Mutants of the Y locus differed appreciably in their basic conversion frequencies (frequencies of conversion in one-point crosses) to wild type. The differences in the basic conversion frequencies in the opposite direction, i.e. from corresponding wild-type allele to mutant, were in general not pronounced. For some alleles frequencies of conversion in both directions were similar, but for the others they differed markedly. No evident correlation between the position of mutants on the map and their basic conversion frequencies was observed.

In two-point crosses in repulsion, the great majority of recombinant octads were of conversion type. In these crosses symmetry or asymmetry of conversion depended mainly on similarity or differences in basic conversion frequencies of mutants crossed. In crosses between mutants from different clusters the recombination frequencies were near to the sums of their basic conversion frequencies. Such ‘mutant specificity’ makes it impossible to establish the linear order of mutants on the basis of recombination frequencies in two-point crosses.

The results of two-point crosses in repulsion between mutants within clusters pointed to the influence of one allele on the frequency of conversion of another one. This ‘marker effect’ was also evident in some three-point crosses.

The frequencies of simultaneous conversions in two-point crosses in coupling did not show negative correlation with the distances between the mutants involved.

It seems that many of the data presented here are most easily explained by recently developed hybrid DNA models.

An allotypic specificity (A1) of sheep serum is described. The antigenic determinant is located on a low molecular weight glycoprotein which has its isoelectric point at pH 5 and is capable of interacting with the lectin concanavalin A.

Family studies showed that the allotype is inherited in a simple Mendelian manner.

Selection for increased and decreased ratio of eye span to body length was exerted on male stalk-eyed flies (Cyrtodiopsis dalmanni) from Malaysia using replicate selected and unselected lines. Response to selection was symmetrical. After 10 generations high line male eye span increased to 1·3 body lengths while low line male eye span declined to 1·1 body lengths. Realized heritabilities for eye span to body length ratio, estimated using regressions of deviations from unselected controls on cumulative selection differentials, were greater than zero for all four selected lines with average h2 = 0·35 + 0·06. The static linear allometric relationship between eye span and body length diverged between selected lines and rotated among selected line males in the same direction as among males in other sexually dimorphic diopsid species. Crosses between lines after 13 generations of selection indicate that the genes which influence relative eye span combine additively and do not exhibit sex linkage or maternal effects. The genetic correlation between the sexes, 0·29 + 0·05 as estimated by the regression of female on male change in eye span, did not prevent sexual dimorphism in eye span from diverging between lines. These results suggest that the exaggerated eye span of male C. dalmanni is maintained by natural selection opposing sexual selection rather than by lack of or asymmetry in additive genetic variation. Furthermore, the variation in sexual dimorphism for eye span-body length allometry observed among extant diopsid species is consistent with sexual selection of variable intensity acting on relative eye span.

In Drosophila melanogaster and closely related species, polymorphism has been shown to be reduced at loci located in regions of low recombination on the X chromosome and on the fourth chromosome, which does not normally recombine. Thispositive correlation between nucleotide polymorphism level and recombination rate is not predicted by standard neutral theory and therefore must result from natural selection and genetic hitchhiking along the chromosomes. We report here the near-complete absence of variation at concertina (cta), a locus located in the β-heterochromatic base ofchromosome 2L, a region of strongly reduced recombination. A 1.2 kilobase region containing coding regions and introns was sequenced from each of nine lines of D. melanogaster and nine lines of D. simulans representingworldwide collections. Variation is significantly reduced in cta in both species compared with other available loci on the same chromosome. Two analyses of background selection demonstrate that the reduction in variation at cta, considered in combination with other loci on chromosome 2L or alone, is consistent with the background selection model.

The selection of animals or plants for high values of a certain character may favour not only genotypes associated with these high values but also genotypes associated with high variability. Any differences between genotypes in variability may therefore be of considerable importance in plant and livestock improvement programmes as well as in evolution. The effects of various selection procedures on variability have been studied in three recent experiments [Falconer & Robertson (1956) Falconer (1957) and Prout (1962)]. In these experiments one line was continued by selecting, in each generation, parents with values of a particular character near the population mean. Manning (1955, 1956) has described the effects of this kind of selection applied to cotton. Robertson (1956) derived and discussed the theory of such selection procedures when certain simplifying approximations can be made We shall obtain some more general results and show that Robertson was incorrect in saying that the selection procedure would lead to gene fixation even if the heterozygotes are less variable than the homozygotes. The importance of the results is discussed in section 8.

A method is now available for synchronizing the division of populations of cells of the smut fungus Ustilago maydis. In two experiments carried out with a diploid strain heterozygous for several biochemical markers, samples of cells were removed at intervals through the synchronized division cycle and treated with a constant dose of ultra-violet light. Cell survival and the frequency of the various recombinants resulting from induced mitotic crossing-over were recorded. In addition the period of DNA synthesis in the unirradiated population has been measured. During this period the cells are very sensitive to ultra-violet light and those that survive contain the highest proportion of induced recombinants. In so far as the markers make it possible to locate the position of cross-overs, cells which are irradiated early in the period of genetic replication show most crossing-over towards the ends of the chromosome arms, whilst cells which are treated late in this period show crossing-over near the centromeres. The data are most easily interpreted by supposing that chromosome replication begins at the ends of the arms and proceeds to the centromere, and that the temporary interruption of this process by ultra-violet light can result in pairing and crossing-over in the vicinity of the points of interruption.

