The high XO production in Bpa+ mouse stocks appears to be due, not to Bpa itself, but to a closely linked factor for high XO production (Fxo) which may be a gene or a chromosomal aberration. The change of Bpa Fxo to Bpa non-Fxo occurs with a frequency of about 6%; the reciprocal change from Bpa Fxo to Fxo non-Bpa has not been found. There is some evidence that Fxo might involve a structural alteration such as an inversion or a deletion. The X-chromosome loss is due to non-disjunction which occurs in about one-third of the oocytes, probably mainly at meiosis I. Frequencies of other chromosomal abnormalities found in the stocks are given.

Matings of Drosophila pseudoobscura were observed in Elens-Wattiaux chambers, using individuals of three karyotypes: AR/AR, AR/CH and CH/CH. In each chamber two karyotypes were represented, with frequencies 10:10, 2:18 or 18:2. The males of the heterokaryotype AR/CH tend to have an advantage in mating compared to the homokaryotypes, provided that both kinds of males are equally frequent, i.e. the ratio 10:10. This advantage is further increased when the heterokaryotype is a minority, the ratio 2:18. When the homokaryotype, AR/AR or CH/CH, is a minority (18:2) it is equally or more successful than the heterokaryotype in securing mates. Among females, the mating success is independent of frequency, or the minority females have sometimes only a slight advantage.

Sterility resulting from treatment of male rats with methyl ethane sulphonate and ethyl methane sulphonate was found to be due to the induction of dominant lethal mutations in spermatozoa and spermatids. Spermatids in the acrosome phase of development were found to be the most sensitive cells and at low doses of both compounds the majority of deaths occurred after implantation. The earlier cell stages, i.e. spermatogenia and spermatocytes, were the most sensitive to isopropyl methane sulphonate and Busulphan (Myleran). The action on spermatogonia was confirmed by spermatid counts and the time of occurrence of oligospermia or aspermia was calculated. Pre-implantation deaths occurred mainly in the early cleavage stages after both isopropyl methane sulphonate and Busulphan; at least part of these losses can be attributed to the induction of dominant lethal mutations in spermatocytes and early spermatids. Isopropyl methane sulphonate appears to be the more effective of the two drugs in this respect.

Mutations in the structural genes for triosephosphate isomerase and glyceraldehyde-3-phosphate dehydrogenase activity in the mouse, selected after mutagen treatment, were used to estimate the map distance between the two loci. It is shown that Tpi-1 and Gapd are closely linked on chromosome 6, with a recombination frequency of 0·1±0·1%.

The isolation of single inheritance units affecting body size in mice has been attempted. By using Large and Small mice as the parental strains, a breeding scheme has been carried out by repeated backcrossing to each strain with selection for both large and small body sizes in each backcross. The selections were started from the second backcross generations. The Small backcross was carried to the seventh generation and the Large to the fifth. Based on analysis of the means and variances for the parental strains and the backcross generations, it is tentatively concluded that a small number of, if not single, inheritance units may have been introduced from the Large to the Small mice. The ‘large’ genes appear to be dominant over the ‘small’ genes.

The position and orientation of an interchange quadrivalent in flattened, lateral views of metaphase I were studied in pollen-mother-cells of an interchange heterozygote of Allium triquetrum. The quadrivalent is most often located in marginal rather than central positions in the equator. Moreover, when positioned marginally the quadrivalent is more often than expected found in adjacent orientation, whilst when positioned centrally it is more often than expected found in alternate orientation. Consequently, the frequency of alternate (genetically balanced) orientation in the quadrivalent varies sharply according to whether data are obtained from more marginal or more central positions in the metaphase plate.

Some features of gene conversion in fungi and their bearing on the hybrid DNA models are discussed. Available experimental data from tetrad analysis seem to give a more complex picture of polarity in intra-genic recombination and of the relations between conversion and post-meiotic segregation, and between conversion and crossing-over, than predicted by the models.

A new hypothesis of the mechanism of gene conversion with special attention given to the aspect of asymmetry in this phenomenon is proposed as an alternative to the mechanism suggested by the DNA hybrid models.

Mutants resistant to growth inhibition have been obtained by selecting on media containing inhibitory concentrations of either d-glucosamine or l-sorbose. All of the mutants isolated were found to be sugar transport-defective and cross-resistant to growth inhibition by the three analogues 2-deoxy-d-glucose, d-glucosamine and l-sorbose. Like the mutants previously selected for resistance to 2-deoxy-d-glucose, the new mutants described are all shown to be alleles of the ftr cistron.

