Computer simulations have been done to study the effects of stabilizing and disruptive selection on a polygenic character. The results are reported in terms of three components of genetic variability which represent changes in gene frequencies, departures from Hardy–Weinberg equilibrium and linkage disequilibrium respectively. Under random mating the first and third components are the most important. The observed changes in gene frequencies are interpreted in the light of previous theoretical work on the stability of equilibria under selection. In addition, large and rapid changes in the genotypic variance result from the generation of linkage disequilibrium under selection; the observed changes are in good agreement with those predicted on theoretical grounds.

The transient distribution of allele frequencies in a finite population is derived under the assumption that there are k possible alleic states at a locus and mutation occurs in all directions. At steady state this distribution becomes identical with the distribution obtained by Wright, Kimura and Crow when k = ∞. The rate of approach to the steady state distribution is generally very slow, the asymptotic rate being 2v + 1/(2N), where v and N are the mutation rate and effective population size, respectively. Using this distribution it is shown that when population size is suddenly increased, the expected number of alleles increases more rapidly than the expected heterozygosity. Implications of the present study on testing hypotheses for the maintenance of genetic variability in populations are discussed.

The incidence of many minor skeletal variants in the inbred mouse strain C57BL can be influenced by the diet on which the parents live: in many cases, the effect is mediated by a correlation with body size. This also seems to be true in Falconer's (1973) Q-strain in which body size has been increased or decreased by selection. However, there was so much heterogeneity between replicates within selection lines that variants influenced by body size could be detected as a group, but not identified individually.

Equations are presented relating the number of generations elapsed to the fractions of bacteria carrying one or both plasmids in a clone derived from a cell containing two incompatible plasmids.

Recessive transformants (col) obtained from v (vermilion) embryos treated with v+ DNA are shown to map at 1−0·02, a position not distinguishable from that of su(s) (suppressor-of-sable) and in agreement with observations indicating phenotypic allelism of col with su(s) mutants. Recombination in the y–col–gt segment of the X chromosome, over a total map length of O·3 units, was studied among the progeny of colv1/y1gtE6cvv1f and y1colgtE6cvv1f/v1 females. The data from both crosses exhibit the following features: (1) recovery of reciprocal recombinants between y1 and col; (2) recovery of reciprocal recombinants between col and gtE6 and (3) striking negative interference in the y–col–gt segment. These results allow three alternative interpretations: (1) that recombination in the y–col–gt segment results from conventional crossing-over, with high coincidence of crossovers in the two subsegments; (2) that it results from symmetrical gene conversion at the col site (col to col+, and col+ to col), which may be accompanied by single cross-overs in either of the adjacent regions; (3) that col behaves like a transposable element, formally symbolized su(s)+·col, and that recombination in su(s)+·col/su(s)+ (i.e. col/col+) heterozygotes results from transposition of the col element from homolog to homolog, accompanied sometimes by crossing-over, either in the y–col subsegment or in the col-gt subsegment.

Wehrhahn (1975) introduced the method of probability generating function to study the distribution of charge differences between homologous proteins in a population but considered only the special case where the population starts with a single allele. Some of his results, however, contained errors. In this paper, all the formulae are presented in general, correct yet much simpler forms. It is also shown that the method of diffusion equations (Ohta & Kimura, 1973) can produce the same results. Numerical computations show that the difference between the one-step and two-step models of charge changes is practically negligible. The results obtained have also been applied to study Nei's genetic distance. Numerical computations indicate that the genetic distance computed from electrophoretic data is about 10% smaller than the expected number of amino acid substitutions involving charge changes in the early stage of divergence of populations and may give a serious underestimate in comparisons between species.

The survival of the plasmid RP 1–1 in E. coli depends on the presence of a functional polA gene. Unlike other plasmids that have this requirement, the replication of RP 1–1 is inhibited by chloramphenicol. RP 1–1 cannot be inherited by E. coli recA mutants, and inactivation of the recA gene product in a recAts mutant leads to destruction of the plasmid. RP 1–1 cannot be inherited by recB or recC E. coli unless the strains also carry a suppressor of these genes, such as sbcA or sbcB. The pattern of replication of RP 1–1 in E. coli K12 and its mutants suggests that the survival of this plasmid in this species is the outcome of a balance between DNA polymerase I-specified replication and exonuclease destruction.

Eighteen mutations of Saccharomyces cerevisiae, at eight loci, isolated as antisuppressors of SUPQ2, an ochre-suppressing allele of SUP11, were crossed with three other suppressors.