It is generally accepted that reproductive isolation leading to the irreversible division of a Mendelian population into two species must be initiated during a period of geographical isolation (see Mayr, 1963). Thoday & Gibson (1962, Gibson & Thoday, 1963) obtained partial isolation within a population of Drosophila melanogaster by artificial disruptive selection for the number of sternopleural chaetae. This occurred in two experiments from the same base population after seven and twelve generations respectively. On the strength of these results they concluded that speciation does not require geographical isolation and that sympatric speciation by disruptive selection in a heterogeneous habitat is at least a theoretical possibility.

Peripheral blood lymphocytes from normal human volunteers or from Down syndrome patients were pre-treated with sodium butyrate (a compound which is known to induce structural modifications in the chromatin through hyperacetylation of nucleosomal core histones) and exposed to X-irradiation or treated with bleomycin in vitro in the G0 and/or G1 stage(s) of the cell cycle. The frequencies of chromosomal aberrations in the first mitosis after treatment were scored.

The results show an enhancement in the yield of aberrations in the butyrate pre-treated groups. However, the absolute frequencies of chromosomal aberrations as well as the relative increases with butyrate pre-treatment varied between blood samples from different donors suggesting the existence of inter-individual variations. There is a parallelism between the effects of X-irradiation or of combined treatments in G0 and G1 stages and between effects observed in the X-ray and bleomycin series. The increase in the yields of chromosomal aberrations in butyrate-treated and X-irradiated lymphocytes (relative to those which received X-irradiation alone) is interpreted as a consequence of the inhibition of repair of DNA damage by butyrate.

Data from Neurospora, Drosophila, and the mouse support the mapping parameter conventionally used for man, exclude the Haldane, Kosambi, and Carter—Falconer functions, and suggest a refinement for centromere mapping. Different sexes, chromosome arms, and types of data are surprisingly consistent. Double recombination frequencies are accurately predicted, but triple recombination frequencies are overestimated. The centromere appears to act on interference as an obligatory chiasma. Recombination across the centromere conforms to a simple approximation, based on the interval Markov assumption with a common mapping parameter. These results imply that mapping of n loci requires estimation of at most n parameters, and the relation between map distances and all recombination frequencies is explicit.

We have examined mice doubly homozygous for both pale ear (ep/ep) and beige (bgJ/bgJ) mutations in order to detect genetic interactions between these 2 loci affecting pigmentation and lysosome physiology. The doubly homozygous mouse has a new pigmentation phenotype consistent with independent effects of ep and bg. The beige (Brandt, Elliott & Swank, 1975) and pale ear (Novak & Swank, 1979)genes have abnormal kidney lysosomal enzyme accumulation caused by defective secretion into urine. No cumulative effect on these functions was observed in the new double mutant phenotype. The new phenotype has giant lysosomes typical of the beige mutation. Unexpectedly, the beige gene corrects the effect of the pale ear on serum lysosomal enzyme concentration. There is also a gene dosage effect of the beige gene on this serum lysosomal enzyme phenotype. The results suggest that the beige and pale ear genes affect the same pathway(s) of lysosome biosynthesis and/or processing. The action of the beige gene may precede that of the pale ear gene in lysosome physiology.

During an extensive survey of drosophilid fauna in 1977, D. melanogaster was not collected in the Seychelles. However, a population was found in 1981 in Victoria city, suggesting a recent introduction of this species. With respect to allozyme frequencies or ethanol tolerance, this population is almost identical to European ones and very different from those living under a similar equatorial climate on the African continent. The frequencies of rare biochemical alleles perhaps suggested that this population was founded by a small number of flies, less than ten inseminated females. For various biometrical traits, the situation was not so clear: according to the trait considered, Seychellian flies are either intermediate between European and African populations or closer to the latter. These data suggest that a few flies, recently introduced from a temperate (European?) country, built up a big population which is now on the way to adapting itself to new tropical conditions. Such an involuntary experiment should afford a unique opportunity to distinguish the respective roles of drift and adaptation in the evolution of D. melanogaster geographic races.

Populations of linear asci are classified according to the number of spores in an ascus. The resultant five numerical classes are further classified into ten spatial classes according to the arrangement of the spores in an ascus and, by inference, into ten historical classes according to the number and origins of failures during the developmental process. An analysis of the observed frequencies of numerical classes allows derivation of the efficiencies of the first meiotic division, the second meiotic division and sporulation in a fission yeast. The analytical method may be useful in locating the site of action of sporulation inhibitors and in identifying meiosis mutants from sporulation mutants.