It is hypothesized that the products of Neurospora rec+ genes mask recombinators such as cog by modifying DNA and that unmodified recombinators act as recognition sites for an endonuclease with scission properties like those of the type I restriction enzymes found in E. coli. These cut the DNA in both strands at some variable distance from a recognition site. Repair of a two strand gap initiated in this way would require DNA synthesis using the information contained in the homologous DNA duplex, leading to gene conversion. Crossing over could follow from resolution of two Holliday structures formed during gap repair. The hypothesis explains the polarity in the frequency of conversion events across genetic loci, the observation that chromosomes carrying recombinators are more often converted than is the homologue, and how recombinators can initiate conversion at a distance, as suggested by the pattern of conversion events in the his-3 locus in crosses heterozygous for the translocation TM429.

Although the fungus Aspergillus nidulans has a multiplicity of phosphatases and of genes where mutations affect one or more phosphatases, we have succeeded in identifying structural genes for three phosphatases as well as one other gene which might encode a fourth. Using both conditional and non-conditional mutations, palD has been shown to be the structural gene for a phosphate-repressible alkaline phosphatase, palG to be the structural gene for a non-repressible alkaline phosphatase which apparently exists in two electrophoretically distinct forms (but whose rates of thermal inactivation are apparently very similar) and pacA to be the structural gene for both intracellular and secreted forms of a phosphate-repressible acid phosphatase. Colony staining techniques for the enzymes specified by palD and pacA have been described previously but we have now shown that the enzyme specified by palG can be detected by staining toluene-permeabilized colonies. Mutations in pacG lead to loss of non-repressible acid phosphatase as judged by colony staining and electrophoretic patterns but their effects on assays of activity in cell-free extracts are only marginal. Under phosphate-limited, but not phosphate-starved or phosphate-sufficient, conditions, pacG− mutations also affect the regulation of other, phosphate-repressible phosphatases. None of these phosphatases, alone or in combination, plays an essential role.

The conjugation regions of IncF plasmids are closely related in that they share extensive DNA homology, and that they specify related pili. Variations between individual conjugation gene products of different IncF plasmids have, however, been noted. We have extended these observations by carrying out a systematic survey of twelve such plasmids, to examine the numbers and the groupings of the plasmid-specific alleles of several genes required for conjugation and its control.

Using vector plasmids carrying cloned origins of transfer (oriT), four different specificities were recognized, and these were correlated with the specificities of the genes with products that may act at this site (traM, traY and traZ). The traY gene is the first gene of the major transfer operon, and is therefore located close to the site at which the traJ protein acts to induce expression of the operon: correspondingly, correlation was observed between the oriT/traMYZ and traJ specificities in most of the plasmids. In turn, traJ is negatively regulated by the finO and finP products acting in concert: the finO product was relatively non-specific, but six finP alleles were identified, again with specificities correlated with those of traJ. Our explanation for this unexpectedly large number of finP alleles derives from the concept that the finP product is an RNA molecule rather than a protein. Although the conjugative pili encoded by IncF plasmids are closely related, they confer different efficiencies of plating of the various F-specific bacteriophages. We distinguished four groups on this basis, presumably resulting from differences in the primary amino-acid sequences of the pilin proteins. These groups could be related to the surface exclusion system specificities, consistent with the hypothesis that surface exclusion acts at least in part by preventing interaction between the pilus and the recipient cell surface.

From these data, information about the evolutionary relationships between the twelve IncF plasmids can be deduced.

In eukaryotic chromosomes, large blocks of satellite DNA are associated with regions of reduced meiotic recombination. No function of highly repeated, tandemly arranged DNA sequences has been identified so far at the cellular level, though the structural properties of satellite DNA are relatively well known. In studying the joint action of meiotic recombination, genetic drift and natural selection on the copy number of a family of highly repeated DNA (HRDNA), this paper looks at the structure–function debate for satellite DNA from the standpoint of molecular population genetics. It is shown that (i) HRDNA accumulates most probably in regions of near zero crossing over (heterochromatin), and that (ii), due to random genetic drift the effect of unequal crossover on copy numbers is stronger, the smaller the population size. As a consequence, highly repeated sequences are likely to persist longest (over evolutionary times) in small populations. The results are based on a fairly general class of models of unequal crossing over and natural selection which have been treated both analytically and by computer simulation.