Homozygotes for recessive visible genes have often been discovered in lines under artificial selection, sometimes many generations from the start. As a help in the interpretation of this phenomenon, the distribution of the time to first detection as a homozygote of a recessive gene occurring only once in the initial generation has been obtained. Alternatively the results may be considered as referring to the time of first appearance as a homozygote of a new mutation occurring in a finite population. For a monoecious random mating population of size N with selfing permitted, the mean time to detection is very close to 2N⅓ over a range of N from 1 to 500 with a coefficient of variation of roughly 2/3 and a 95% upper limit about 2·5 times the mean. If selfing is prohibited, the mean time is increased by a little over 1 generation. The treatment is extended to cover the effects of artificial selection in favour of the heterozygote, of the frequency of occurrence in the initial generation and of the examination of more individuals each generation than are used as parents.

Male sterility, male recombination, and transmission ratio distortion – all examples of a syndrome known as hybrid dysgenesis in Drosophila melanogaster – were found to involve chromosome–cytoplasm interactions. The latter two have temperature optima near 25° and involve pre-meiotic events. In addition, sex ratio distortion, and induction of certain translocations of the X and Y chromosomes (but not the autosomes) were found to be part of hybrid dysgenesis. Both are caused by chromosome–cytoplasm interactions with pre-meiotic events playing a crucial role. The results agree with previous data on female sterility in hybrid dysgenesis, which also has cytoplasmic components and premeiotic origins.

The organization of the genetic material in the macronucleus of ciliates has been the subject of considerable controversy. Two of the four models of macronuclear structure predict assortment of the alternative phenotypes of heterozygotes during vegetative growth. Early studies of the phenotypic behaviour of heterozygotes after macronuclear regeneration (Sonneborn, 1947) had supported a diploid subunit model. The availability of quantitative predictions of the rate of assortment for the haploid and chromosomal models (Preer, this volume) and the existence of two alleles controlling a quantitative trait, copper tolerance, in Paramecium tetraurelia, has provided an opportunity to test these models. The median tolerance limits to copper of unselected sublines were measured as a function of age. There was no increase in the variance among sublines, as the haploid and chromosomal models predict. Quantitative evaluation shows that subdiploid models with a kinetic complexity of 860 or less are not compatible with the results. This experiment was not sensitive enough, however, to exclude subdiploid models if the kinetic complexity is 2000 or greater. Selection on heterozygotes also failed to provide evidence in favour of assortment. All the results are consistent with and support the diploid subunit model of the Paramecium macronucleus.

Young mice of the strains A/Y and B10.D2 were repeatedly treated with diethylsulphate (DES) in different doses (36 to 1100 mg/kg). At the age of 9 weeks they were mated to females of the strains A. CA (‘A’ group) and C57BL/10Eg (‘D’ group) respectively. 2101 progeny of these matings were tested for histocompatibility by skin grafting. The spontaneous H-mutation rates were 6·96 × 10−4 per gamete in the A group and 9·6 × 10−4 per gamete in D group. In progeny of treated males the H-mutation rates were 0 in A group and 5·79 × 10−3 per gamete in D group, showing apparent effect of paternal DES treatment on mutation frequency in the last group. Two mutations of the H-2 locus were found, which together with the other three H-2 mutations published so far yielded a mutation rate of 5·18 × 10−4 per gamete. The mutation rate of the H-2 locus is higher than the expected rate per H-locus, indicating a great genetic complexity of H-2.

Age-related reactivation of an X-linked gene which maps close to Xce, the X chromosome inactivation centre, has been observed. In five female mice which carried the X-linked coat colour gene Moblo on the reciprocal translocation T(X;16)16H (Searle's translocation), and the wild-type gene on the normal X chromosome, and therefore expressed the Moblo phenotype due to the non-random inactivation characteristic of Searle's translocation, progressive darkening of the coat was observed as the animals aged. This is due to reactivation of the previously inactivated wild-type gene at the Mo locus on the normal X chromosome. As the Mo locus is located 4 cM distal to Xce, the X chromosome inactivation centre, these observations provide evidence of age-related instability of inactivation of an X-linked gene close to the inactivation centre.

Two hitherto unmapped recessive autosomal mutations, ‘arrested development of righting response’ (adr) and ‘myotonia’ (mto, now adrmto), were found to be allelic, so that preliminary linkage data on adr and adrmto can now be combined. No linkage was found to the glucose phosphate isomerase (Gpi-1) locus on Chr 7 that marks a region of homology between mouse Chr 7 and human Chr 19, and in man is closely linked to the myotonic dystrophy (DM) gene. The adr gene was expressed on diverse genetic backgrounds including the athymic nude mouse.