The complex anatomy of the adult limb skeleton of Os/+ mice is attributable to a reduction of the preaxial margin of the foot plates. This is detectable, in the fore limbs, in the 11-day stage, i.e. before condensations of mesenchyme have taken place. The involvement of the skeleton is thus secondary to an earlier defect in the foot plates. The cause of the latter has not been discovered.

This paper examines models of the population dynamics of transposable elements when chromosomal sites vary with respect to the effect on fitness of mutations caused by element insertions. Element abundance is assumed to be stabilised solely by the joint results of transposition, excision, and selection against insertional mutations. When there are only two classes of site, selected and neutral, it is hard to find parameter values for which numbers of elements are maintained that match the findings from surveys of Drosophila populations, as elements tend to accumulate at high frequencies at the neutral sites. It is similarly hard to produce realistic equilibria with three classes of site (strongly selected, weakly selected, and neutral), when elements can transpose out of the neutral sites. If transposition from neutral sites is impossible, as might be the case for elements inserted into centric heterochromatin, then realistic equilibria can be generated if there is very weak selection against elements inserted into the majority of non-neutral sites. This model predicts a modest over-representation of elements at the neutral sites. It also predicts that elements should be under-represented on the X chromosome compared with the autosomes, but this is not generally found to be the case. It is concluded that selection against insertional mutations is unlikely to be the major factor involved in the containment of element abundance.

The effect of sub-dividing a population during selection was studied by selecting downwards for sternopleural bristles in Drosophila melanogaster. The four structures used all involved the selection of 40 individuals out of 200 measured in each sex. The alternatives were:

(i) one large line with the same selection procedure in all generations,

(ii) eight small lines with the same procedure in all generations,

(iii) the ‘single cycle’ structure in which large lines derived from crosses between selected small lines at either the sixth or twelth generation, and,

(iv) the ‘repeated cycle’ structure in which the best five small lines were mixed at the sixth generation of selection to produce a new set of small lines on which the procedure was repeated for a total of three cycles.

Of the first two methods, the large line had a higher final response than did any of the small lines. The latter, selected with an intensity of 5 out of 25 in each sex, had lower average final response than had a similar group of lines selected from the same base population with the lower intensity of 10/25. The results showed no clear effect of the sub-dicision of the population, followed by selection between lines, though they were in general agreement with theortical expectations. Several second chromosome recessive lethals were found at high frequencies in different selected lines. One of these, found in eight of the lines, had an effect on bristles in the heterzygote of about 1·5 phenotypic standard deviations. The evidence strongly suggests a pleiotropic effect on bristle score.

This study reports on exceptional males which are obtained by using Drosophila melanogaster mothers carrying the balancers In(l)FM6 or In(l)FM7 as one of their X chromosomes. The phenomenon was first observed in interspecific crosses between D. melanogaster females and males of its closest relatives which normally produce unisexual female hybrid progeny. Whereas hybrid sons from these crosses die as third instar larvae, the presence of the particular X balancers in the mother allows a low percentage of sons to survive. Similar sterile males are also observed among non- hybrid flies. Data are presented which suggest that the males thus generated could be hyperploid for part of their X chromosome as a result of a meiotic event in their mothers or else they could start life as female zygotes and change sex through a mitotic event at an early stage.

Two hundred and two house mice (Mus musculus L.) from 29 populations in Europe and North Africa were typed for 16 H-2K and 17 H-2D antigens, each antigen defining a different allele. Among the 13 best characterized populations, 1 to 4 common and 3 to 20 rare antigens were observed. However, an average of 37% of the H-2K and 39% of the H-2D antigens remain to be identified. Ninety-four percent of the 50 mice tested were heterozygous for H-2K antigens and 89% for H-2D antigens. In 4 of the 8 populations tested, the most common H-2K and H-2D antigens occurred in the same individual more often than if randomly associated. Associations between common H-2K and H-2D antigens and excess heterozygosities may be the consequence of the small size and instability of populations composed primarily of related individuals. Estimates of the genetic distances between populations revealed that Danish, Egyptian, and several of the Orkney Island populations were related. These were the only populations in which metacentric chromosomes were not found. In contrast, populations which were antigenically different were also karyotypically different, regardless of taxonomic status of allozymic similarity.

When zygotes formed by conjugation between Hfr and F- cells of E. coli are transferred from an amino-acid rich medium to one lacking amino-acids the probability of genetic recombination is reduced. This result is interpreted as indicating that recombination can only occur during a limited period after zygote formation and that the metabolic imbalance caused by the change of medium leads to temporary inhibition of recombination. By an hour after zygote formation the yield of recombinants is no longer affected by transfer to amino-acid free medium.

In independent investigations, reported almost simultaneously (Cohen, 1960; Shreffler, 1960), the authors have described a protein variant system in the serum of mice, involving an electrophoretic difference in the iron-binding beta-globulin component, transferrin, and have shown the difference to be genetically determined by a pair of co-dominant alleles at a single locus. Both reports introduced the same gene symbol, Trf, for this locus, but in one, letter designations were used (Cohen, 1960), making the allele symbols TrfA and TrfB, and the three phenotypes TrfA, TrfAB and TrfB, while in the other, number designations were used (Shreffler, 1960), with the corresponding symbols Trf1, Trf2, Trf-1, Trf-1·2 and Trf-2. To avoid confusion in future reference to this locus, due to the two differing systems of nomenclature, the authors have agreed upon the revised nomenclature proposed in this communication.