They were found to abolish the ability of SUP2 (inserting tyrosine), to suppress the ochre mutations ade2.1 and can1.100, but not its ability, to suppress his5.2 or lys1.1. When coupled with any antisuppressor, SUPQ5, inserting serine, was also unable to suppress ade2.1, but the suppression of other ochre mutations varied from one asu-SUPQ5 strain to another. No antisuppressor affected the ability of SUP11-am, an amber-suppressing allele of SUP11, to suppress trp1.1, an amber mutation.

The inheritance of resistance to the rodenticide, warfarin, in the Norway rat, Rattus norvegicus, derived from a wild rat population in Scotland was studied in the backcross, intercross and testcross. The resistance was found to be due to a major gene with about the same map position in Linkage Group I as the warfarin-resistance gene, Rw2, which occurs in the wild rat population in Wales. In heterozygotes, the Scottish resistance gene, unlike the Welsh gene, is incompletely penetrant in expression, though the penetrance was found to increase markedly in response to selection. Differences between the Scottish and Welsh types of resistance suggest that the two resistance genes are allelic.

Variation in spermatozoal β-glucuronidase levels of inbred strains of mice was found to parallel the known variation usually studied in liver with one outstanding exception: BALB/cJ spermatozoa, but not other tissues, showed an anomalously low level. This difference in spermatozoal β-glucuronidase activity was studied in hybrids and backcrosses but the data did not establish the number of loci affecting the difference. The levels of β-glucuronidase in F1's between C3H/HeJ and A/J depended markedly on the sex direction of the cross.

The relationship between parental age and recombination frequency in the offspring of mice has been studied using backcrosses to heterozygous males or females. The mice were allowed to breed until they became infertile. Data for several chromosomes were analysed, mostly from a stock carrying five markers covering approximately three-quarters of the length of chromosome 2.

Analyses of two-point recombination ratios or of multi-point interference ratios revealed no consistent age-related heterogeneity in offspring of male or female heterozygotes. A significant age-related heterogeneity was detected in the offspring of female heterozygotes but not of males, when the number of offspring with no detectable recombination was compared to those with one or more. This difference between the sexes could be related to earlier cytological observations on chiasma frequency in oocytes and spermatocytes. The significance of these findings for the analysis of follicular growth in mammals and in the origin of human trisomic conditions is discussed.

The genetic variability at 16 enzyme loci in 52 Scandinavian triploid parthenogenetic populations of the beetle Adoxus obscurus has been studied by starch-gel electrophoresis. The overall genotypes of different beetles have been compared with each other and with a sample from a diploid bisexual population from Canada. Eighty per cent of the parthenogenetic beetles have the same overall genotype. The remainder belong to six genotypes, three of which are found in only one population. The variability within and between parthenogenetic populations of A. obscurus is much lower than in other comparable parthenogenetic insects studied by us. This is interpreted to be a consequence of the efficient migration of the genotypes with the highest fitness, leading to a replacement of less-adapted genotypes. A. obscurus is a flying insect, whereas the other parthenogenetic insects studied by us are sluggish flightless forms.

Recently collected strains from Malay peninsula, Taïwan and Japan proved to be similar to previously studied Japanese strains kept for long time under laboratory conditions. It is therefore possible to speak of a Far East race, characterized by slow growth, very high fresh weight and small ovariole number. High heterogeneity between laboratory strains founded from wild caught flies seems also typical. Among the three traits studied, a positive genetic correlation was observed only between duration of development and adult weight. No correlations were found between biometrical traits and the latitude of strain origin. The problem of the origin of the Far East race is discussed.

We have investigated a mathematical model of the process of activation of the X chromosomes in eutherian mammals. The model assumes that the activation is brought about over some definite time interval T by the complete saturation of N receptor sites on an X chromosome by M activating molecules (or multiples of M). The probability λ of a first hit on the receptor site is considered to be very much lower than that of subsequent hits; that is, we assume strong co-operative binding. Assuming further that an incomplete saturation of receptor sites is malfunctional, we can show that for proper activation of X chromosomes in normal diploid males and females, we must have λMT ≥ 3 and 0·96 ≤ N/M ≤ 1. An extension of this analysis for the triploid cases shows that under these conditions, we cannot explain the activation of two X's if the number of activating molecules is fixed at M. This suggests that there must be two classes of triploid embryos differing from each other in a step-wise manner in the number of activating molecules. In other words, triploids with two active X chromosomes would require 2M activating molecules as opposed to M molecules in triploids with a single active X. This interpretation of the two classes of triploids would be consistent with differing imprinting histories of the parental contributions to the triploid zygote.

Euploid and aneuploid plants of Triticum aestivum, variety Chinese Spring were pollinated with, pollen of Hordeum bulbosum. Euhaploids and aneuhaploids of Chinese Spring were obtained from the crosses. Meiotic chromosome pairing was analysed in 25 different aneuhaploids and the results were compared with those obtained from euhaploids. The evidence provided by the meiotic studies was used to identify chromosomes whose activities affected the genetic control of chromosome pairing.

