Six hundred and ten gynandromorphs were produced in which an X chromosome loss uncovered the vermilion mutation. The mosaic patterns observed indicate that wild type ocelli are incapable of kynurenine production and that, in addition to the eyes, postembryonic kynurenine producing cells originate from two separate regions of the blastoderm. The positions of these regions on the genetic fate map of Drosophila melanogaster correspond to the embryonic precursors which give rise to the kynurenine producing cells of the larval fat body and Malpighian tubes.

Crossover frequencies within three overlapping heterozygous paracentric inversions of differing length in the long arm of chromosome 1 of maize were estimated from cytological data. These frequencies were found to be close to normal for the regions involved and directly proportional to the respective physical lengths of these inversions. It is suggested that if pairs of centres which are specialized to regulate homologous pairing of the region between them exist in this part of the maize genome, they are most probably numerous and widely distributed.

Twenty-three exotic strains of four species of Neurospora (N. crassa, N. sitophila, N. intermedia and N. tetrasperma) were grown in minimal medium and their enzyme phenotypes were compared. The exotic strains did not differ substantially with respect to the specific activities of the biosynthetic enzymes, ornithine transcarbamylase (OTC), carbamyl phosphokinase and aspartate transcarbamylase. Exotic strains of N. crassa and N. sitophila were crossed with laboratory mutants of N. crassa to determine whether there were significant differences among the exotic strains in the gene complexes underlying their enzyme phenotypes. Most of the exotic strains carried similar OTC structural genes and similar genetic modifiers of OTC activity: their OTC structural genes were expressed normally in the genomes of the parent strains, in the alien genomes of standard laboratory mutants and in the mixed genomes of parent × mutant hybrids. One exotic N. crassa carried a distinctive OTC structural gene that elevated OTC activity when transferred into the genome of a standard laboratory mutant. A second exotic N. crassa carried distinctive genetic modifiers of OTC activity: two mutations, c- and hi, interacted in the exotic parent to normalize OTC activity and rate of growth, but hi led to extremely high OTC activities and to a reduction in rate of growth when separated from c- by interstrain recombination.

Following the transfer to the common pigeon (Columba livia) of a cellular antigen (Cg) from C. guinea, and to Streptopelia risoria of respective members of the group-8 system from chinensis and senegalensis, new combinations of these two antigenic systems were produced from matings of livia and risoria. The hybrid substances associated with the cells of each of the five new combinations of the cellular antigens were related to each other and to the usual F1-livia/risoria cells but, with the possible exception of two, were definitely differentiated from each other. Cross-reactivities of the reagents for the hybrid substances were also observed, primarily with heterozygotes of the group-8 system of Streptopelia. Evidence is thus provided that the interaction takes place primarily if not entirely between the genes of the C-system of Columba and the group-8 system of Streptopelia.

Virus-like particles can be seen in sections of kappa of stocks 7 and 562. The virus-like particles are specifically associated with R bodies and are rare or absent in kappa particles lacking R bodies. This observation clearly links the virus-like particles with kappa's toxic activity; it also supports the suggestion that kappa is infected with a lysogenic virus whose induction results in the production of the R body and viruses. Many ‘unfilled’ virus-like particles are present in whole kappa particles. Wound R bodies of stock 7 are surrounded by a sheath, whose breakage may provide the mechanism which causes the unrolling of the R body. One edge of the R body ribbon stains more intensely than the other.

Variability in fertility in compound chromosome (CC) strains of Lucilia cuprina (Wiedemann) is directly correlated with the segregation properties of the CCs in those strains. In compound fifth-chromosomes, high fertility (up to 50% ) is associated with high frequencies of segregation of the left element (C(5L)) from the right element (C(5R)) in males, while low fertility (up to 25%) is associated with random assortment of the CCs in males. Regular segregation of C(5L) and C(5R) chromosomes in males is associated with particular C(5L) elements, which contain cytologically visible duplications of 5R chromatin. These duplications may contain a site which is normally involved in the pairing of the fifth chromosomes in males.

In females the CCs segregate from one another with a frequency of 89%. None of the C(5R) elements studied affected segregation in males, and neither C(5L) nor C(5R) elements had any demonstrable effect on segregation in females. Transmission of certain CC elements through one or both sexes is significantly lower than expected. This could have a variety of causes, such as meiotic drive, inviability associated with position effects, or homozygosis of deleterious mutations on homologous CC arms during meiotic crossing over in females. The CC strains released during a field trial of genetic control contained both high-fertility and low-fertility C(5L) chromosomes. Both types of CC were subsequently recovered from individuals which had overwintered in the field. Over a period of several generations following this recolonization one particular high-fertility C(5L) chromosome increased in frequency to near-fixation, despite the presence in this chromosome of deleterious mutations.