Marker Assisted Selection (MAS) based on additive effects associated with alleles at marker loci, estimated by linear regression of individual phenotype on the markers, was applied to characters with non-additive gene action and non-additive environment. The base population was the F2 generation of a cross between two inbred lines. Computer simulations of MAS were conducted for characters with dominance, epistasis and genotype-environment interaction approximated by the ‘additive-multiplicative’ model. MAS was more effective than purely phenotypic selection in all cases. The efficiency of MAS for characters with non-additive gene actionis comparable to (and for negative dominance even higher than) the efficiency of MAS for strictly additive characters. Environmental non-additivity, however, lowers the efficiency of MAS. Almost all results concerning the efficiency of MAS in our previoussimulations of purelyadditive traits are applicable to non-additive traits.

R factors present in Pseudomonas aeruginosa strains of clinical origin can be transferred to other strains of P. aeruginosa and may act to promote host chromosome transfer. In general, their properties are similar to those R factors in Enterobacteria. The different R factors studied vary with respect to transferability, transfer of specific resistance properties, repressibility, and ability to promote chromosome transfer.

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.

Two pairs of lines of mice, selected for High and Low weight at 5 weeks of age (H5 and L5) and at 10 weeks of age (H10 and L10) over 15 generations, were compared with each other and an unselected control line (QC) at three different ages, i.e. at 5, 10 and 21 weeks of age. Differences in percentage fat between High and Low lines were small at 5 weeks, clearcut at 10 weeks and at 21 weeks the High lines were almost twice as fat as the Low ones. The H5 line was slightly fatter than the H10 line at all three ages and the L10 line was fatter than the L5 line at two ages. The developmental pattern of fat deposition was unaffected by selection for high or low weight at 5 weeks, i.e. the relationship between fat weight and carcass weight was similar in the H5, L5 and QC lines. Selection at 10 weeks affected the level of fat at low carcass weights and the relative rate at which fat was subsequently deposited. The different correlated responses resulting from selection at 5 and 10 weeks are explained in terms of genetic variation in (1) the rate of food consumption, (2) the efficiency of energy utilization for growth, and (3) the relative amount of fat deposited up to the age at selection.

Mutations giving rise to sexual sterility were induced in Neurospora crassa macroconidia by ultraviolet-light irradiation. Thirty mutants were isolated on the basis of their male sterility in crosses with a wild-type strain. When used as the male parent these mutants exhibited a wide spectrum of sexual behaviour patterns ranging from the production of only small brown protoperithecia (complete male sterility) to the production of large and normally pigmented perithecia but with an undeveloped ostiole and very few if any spores. For many of the mutants the behaviour pattern is different when the strain is used as the female parent. Segregation data reveal that none of these mutants represent mutations of the mating-type locus. These findings suggest that the sexual development cycle is blocked at various stages in the different mutant strains. All attempts to restore fertility by supplying various additives to the medium or by varying the incubation time and temperature were unsuccessful. Conidial viability tests carried out on many of the strains revealed no abnormality in this respect. The aberrant segregation patterns exhibited by many of the mutants are discussed.

Two codominant alleles, Mup-a1 and Mup-a2, controlling electrophoretic variation of one of the components of the major urinary protein (MUP) complex of the mouse Mus musculus have been found to be linked to the black–brown coat color alleles (linkage group VIII).

Transmission ratio distortion (TRD) is a property of the complete t-haplotype which results in the preferential transmission of the t-haplotype chromosome from heterozygous t/+ males to the majority of the progeny. Available data suggest that in t/+ males, a dysfunction of the wild-type sperm within the female reproductive tract is responsible for the observed deviation from Mendelian segregation ratios. Genetically, Lyon has shown that multiple loci within the t-complex are required for maximum levels of TRD. These loci include multiple t-complex distorters (Teds) which act upon a single t-complex responder (Ter). Testis-expressed genes have been cloned which map to the same subregions of the t-complex as the Teds and Ter and are thus considered candidate genes for the products of these loci. To begin to understand how the products of these loci biochemically control TRD, the expression of three TRD-candidate genes (Tcp-1, D17Leh117c3, and D17Leh66) has been determined in populations of spermatocytes and differentiated spermatids purified to near homogeneity by unit gravity sedimentation. Fractions covering the entire gradient were analysed resulting in a more accurate picture of the precise timing of expression than previously reported. Transcription of all three genes was up-regulated in pachytene primary spermatocytes and persisted at stable levels through the haploid spermatid stages. Significantly, only levels of mRNA encoded by D17Leh66, the candidate gene for Tcr, increased from early round to elongating-stage spermatids. If this pattern of expression does, in fact, represent Tcr, these data provide the first direct evidence that wild-type and t-haplotype Tcr elements could be differentially expressed in haploid spermatids.

A new method for the isolation of temperature-sensitive mutants of Physarum polycephalum is described. It involves enrichment and prescreening of mutagenized amoebae followed by screening at both the plasmodial and amoebal stage. A total of 74 temperature-sensitive strains were recovered of which 26 were temperature-sensitive only as plasmodia, 35 only as amoebae and 13 in both stages. After a shift to the nonpermissive temperature, DNA and protein synthesis were followed in temperature-sensitive plasmodia to discover if the lesion affected functions of the nuclear cycle.

When mating pairs of Escherichia coli K12 are separated by physical means, e.g. by violent agitation, the Hfr cell is unable to mate with a second F− cell. This effect can be simulated by agitation of the Hfr cells prior to mating and, in addition, such cells can be shown to have an impaired ability to adsorb the male-specific coliphage μ2. Inhibitory concentrations of chloramphenicol block the repair of the damage.