Glycine tastes both bitter and sweet to mice but there are differences between strains in their ability to detect each taste. With respect to the bitter taste, fifteen strains were classified as tasters and twelve strains as non-tasters. The difference is due to a single gene, Glb (glycine bitterness). Cycloheximide tastes bitter to all mice at a concentration of 8 μM, but strain differences in sensitivity to the taste of cycloheximide can be detected at lower concentrations. The BXD RI strains can be classified into two groups with respect to sensitivity to cycloheximide. This is probably due to the segregation of two alleles of a single gene, Cyx. A comparison of the distribution in RI strains of alleles of four bitterness-tasting genes shows that the loci are all closely linked and are probably in the order Cyx–Qui–Rua–Glb.

A simple technique for the selection of auxotrophs of Penicilhium chrysogenum after mutagenic treatment has been devised using nystatin. It relies on the differential sensitivity of germinated and dormant conidia to nystatin and on the gradual loss of antibiotic activity which occurs on complex media at laboratory incubation temperatures. After mutagenic treatment, conidia were plated on MM agar to allow germination of prototrophs and then overlaid with CM agar containing enough nystatin to kill germinated conidia but not ungerminated auxotrophic conidia. When nystatin activity had decreased sufficiently the latter germinated and grew and thus were enriched relative to prototrophs.

Two strains of S. commune characterized by different levels of high recombination frequency in a particular region of the genome (between the two subunits, α and β, of the B incompatibility factor) were crossed, and their progeny tested for recombination frequency in the same region. The difference between the strains in recombination frequency is found to be due to some factor located within the recombining region itself.

The segregation among the progeny indicates that the factor consists of a number of sites, with additive effects. This and the dominance relationships suggest that these sites may be recognition sites which comprise a part of the fine control of recombination.

I report the results of an experimental study of the effects of sex ratio and density on the mean and variance in offspring numbers in both sexes of the flour beetle, Tribolium castaneum. The variance-effective number is estimated from the observed variance in offspring numbers using the methods of Crow & Morton (1955).

Both the mean and the variance in offspring numbers were found to vary with sex ratio and density; as was found in previous studies (Wade, 1980), males were generally more variable in offspring numbers than females. The ratio of variance to mean progeny numbers in each sex was approximately unity at all sex ratios.

It has been suggested (Berry & Searle, 1963) that the discontinuous (‘quasi-continuous’) variants studied by Grüneberg et al. in the skeleton of rodents can be regarded as constituting epigenetic polymorphism in different populations. Comparisons have been made between the incidences of skeletal variants in house mouse populations collected from: corn ricks on a single farm in Hampshire; eleven separated localities in different parts of the British Isles; and nine other places throughout the world. These showed that the method could profitably be used for genetically characterizing and hence comparing populations. There was evidence suggestive of genetical drift between local populations and stabilizing selection over a larger area.

Within the genus Apodemus, DNA from A. sylvaticus and A. flavicollis could be distinguished on the basis of reciprocal DNA/DNA annealing in agar, even when unfractionated DNA was used. In preliminary experiments, DNA from geographical races of A. sylvaticus could not be distinguished, nor could DNA from two closely related species of Peromyscus.

Within the species Mus musvulus, the fast-renaturing fraction of L-cell DNA, separated on a hydroxyapatite column, was able to discriminate between L-cell (originating from the C3H inbred strain) and NZB (New Zealand Black) inbred strain DNA in agar.

Structural genes for the isozymes of shikimate dehydrogenase (E.C. 1.1.1.25) have been located on chromosome arms 5AS, 5BS and 5DS of wheat, 5RS of cereal rye and on chromosome 5Cu of Aegilops umbellulata by electrophoretic techniques. This character provides a useful genetic marker for these previously unmarked chromosome arms, and the results support the notion of the conservation of gene synteny groups within the Triticinae.

Two sex-reversed males and eight intersexes have been found in a natural population of the mole species Talpa occidentalis. All individuals of karyotype 34,XY were normal males, while the 34,XX karyotype was found in normal females, intersexes and sex-reversed males. Small testes were present in XX males, and ovotestes in intersexes. Intersexes showed male antigen levels higher than for females and lower than for males (including XX males), as judged by cytotoxicity tests. The X chromosome of sex-reversed males and intersexes and the Y chromosome of males appeared morphologically normal.