Linkage data and revised maps for 52 autosomal loci in L. cuprina are presented. Examination of the linkage relationships of biochemically and morphologically similar mutations in L. cuprina, Musca domestica L. and Drosophila melanogaster (Mg) suggests that the major linkage groups have survived largely intact during the evolution of the higher Diptera.

Breeding tests of wild house mice, trapped from an isolated population from Sanday in the Orkney Islands, have demonstrated the presence of a semilethal t-haplotype designated tw106. Microscopic examination of sperm and testes from a sterile male obtained from this population revealed the histological characteristics typical for homozygotes for semilethal t-haplotypes. This report is the first description of the recovery of a t-haplotype from an island population of wild mice.

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.

Tetrad data from five-point crosses involving linkage group I (mating-type chromosome) and three-point crosses involving linkage groups VI and VII of Neurospora crassa and N. sitophila have been analysed in order to detect the phenomena of chromatid and chiasma interference on a comparative basis. Marker genes were transferred from N. crassa to N. sitophila by hybridization and repeated back crossing. The details of the methods of transfer and of making crosses have been described in a previous paper (Scott-Emuakpor, 1965).

The two clones produced by the two members of a conjugating pair are normally of the same mating type, about 50% of such synclones being mating type I, 50% type II. This ratio suggests (1) that mating type is determined by a pair of alleles at one locus or by a pair of mating type determining chromosomes, and (2) that one mating type is homozygous, the other heterozygous or hemizygous. Mating type ratios are unaffected by temperature or parental age. Exceptions are of three types: (1) mates that produce one clone of type I, the other of type II, (2) mates which fuse permanently and yield a clone pure for either mating I or mating type II, and (3) conjugation of three mates one of which is regularly non-viable, the two survivors producing either two clones of type I, two clones of type II or one of each type. The bases of these exceptional results remain obscure.

The pattern of changes of the genetic covariance between two characters on selection was examined in an effort to explain the asymmetry of correlated responses in two traits, or of the same trait in two environments, frequently observed in experimental results.

The algebraic conclusions were further examined by model selection experiments using a computer. The computer was programmed to calculate the change in gene frequency from generation to generation and to calculate from it the expected changes in genetic variances and covariance as selection proceeded. This procedure was carried out with several models of gene effects and gene frequencies.

Asymmetry of the genetic covariance, and consequently of the correlated responses, resulted when the relative change in gene frequency at the loci contributing positively and negatively to the covariance depended on the trait selected. The conditions necessary for the development of asymmetry were examined and the results suggest that any symmetry found in an experiment is perhaps more surprising than asymmetry. Probably the most frequent contribution to asymmetry in practice will be from loci contributing negatively to the covariance and having frequencies other than 0·5.

Accurate prediction of correlated response over many generations is therefore not possible without prior knowledge of the composition of the genetic covariance, as well as its magnitude. The validity of existing theory for the prediction of correlated responses is likely to be much poorer than for the prediction of direct responses. Predictions would then have to be based on the genetic parameters estimated in each generation.

We constructed a cloning vector (pMT603) based on the low copy number plasmid SCP2*. pMT6O3 is unstable because it lacks the SCP2* stability region and carries the selectable marker thiostrepton-resistance and a tyrosinase gene which results in melanin production. This allows easy testing of plasmid stability and we demonstrated its usefulness by cloning a plasmid stability function.

Experiments on Drosophila suggest that genetic recombination may result in lowered fitness of progeny (a ‘recombination load’). This has been interpreted as evidence either for a direct effect of recombination on fitness, or for the maintenance of linkage disequilibria by epistatic selection. Here we show that such a recombination load is to be expected even if selection favours increased genetic recombination. This is because of the fact that, although a modifier may suffer an immediate loss of fitness if it increases recombination, it eventually becomes associated with a higher additive genetic variance in fitness, which allows a faster response to directionselection. This argument applies to mutation-selection balance with synergistic epistasis, directional selection on quantitative traits, and ectopic exchange among transposable elements. Further experiments are needed to determine whether the selection against recombination due to trie immediate load is outweighed by the increased additive variance in fitness produced by recombination.