the five most frequent genotypes. In four of these, the mortalities in 6D were about half way between those in 6 and 6F. Taken together, these results provide evidence that the selective pressures on the B locus from differential embryonic mortality vary from generation to generation and even between populations differing apparently only slightly in environment and genetic composition.

In progeny of crosses between females from 6D and 6F with males from 5D and 5F respectively, the latter being two related Rhode Island Red/New Hampshire substrains, embryonic mortality was much lower than in the pure matings within 6D and 6F, and only a few associations with B blood-group genotypes of zygotes were found.

The results are discussed in relation to the general lack of association of polyallelic polymorphisms with aspects of fitness reported by other workers; and the importance is stressed of studying associations of genotypes rather than genes or serological factors.

An analysis of populations of Drosophila melanogaster which had ceased to respond to selection for increased scutellar bristle number has disclosed the existence of an overall negative genetic correlation between replicate lines in the frequency of the two major component bristle types, viz. anteriors and posteriors. Negative phenotypic correlations among component bristle sites have also been detected within populations. A model involving competition among sites for the available resources of a particular limiting substrate is therefore proposed. Genetic changes have been effected in exceptional populations which lead either to an increase in the rate of production of the limiting substrate, or to a lowering of the concentration of the substrate necessary for bristle initiation. The allelic substitutions concerned are recessive, and have been described as decanalizing alleles in view of their effects on both bristle number variability and a measure of developmental regulation at individual anterior sites. Genetic variation has also been demonstrated for the mean allocation of resources to each of the four component bristle types, viz. anteriore interstitials, posteriors and apicals. A brief discussion is given of the implications of the model for breeding practice in domestic species.

The functional morphology and the replication pattern of the male X-chromosome in an autosome-X insertion stock (T(1;3) 05) of Drosophila melanogaster have been examined. In larval salivary glands carrying this insertion neither the enlargement and pale staining of the single male X, nor the characteristic early completion of replication cycle, as revealed by 3H-TdR autoradiography is in any way changed. The normal properties of the inserted autosomal segment are also unaltered. The results appear to support a ‘piecemeal’ type of dosage compensation mechanism in Drosophila operating through the male.

A new spontaneous mutant bubble (bu) with a marked effect on early morphogenesis in the multicellular green alga, Ulva mutabilis is described. It behaves as a recessive chromosomal mutant. The bu+ gene product formed in heterozygous sporophytes exerts a predetermining effect on morphogenesis of haploid gametophytes with the bu genotype. It is suggested that the product of the bu+ gene affects the orientation of the mitotic spindles during early development.

Translocation T(III–VIII) in Aspergillus nidulans has been analysed by the detection of meiotic linkage between markers previously located separately on linkage groups III and VIII. The breakage points have been mapped by the detection of linkage between the crinkled type and genetic markers in the region of the break. A segment from linkage group III, approximately 43 units long and including the markers moC96, sC12, sA1 and cnxH3, has been translocated into linkage group VIII. The breakage point is between su6proA and moC96 and the attachment point is close to cha in linkage group VIII. It seems probable that the segment has been inserted into linkage group VIII.

The F factor and the K resistance determinant form an R factor (KF) in E. coli K12. In this R factor the two plasmids are independent of each other and occupy different attachment sites in the host cell.

Treatment of an F+(K) strain with acriflavine and acridine orange yielded the following classes: F+K−; F−K+; and F−K−. The K determinant was thus curable independently of F, which supports the hypothesis of independence of the two plasmids. In Hfr(K) strains, K remains curable with acridines while F is immune. Thus, K remains independent of F in both the F+ and Hfr states.

The use of higher concentrations of acridine orange than those needed for cure of K in the F+(K) strain resulted in no elimination of K, although the F factor was absent from all surviving lines tested. It is suggested that this is caused by equal inhibition of replication of both the chromosome and K, and greater inhibition of that of F.

A new type of apparatus for replica plating is described, which reduces operator error and fatigue. Several replica plates can be made immediately, thus eliminating the need for a master plate in many cases.

Sarcina flava ATCC 540 (ade) and Micrococcus flavocyaneus ATCC 8673 (ade), two related micrococci, were transformed to prototrophy at frequencies as high as 0·02% and 0·005% of colony-forming units, respectively. Both of these organisms were transformed by selected prototrophic strains of Micrococcus lysodeikticus, M. flavocyaneus, S. flava and Sarcina lutea.

The mutagenic effects of feeding calf-thymus DNA to larvae of Drosophila melanogaster have been studied on second chromosome recessive lethals, sex-linked dominant and recessive visibles and dominant autosomal visible mutations. While a positional specificity of DNA-induced lethals has been confirmed, no evidence has been obtained for a preponderance of visible mutations or for a specificity of phenotype among the visibles.

Three new autosomal mutants of the milkweed bug, Oncopeltus, obtained by treatment with ethylmethane sulphonate, are described and compared with the two other previously known variants. Preliminary data are given on the results of testcrosses and of chromatographic analysis of some pteridene pigments.

A genetic fine-structure experiment on Drosophila melanogaster is described, which makes use of a lethal selective system which permits survival only of recombinants in the ma-l region, including exchanges between site mutants of ma-l. On the basis of prior mapping information, the experiment was designed to select for ma-l double mutant recombinants. Utilizing a pair of complementing ma-l mutants, double mutant meiotic products were recovered whose complementation properties provide independent support for the single-cistron, allele complementation model of organization of the maroon-like region inferred from other experiments described earlier.