Seventeen non-motile strains of Chlamydomonas reinhardii isolated by Dr Lewin and thirty-six newly isolated strains have been examined in the electron microscope for structural abnormalities of the flagella. Fourteen of them have straight, paralysed flagella in which the central fibres are replaced by an irregular core of disorganized material. The fourteen mutations map at four unlinked loci. Some of them are leaky; light and electron microscope observations on leakiness are described. In the former case leakiness is measured by the proportion of motile cells, and in the latter by the proportion of intact centre fibres seen in transverse sections. The degree of leakiness is to some extent characteristic of particular loci.

A partial suppressor of some of these mutations has been isolated which acts on all the mutant alleles at two loci and to a much lesser extent on four out of five alleles at a third locus.

Evidence is presented for the existence of a new allele at the patch locus called patch-extended (symbol Phe). Crosses of Phe with Ph, Wv and Rw produce double heterozygotes that are different from those resulting from the original patch allele. Besides the different coat colour in these mice, all the Wv + / + Phe animals that were tested resembled Wv + / Wv + and proved to be sterile. However, the two patch alleles interact to produce a split-face phenotype which resembles the presumed Ph/Ph.

A combined cytological and genetic analysis has been carried out to determine whether the spontaneous male recombination associated with a line of Drosophila melanogaster derived from Southern Greece (31.1 MRF) involves chromosome breakage and random reunion. In all crosses showing male recombination, extensive abnormalities involving anaphase bridges and fragments were found at first and second meiotic divisions. This confirms that the low level of recombination associated with male recombination lines is not produced by normal, controlled crossing-over of the type found in females, but by chance reunions, following more erratic breakage events at first and second anaphase. In addition the occurrence of some limited premeiotic recombination cannot be excluded. The 31.1 MRF system shows higher recombination levels at elevated temperatures (29 °C) and chromosome abnormality frequency is also greatly increased. The 31.1 MRF induces both male recombination and chromosomal abnormalities at meiosis when it is inherited from females deriving their cytoplasm from stock other than CyL4/Pm. It is concluded that this factor acts independently of sex and that the reciprocal cross effect is caused by a cytoplasmic factor which the 31.1/CyL4 strain inherited from the CyL4/Pm stock.

Mutants in the ftr cistron of the Basidiomycete Coprinus lagopus have a lesion in sugar transport. Fifty-two alleles are placed in an allele map using recombination frequencies obtained from over 400 heteroallelic crosses. The mutant sites in the allele map are distinctly clustered into three approximately equally spaced regions. It is demonstrated that the clustering is not due to any mutational specificity. Evidence is presented which indicates that the clusters are functionally differentiated both within themselves and from one another. Additivity of recombination frequencies was good over the whole of the allele map and there was no overall map expansion. However, specific marker effect sites could be recognized. The data indicate that marker effect may act to enhance or reduce recombination frequency and that enhancement is equal and opposite to reduction. It is also shown that marker effect enhancement is only observed when the interval being mapped exceeds a certain minimum value, but that there was no upper limit to the size of the interval. Expression of marker effect was influenced both by the background genome and by the nature of the mutation at the second site in the heteroallelic cross. It is suggested that current models of recombination do not adequately explain these data and, more importantly, that then-reliance on initial breakage of DNA strands in the formation of hybrid DNA creates grave mechanical difficulties. A model for genetic recombination is proposed in which the sequence of events is: (i) separation of intact double helices into single strands; (ii) illegitimate pairing between single strands from non-sister chromatids; (iii) strand breakage and exchange of covalent links so as to legitimize the illegitimately paired regions. The model allows for the formation of hybrid DNA either with or without coincident chiasmata. It is envisaged that the error recognized by any excision-repair system involved in recombination is the tangled strands at each end of the illegitimately paired region rather than base mismatching; and that the exchange of covalent links in stage (iii) involves the excision and replacement of segments of DNA.