The regulation of the syntheses of a number of phosphatases in the fungus Aspergillus nidulans has been examined. Levels of the intracellular alkaline phosphatase P11 are increased by starvation for carbon, nitrogen, phosphorus or sulphur. There is, however, no evidence that any of the wide domain regulatory genes which mediate sufficiency-triggered repression for each of these elements involved. A possible interpretation is that all four forms of starvation result in accumulation of an inducing metabolite. The palcA gene has been identified as a wide domain, probably positive-acting regulatory gene mediating phosphate repression. The palcA product controls the syntheses of alkaline phosphatase PI, acid phosphatases PIII and PV, a phosphodiesterase lacking phosphomonoesterase activity and probably also a phosphate permease. Mutations resulting in derepression of phosphate-repressible activities at acid but not alkaline growth pH define a gene designated pacJ. pacJ mutations also confer arsenate resistance at low but not high pH. It is likely that phosphate derepression and arsenate resistance result from reduced uptake of H2PO4−. Finally, phosphatase regulation might be less complex than previously thought. Mutations designated r and mapping at several loci apparently have no effect on phosphatase. They enhance phosphatase colony staining but this occurs even if the phosphatase substrates are omitted from the staining mixtures. r mutations appear to promote reactions converting the diazonium salts used for phosphatase staining to coloured precipitates.

Despite recent advances in our understanding of chromatin ultrastructure, little is known of the arrangement of chromosomes during interphase, the portion of the cell cycle associated with somatic gene transcription. An experimental procedure is described which has allowed the determination of the nature of the relative arrangement during interphase of chromosomes in a specific diploid cell type of Drosophila, the salivary gland anlage of the 10–14-h-old embryo. At this stage of development the salivary gland cells have ceased mitotic divisions. Embryos of 10–14 h in age were irradiated with 12000 rads of gamma radiation and then allowed to develop into third instar larvae. The polytene chromosomes of these larvae were examined for radiation-induced interchanges. From the distribution of observed interchanges, three major features of interphase chromosome arrangement were inferred. (1) Each euchromatic chromosomal arm occupies a specific domain within the interphase nucleus which does not appreciably overlap with those of other arms. (2) Within these chromosomal domains DNA folding is very extensive. (3) The heterochromatic regions of each chromosomal arm are sequestered from the euchromatic regions. An additional point of interest concerns the nature of the interchanges observed. No reciprocal interchanges were observed – all appeared to be partial exchanges, possibly subchromatid interchanges involving only one DNA strand from each of the two exchange sites.

Structural genes for the leaf isozymes of peroxidases (E.C.1.11.1.7) have been located on chromosome arms 2BS, 2DS and probably 2AS of wheat, 2RS of cereal rye and on chromosome 2H of Hordeum vulgare. This character provides a useful genetic marker for these chromosome arms, and the results supports the notion of the conservation of gene synteny groups within the Triticinae.

Genetic and cytological analyses of house-flies collected from 12 pig-breeding farms throughout the British Isles demonstrated that the non-standard sex determination mechanism prevailing in South-East England, involving a dominant female determinant (F) and virtual homozygosity for a male determinant on the X chromosome (Xm, both males and females morphologically XX), was not typical of the country as a whole. Instead there was a gradual decrease in the frequency of F, Xm and a rarer male determinant M III, and a concomitant increase in the standard male determining Y chromosome, on moving north, east and west of this region. Only the Scottish and probably the Irish populations were fully standard (XX females XY males), although one from the East Anglian coast in which non-standard determinants were rare was predominantly of this type. Populations from intermediate areas possessed complex multifactorial mechanisms in which Y, F Xm and M III coexisted. It is hypothesized that this radial cline in sex determinants, like the latitudinal cline known from mainland Europe, represents a transient polymorphism caused by the recent and continuing invasion of non-standard determinants into originally standard populations. The cause(s) of this apparently rapid evolutionary change, however, remain unclear.

Over one hundred backcross-one progeny were analysed by two dimensional electrophoresis to locate the genes on the short arm of chromosome 1D which code for, or control, the D group of LMW glutenin subunits. Recombination between these genes and Glu-D 1, encoding HMW glutenin subunits, was frequent but none could be detected between the D subunit genes and Gli-D1, coding for ω-gliadins, γ-gliadins and the B group of LMW glutenin subunits. These results are in contrast to those for chromosome 1 B because apparently homoeologous D subunit genes occur at a different position, being equidistant between Glu-B 1 (homoeologous to Glu-D 1) and Gli-B 1 (homoeologous to Gli-D 1). A range of 18 genetically-diverse wheat varieties, each containing one of two allelic groups of 1 D-encoded D subunits, were also analysed by two-dimensional electrophoresis. Consistent with the genetic analysis above, unbreakable linkages were found between these alleles and alleles of Gli-D 1. The results are discussed in relation to the evolution of the distribution of prolamin genes in the wheat genome.