Meiosis was abnormal in a 23-chromosome aneuhaploid and in the 22-chromosome sectors of a chimaeral plant. Both plants were thought to have resulted from the incomplete elimination of the genome of H. bulbosum from hybrid embryos. It is suggested that the meiotic abnormalities in the two aneuhaploids were caused by the residual barley chromosomes.

In a previous paper the theory of a model of electrophoretic profiles due to Ohta & Kimura was considered. This model assumes a finite population with a linear series of possible alleles with mutation between nearest types but no selection. In the present paper a model with both mutation and selection is constructed which results in a stable population distribution closely fitting empirically observed features of the Ohta–Kimura model. The problem of discriminating between selective and non-selective models for electrophoretic models is considered.

The role of choline-O-sulphate (COS) as a sulphur storage compound in Aspergillus nidulans was examined by comparing a normal strain and one unable to utilize COS in a sulphur-starvation experiment designed to measure the mobilization of sulphur stores. Efforts to isolate the necessary mutants deficient in choline sulphatase activity produced two nutritionally distinct classes of mutants unable to utilize COS. They were found to be allelic on the basis of genetic complementation and fine structures mapping and represent either leaky or tight mutants with respect to choline sulphatase activity. One of these mutants with no detectable choline sulphatase activity was selected for a growth experiment which demonstrated that COS is a major, though not the only source of the endogenous sulphur supply which can be mobilized during growth in sulphur-limiting conditions.

Four species of Drosophila (melanogaster, simulans, mauritiana and virilis) were tested for their ability to survive on erg strains of Saccharomyces cerevisiae each having a mutant block at a different reaction in the synthesis of ergosterol. Species capable of completing development on a given yeast mutant strain were tested for egg production and viability, and also for their ability to survive on the mutant yeast for five complete generations. Sterol analyses using gas–liquid chromatography show that the sterol composition of flies closely resembles that of the yeasts on which they are grown, confirming that the steric criteria for sterols capable of a structural role in lipoprotein membranes are relatively broad. Even so, different sterols are not equally efficient in this respect and there are differences between species in their tolerance to the sterols available from each of the yeast mutants. The range of sterols capable of satisfying the micronutrient, as distinct from the macronutrient, requirement is narrower. Growth on two of the yeast mutants (erg-2 and erg-6) leads to developmental arrest in the larval stage due, it is suggested, to a block in the synthesis of ecdysone. The four Drosophila species lack an effective level of Δ8−Δ7 isomerase which is necessary for utilization of zymosterol and other Δ8 sterol precursors of ergosterol. Ergosta-5,7-diene-3β-ol appears to be capable of substituting for cholesterol in the metabolism of Drosophila.

Evidence is presented that the distribution of the combinations of the sexes in twin lambings has subnormal dispersion: there are more opposite-sexed pairs than same-sexed pairs. When adjustment is made for the occasional presence of monozygotic twin pairs, the departure from expectation (on the assumption that the sexes of DZ twins are distributed binomially) is highly significant. Thus sheep litters seem to show a feature characteristic of litters of pigs and (probably) mice and rabbits. This is consistent with the suggestion that p, the probability of a male zygote, varies systematically from one zygote to another within a litter.

By means of rye chromosome CR from Imperial added to Chinese Spring wheat the alien substitutions 4A/CR, 4A/CRS, 4B/CR, 4D/CR, 4D/CRS, 7A/CRL, 7B/CR, 7B/CRL, 7D/CR and 7D/CRL were isolated. Substitutions 4A/CRS and 4D/CRS had a better fertility and vegetative vigour than the corresponding substitutions with the complete chromosome CR. In substitutions the entire chromosome CR did not compensate for the absence of wheat chromosomes of group 7, whereas the substitutions 7A/CRL, 7B/CRL and 7D/CRL were more fertile than the corresponding nullisomics 7A, 7B and 7D. It was shown that the short arm of CR is homoeologous to wheat telosomes 4Aα, 4BL and 4DS, whereas most of the long arm of CR is homoeologous to the wheat arms 7AS, 7BS and 7DS. Rye chromosome CR is designated 4R. Only three substitutions with rye chromosome DR were produced. The fertility of substitutions 7A/DR and 7B/DR were considerably lower than that of nulli-7A and nulli-7B. The 4B/DR substitution was sterile. Rye chromosome DR is believed to be a double interchanged chromosome in comparison to the corresponding Secale montanum chromosomes. Rye chromosome DR is designated 7R. On the basis of substitution ability of CR and DR and several homoeologous gene loci on these chromosomes an evolutionary scheme for the derivation of the Secale cereale genome from Secale montanum is outlined.