Two series of mouse chimaeras were produced by aggregating pairs of eight-cell embryos that differed at the Gpi-1s locus, encoding glucose phosphate isomerase (GPI-1); the paired embryos were respectively homozygous Gpi-1sa/Gpi-1sa and Gpi-1sb/Gpi-1sb. Chimaeric blastocysts were transferred to pseudopregnant females, that were homozygous Gpi-1se/Gpi-1se and produced only GPI-1 C enzyme. Quantitative electrophoresis of GPI-1 was used to estimate the contribution of each embryo (GPI-1 A and GPI-1B enzyme activity) to the foetus, placenta and other extraembryonic tissues of 12½ day chimaericconceptuses. For both series of chimaeras, the distributions of %GPI-1A in different tissues were classified as (1) balanced and typical, (2) balanced but atypical or (3) unbalanced. One series of chimaeras was clearly unbalanced, so that the cells derived from the (C57BL × CBA/Ca)F2 embryo (Gpi-1sb/Gpi-1sb) predominated over those derived from the BALB/c inbred strain (Gpi-1sa/Gpi-1sa) in most foetuses. Two significant observations were made concerning this unbalanced series. Firstly, the mean composition of the placenta and other extraembryonic tissues was similar to that in the foetus i.e. also unbalanced with (C57BL × CBA/Ca)F2 (abbreviated to BF2) cells predominating. Secondly, despite this generalizeddeficiency of BALB/c cells, there were differences in the frequency of non-chimaeric tissues between different developmental lineages. In 20/34 chimaeric conceptuses in the unbalanced series only BF2 cells were detected in the foetus, whereas both BF2 and BALB/c cells were present in at least one of the extraembryonic tissues. This group of chimaeras, therefore, shows some similarities to human confined mosaicism. Although chimaerism occurred more often in the primitive endoderm (hypoblast) lineage (yolk sac endoderm and parietal endoderm) than in the placenta, this may also be the case in human mosaics. The mosaic status of the human yolk sac endoderm is usually unknown so it is possible that mosaicism often occurs in the yolk sac endoderm as well as the trophectoderm in human ‘confined placental mosaicism’. The uniformly unbalanced phenotype seen in the mouse chimaeras may be a result of generalized cell selection against BALB/c cells in all tissues. As an alternative explanation, we propose that most of the BALB/c cells in the blastocyst are allocated to the mural trophectoderm, which has a limited mitotic potential and so contributes little to the mid-gestation conceptus. Further work is required to test these possibilities.

Assortative mating is generalized to include social homogamy and phenotypic homogamy as two special cases. This generalization, called mixed homogamy, enables tests of hypotheses on the nature of assortment. Cultural inheritance is also extended to include two components: transmitted from parental environments, and non-transmitted sibship environment. Familial correlations are derived for a variety of relationships under mixed homogamy.

The coats of Ta–Ta+ and c–c+ chimaeric mice have been compared with those of Ta heterozygotes and animals heterozygous for an X-autosome translocation which causes c-variegation. With one exception all those features typical of the heterozygotes were found in their chimaeric counterparts, thus showing that the creation of two cell populations by X-inactivation can ultimately lead to the observed heterozygous phenotypes. The one exceptional feature not found in c-variegated chimaeric mice was the progessive increase in the proportion of pigmented hairs in c areas with age which occurs in X-autosome translocation heterozygotes.

The coat of a Modp / + – Ta chimaeric animal was also investigated. Ta hairs showing only the colour effect of Modp were numerous, suggesting that Modp and Ta affect different cell populations. However, Ta hairs showing the structural effect of Modp were very rare. The two observations suggest Modp can operate independently upon two different cell types – melanocytes and hair follicle cells.

Temperate phages φ 80 and λ grown in E. coli strain C differ markedly in the degree to which they are restricted by E. coli strain K. The basis of this difference is subject to genetic recombination and deletion. The loss of restriction sensitivity by deletion shows that sensitivity is determined by loci in the phage genome. These loci are interpreted to be targets of the restriction mechanism, in agreement with previous biochemical evidence.

Little information exists about the mechanisms that determine the fate of mobile elements in natural populations. In this study we catalogue the distribution of 638 P-elements across 114 X chromosomes in samples drawn from three natural populations of Drosophila melanogaster. There is an extremely high occurrence of elements at the tip relative to the rest of the euchromatic chromosome. We demonstrate that the distribution of de novo insertions of the P-element on a specific laboratory chromosome is markedly different; no P-elements were recovered at the tip in the 243 insertion events recorded. In contrast, insertion data for the π2 chromosome suggests an elevated rate associated with the tip site although it does not appear sufficient to explain the large differential accumulation on wild chromosomes. This raises the issue of inter chromosome (or tip) variation in relative rates, as well as the possibility that rates of elimination are lower at the tip.