Male mice from congenic lines carry Y-chromosomes derived from two pairs of inbred strains (CBA/FaCamSt and C57BL/FaSt; PHL/St and PHL-YH/St) on various genetic backgrounds were compared. Serum testosterone levels, and the response of target organs in castrated animals to graded doses of exogenous testerone propionate were measured. These comparisons produced evidence for two Y-chromosomal loci influencing androgen metabolism. One of these affects serum testosterone levels, with variant alleles on the Y-chromosomes derived from the PHL and PHL-YH strains. The other locus influences the response to testosterone of target organs, most significantly seminal vesicle, and variant alleles are found in the CBA and C57 strains. The effects of both loci are modulated by the genetic background. The relationship of these loci to other Y-chromosomal loci in the mouse is briefly discussed.

Several strains have been synthesized which have reactive (R) properties in the I–R system of hybrid dysgenesis and which are also classified as active P strains in the P–M system. The synthesis of this previously unknown combination of types was accomplished by employing a mating scheme which allowed transposition (chromosomal contamination) of P, but not I, factors from IP to RM chromosomes in dysgenic F1 males. The successful synthesis of RP strains provides strong evidence that the apparent absence of this combination in natural and laboratory populations of melanogaster is not due to a biological incompatibility between these two types.

Prior reports from this laboratory have described the experimental basis for our understanding of the rosy locus (ry: 3–52·0) of Drosophila melanogaster as a bipartite genetic entity consisting of a structural element that codes for the xanthine dehydrogenase (XDH) peptide and a contiguous cis-acting control element immediately to the left of the structural element. Although the left end (5′) of the structural element has been well defined, the right boundary (3′) has been given only casual treatment in our prior reports. In our recent studies of rosy locus expression we have been concerned with the production and identification of mutations in the non-structural regions immediately flanking the structural element. An improved definition of the right end of the structural element is essential to this analysis. In addition to producing a better definition of the right boundary of the structural element, this study produced several phenotypically novel mutations. These mutations were classified initially ascontrol element mutations, but upon analysis were found to map within the rosy structural element. No evidence was obtained for the existence of a control element contiguous with the right end of the structural element.

In this study, five separate alleles at the agouti locus in house mice were tested for potential effects on a battery of 13 minor skeletal variants. Six genotypes (aa, ata, atat, Aa, Avya, and Aya) were compared on a standard congenic background (C57BL/6). In log-linear analyses, three of the 13 characters showed significant genotype differences (another three were close to significance), and genotypes also exhibited overall significance in a multivariate randomization test. Both multidimensional scaling and clustering showed an association of aa with ata, Aa with Avya and Aya, and a general separation of atat from the other genotypes. Genotype differences averaged 0·63 in probit standard deviations, 0·09 when assessed by the mean measure of divergence. Since the general magnitude of effects of these major genes was quite similar to those previously estimated for presumptive polygenes from subline divergence studies, it was concluded that major genes may often act as polygenes and make important contributions to the variation in minor skeletal variants.

N-ethyl-N-nitrosourea (ENU) was used to induce mutations within the tw5-haplotype of the mouse to make possible a further study of gene arrangement in t-mutants and to provide potential landmarks for cloning and sequence studies in the region. Two independent mutants were isolated for each of three loci in the t-region, brachyury (T), quaking (qk), and tufted (tf). The new Tkt alleles produce tailless mice when a tct mutation is present in trans. The new qkkt alleles are recessive and homozygous lethal. They are viable, male fertile, and cause seizures and quaking when paired with the qk mutation which previously defined the locus. The tfkt mutations are recessive and phenotypically similar to the mutant alleles available in non-t chromosomes. The mutations were induced in the tw5-haplotype at an average per locus frequency of 1 in 1500. Their isolation demonstrates the power of this technique for obtaining the specific mouse mutants that are needed to genetically dissect a complex mammalian system.

Thirty strains of mice were tested for their ability to taste a 0·4 mM solution of raffinose undecaacetate (RUA). There were large strain differences. Some strains showed little or no ability to taste the RUA. Two strains, SWR and Schneider, could taste RUA because they possess the Soaa allele which enables them to taste a variety of acetylated monosaccharides. Three other strains, BALB/c, DBA/2 and C3H, could taste RUA because they possess the Ruaa allele which enables them to taste some larger structure which is a feature of the molecule as a whole. The gene Rua is tightly linked to the gene for quinine tasting. Qui, but the distribution of their alleles among the strains shows that they are different genes. It is suggested that there is in the mouse a cluster of tightly-linked genes, each one determining a taste receptor for a different bitter substance or chemical group. The relevance of these findings to the physiology of tasting is discussed.

