Four crosses were made between bacteriophages T4 and partially non-excludable T2 exr+4. Each cross was designed as a four-factor cross: in total seven pairs of homologous genes were used as markers, extending over half the circular map from the locus for adsorption specificity (h+) along the map segment for the early functions to osmotic-shock sensitivity (o).

The frequencies of T2 markers recovered in the progeny show polarized exclusion: the frequencies are low for h+2 at the beginning of the map segment for the early functions and increasing to o+2. Recombination of T2 exr+4 is restricted for all loci concerned and the recombinational events are probably non-reciprocal. Restriction of sequential replication from a fixed starting point is proposed to explain polarized exclusion of T2 genes. Non-homology is dicussed in relation to restricted recombination.

1. The gene for Pintail (symbol Pt) shows no dominance. Heterozygotes are viable and fertile; many homozygotes die early in life, but the survivors appear to breed normally. Pt reinforces the effects of Sd and T on the tail; there is no evidence for interaction between Pt and + /vt.

2. The gene produces a shortening of the tail, but the most marked effect on the adult is in the great reduction of the nucleus pulposus of the intervertebral discs, which may lead to tail kinks in some animals.

3. Pintail embryos can be recognized from the eleventh day stage of development onwards by the possession of a much smaller notochord than normal. This reduced notochord is the result of a reduced rate of cell division which first manifests itself in 10-day embryos. Perhaps this reduced rate is the result of altered relations of the notochord with tail-gut and neural tube in 10-day embryos.

In yeast it has been reported that some types of erythromycin-resistance are due to cytoplasmic genetic factors, localized in mitochondria (Thomas & Wilkie, 1968; Slonimski, 1969). In view of the current interest in genetical aspects of mitochondria, and the ease with which cytoplasmic heredity can be demonstrated in Paramecium aurelia, some tests have been made to determine whether erythromycin-resistant variants could be obtained in this ciliate and, if so, to determine their genetic basis.

Puberty was studied in mice of the ninth selected generation of the Q-strain. There were 6 replicate lines selected for large body size (6-week weight), 6 replicates selected for small size and 6 replicate unselected controls. Female puberty was assessed by the opening of the vagina and male puberty by the first copulation plug. The sexes differed in the mean age at puberty, males being older by 13 days in the large, 4 days in the control and 8 days in the small lines. The sexes differed also in the way size affected puberty. In males the large and small lines reached puberty at the same age and both were older than the controls. In females the large lines on average were heavier and younger at puberty than the controls, and the small lines were lighter and older than the controls, though not significantly older. The replicates within each size-group, however, reached puberty at about the same weight, irrespective of their differences in growth rate. Thus, the differences of growth between the large, control and small groups affected both the weight and the age of females at puberty, but the differences of growth between the replicate lines within each size affected only the age at puberty. No explanation was found for this inconsistency between size-groups and replicates. Several lines of evidence led to the conclusion that in females puberty is partly or mainly weight-dependent, whereas in males it is almost wholly age-dependent.

Fixation probabilities and mean times to fixation of new mutant alleles in an isogenic population having an effect on a quantitative trait under truncation selection were computed using stochastic simulation. A variety of population structures and breeding systems were studied in order to find an optimal design for maximizing the fixation probability for recessive genes without impairing that for non-recessives or delaying times to fixation. Circular mating or cycles with repeated generations of close inbreeding alternating with combination of the families proved to be very inefficient. The most successful scheme found, considering fixation probabilities and times to fixation jointly, was to practise individual selection and mate full sibs whenever possible, otherwise mate at random. The benefit was directly proportional to the number of full-sib matings performed, which, in turn, almost exclusively depended on the number of selected individuals with very little effect of selection intensity or magnitude of gene effects. Fixation rates could be well approximated by diffusion methods. When selection was practised in only one sex and, therefore, the proportion of full-sib matings could be varied from zero to one, maximizing the amount of full-sib mating was found to maximize fixation probability, at least for single mutants.

Deterministic simulations have been carried out of populations under normalizing and disruptive selection for a trait determined by genes with additive effects at six loci. In some simulations a pair of alleles at a seventh locus determined the rate of recombination between the seven loci. Normalizing selection with a single optimum, fixed or fluctuating, invariably led to genetic homozygosity. If the optimum fluctuates widely, the approach to homozygosity may be accompanied by a large decline in the mean fitness of the population. Disruptive selection was simulated by having two ‘niches’ with separate optima and separate density-dependent regulation, but with the adult population mating randomly. If the optima are widely separated, this leads to stable polymorphism. Selection produced linkage disequilibrium, normalizing selection causing repulsion and disruptive selection coupling between + and − alleles. This linkage disequilibrium accelerates the phenotypic response to selection, but delays changes in gene frequency. Selection always favoured alleles for low recombination at the expense of alleles for high recombination.

Genetic variation in the amount of binding of dihydroalprenolol (a potent antagonist) to hepatocyte β-adrenergic receptors has been observed among inbred strains of mice. This variation is attributed to a differential effect of magnesium on the receptors between the high and low binding strains. Evidence for a single gene controlling the magnesium effect on dihydroalprenolol binding to β-adrenergic receptors was found using recombinant inbred lines between the high and low strains. We suggest the provisional gene symbol Badm.

By means of grafting skin in which the epidermis of one genotype was combined with dermis of another to a nude host, we have shown that the effect of the mutant at the tabby (Ta) locus on follicle initiation in tail skin was not confined to either the epidermis or the dermis, that the effect of Ta on the morphology of body hairs was determined by the epidermis but that the close proximity of epidermis of a different genotype also affected body hair morphology.