The effect of the Xce (x-chromosome controlling element) genotype on the randomness of X-chromosome inactivation in the mouse was studied by monitoring the expression of the X-linked locus pgk-1. The main aim was to test whether the Xce genotype modified the preferential expression of the maternally derived X-chromosome in the yolk sac endoderm. Quantitative electrophoresis of phosphoglycerate kinase (PGK-1) was used to study Pgk-1 expression in the foetus, yolk sac mesoderm and yolk sac endoderm at 13½ days post coitum. The Xcea/Xcec genotype caused non-random X-chromosome expression in the foetus and yolk sac mesoderm. However, there was no evidence that the Xce genotype moderates the preferential expression of the maternally derived X-chromosome in the yolk sac endoderm, as reported by Rastan & Cattanach (1983).

A locus, Dlb-1, controlling the binding of the lectin from Dolichos biflorus to the intestinal epithelium and vascular endothelium of mice has been located on chromosome 11, 3.1 ± 1.4 centimorgans proximal to Rex, using recombinant inbred strains and conventional backcrosses with three-point linkage tests.

The two alleles so far discovered at this locus behave unusually. The Dlb-1a allele causes binding to the vascular endothelium but not to the intestinal epithelium while Dlb-1b induces the exact reciprocal binding pattern, and the heterozygote shows both patterns.

Isoelectrofocusing two-dimensional polyacrylamide gel electrophoresis (IEF-2D-PAGE) offers the opportunity to detect typical alterations in the protein pattern of trisomic mouse foetuses at a given time of development. The fractionation of the cell lysate by differential centrifugation into various subcellular components (nuclei, membranes, polyribosomes, cytoplasmic proteins) and fractionation of the proteins through DEAE-Sepharose chromatography allows detection of protein differences.

It is possible to detect eight differences in the protein patterns between trisomy 19 (Ts 19) mouse foetuses and euploid mouse fetuses at day 15. Five of these differences are quantitative in nature, three are qualitative. One of these proteins is synthesized in Ts 19 foetuses at a higher level than in euploid mouse fetuses (primary gene dosage effect). The other seven proteins are reduced or not present in trisomic foetuses (consequences of primary gene dosage effects).

The molecular mass of the individual proteins ranges from 13 to 41 kDa.

The histidine decarboxylase (HDC) gene complex, [Hdc], comprises the structural gene for mouse kidney HDC and closely linked regulatory elements which determine enzyme concentration and its response to hormones. One of these regulatory elements, Hdc-e, determines the response (induction or repression) of kidney HDC to oestrogen. HDC is oestrogen-inducible in C57BL/10 and oestrogen-repressible in DBA/2 and C57BL/6; alleles of Hdc-e segregate in crosses between C57BL/10 and DBA/2 and between the C57BL substrains. Two different haplotypes of[Hdc] have been defined previously, B.10 (Hdc-sb, Hdc-cb, Hdc-eb) in C57BL/10 and D (Hdc-sa, Hdc-cd, Hdc-ed) in DBA/2. C57BL/6 represents a third haplotype (B.6) (Hdc-sb, Hdc-cb, Hdc-ed) which differs from both B.10 and D. Hdc-e may therefore be a component of the complex independent of Hdc-s and Hdc-c.

Male mice carrying one complete t haplotype transmit it to virtually all offspring. This implies that either the sperm carrying the t haplotype have an advantage in fertilization, or that the sperm not carrying a t haplotype are dysfunctional. To distinguish between these possibilities, epididymal sperm from tw32/ + and congenic + / + males were artificially inseminated into the uterus of T / + females, and the transmission ratio determined from the phenotype of the offspring. After artificial insemination (AI) of sperm from tw32/ + males, the mean ratio was 0·95 (3 experiments, 5 litters), demonstrating that the transmission ratio remains high after AI into the uterus. After AI of a mixture of equal numbers of motile sperm from tw32/ + and congenic + / + males, the mean ratio was 0·22 (6 experiments, 8 litters). These results suggest that sperm carrying a t haplotype from t/ + males are not superior to sperm from + / + males, and therefore imply that sperm not carrying a t haplotype from t/ + males are dysfunctional.