A study of the structural and functional organization of the rudimentary (r: 1–54·5) locus of Drosophila melanogaster has demonstrated that alleles of this gene reside in a number of recombinationally separable sites, and display a complex pattern of interallelic interaction. Data relating to interallelic interaction have been utilized to construct a linear complementation map consisting of 7 complementation units and 16 complementation groups. Comparison of the genetic fine structure map and the complementation map shows that the two maps are approximately co-linear. Totally non-complementing alleles reside at both ends of the fine structure map. The r locus is best interpreted by the model of a single cistron whose product affects several distinct developmental processes and whose alleles display a complex pattern of interallelic complementation. Intragenic recombination within the r locus is accompanied by the appearance of parental and recombinant flanking marker classes not expected on the basis of reciprocal recombination. Studies with half-tetrads demonstrate that intragenic recombination can occur either by gene conversion or by a reciprocal exchange mechanism. The pattern of organization seen at the r locus is similar to patterns of organization found in work with fungal genes.

Simultaneous selection by independent culling levels of two correlated traits in all four combinations of directions was investigated with Tribolium in a replicated experiment extending over nine generations. In addition to the two primary traits, 13-day larval weight and pupal weight, four secondary traits (pupation time, adult emergence time, adult weight and larval number) were observed.

The observed responses for both selected and unselected traits agreed with theoretical expectations after the latter were adjusted for changes which occurred in genetic and phenotypic parameters. Phenotypio variances for the selected traits were correlated positively with population means, yet genetic variances and heritabilities declined in all selected populations. No change was detected in the genetic correlation between selected traits even though the divergent two-trait selection was designed especially to ‘break’ the positive correlation of + 0·55 ± 0·12 present in the base population.

Striking changes in growth and developmental patterns resulting from the divergent selection were discussed in terms of metamorphic limits and ‘stabilizing’ genetic correlations.

The mutant doublefoot, Dbf, of the mouse arose spontaneously, and was shown to be inherited as an autosomal dominant, mapping 9–13 cM proximal to leaden, ln, on chromosome 1 and showing no recombination with the microsatellite markers D1Mit24 and D1Mit77. In heterozygotes the phenotype includes many extra toes on all four feet, and the tibia and fibula may be reduced and bowed. The head is shortened and broad and the eyes are held half-closed, and some animals develop hydrocephalus. The tail is kinked and abnormally thick, and the soles of the feet are swollen. Growth is retarded, viability is reduced, and reproduction is impaired in both sexes. Only about 30 % of males are normally fertile, and testis weights and sperm counts may be reduced, although this appears not to be the main cause of poor fertility. In females vaginal opening is delayed and oestrous cycles are irregular, although the animals appear to respond to gonadotrophic hormones. Crosses of Dbf/ + × Dbf/ + are very poorly fertile. Prenatally, Dbf/ + heterozygotes can first be recognized at 11½ days gestation by abnormally broad fore limb buds. Putative Dbf/Dbf homozygotes at 12½ days have similar limbs defects and also split face, due to failure of the maxillae to fuse in the midline. Some homozygotes and a few putative heterozygotes have cranioschisis. At 13½ days, the heads of homozygotes tend to bulge in the frontal region and a bleb of clear fluid is visible medially. At 14½ days Dbf/Dbf fetuses may have oedema and some are dead. From 15½ days onwards no live Dbf/Dbf fetuses have been found. The gene maps close to the locus of Pax3, but crossovers between Dbf and Pax3 have been found, ruling out the possibility that a gain-of-function mutation in Pax3 might be involved.

The association of α and β2 subunits of tryptophan synthetase from Escherichia coli and Salmonella typhimurium in homologous and heterologous combinations was studied by sucrose density gradient centri-fugation. Under conditions allowing for optimal association of subunits derived from the same source, subunit association in the mixture E. coli α–S. typhimurium β2 was weaker than normal while in the reciprocal combination of S. typhimurium α–E. coli β2 it was tighter than normal.

These observations suggest that a certain degree of binding between the α and β2 subunits of tryptophan synthetase could have had a selective advantage during the evolutionary divergence of the species of Entero-bacteriaceae, so that a mutation leading to the substitution of an amino acid involved in α–β2 association in one of the subunits could have been compensated by a mutation in the complementary one.

We have studied genetic variation at 27 loci in 42 samples from natural populations of a neotropical species, Drosophila equinoxialis, using standard techniques of starch-gel electrophoresis to detect allelic variation in genes coding for enzymes. There is considerarle genetic variability in D. equinoxialis. We have found allelic variation in each of the 27 loci, although not in every population. On the average, 71% of the loci are polymorphic – that is, the most common allele has a frequency no greater than 0·95 – in a given population. An individual is heterozygous on the average at 21·8% of its loci.

The amount of genetic variation fluctuates widely from locus to locus. At the Mdh-2 locus arout 1% of the individuals are heterozygotes; at the other extreme more than 56% of the individuals are heterozygous at the Est-3. At any given locus the configuration of allelic frequencies is strikingly similar from locality to locality. At each and every locus the same allele is generally the most common throughout the distribution of the species. Yet differences in gene frequencies occur between localities. The pattern of genetic variation is incompatible with the hypothesis that the variation is adaptively neutral. Genetic variation in D. equinoxialis is maintained by balancing natural selection.

The amount and pattern of genetic variation is similar in D. equinoxialis and its sibling species, D. willistoni. Yet the two species are genetically very different. Different sets of alleles occur at nearly 40% of the loci.