Robertson (1960) used probability transition matrices to estimate changes in gene frequency when sampling and selection are applied to a finite population. Curnow & Baker (1968) used Kojima's (1961) approximate formulae for the mean and variance of the change in gene frequency from a single cycle of selection applied to a finite population to develop an iterative procedure for studying the effects of repeated cycles of selection and regeneration. To do this they assumed a beta distribution for the unfixed gene frequencies at each generation.

These two methods are discussed and a result used in Kojima's paper is proved. A number of sets of calculations are carried out using both methods and the results are compared to assess the accuracy of Curnow & Baker's method in relation to Robertson's approach.

It is found that the one real fault in the Curnow-Baker method is its tendency to fix too high a proportion of the genes, particularly when the initial gene frequency is near to a fixation point. This fault is largely overcome when more individuals are selected. For selection of eight or more individuals the Curnow-Baker method is very accurate and appreciably faster than the transition matrix method.

The H2Lc2 linkage region from G. tomentosum was transferred through seven successive backcrosses to G. barbadense and G. hirsutum race punctatum in which these species were used as bulk female recurrent parents. In the last backcross, BC 7, reciprocal crosses were also made. Recombination values diminished with progressive backcrossing. Other workers have shown that values of the order of 20·0% within the pure species were to be expected. In BC 7 joint estimates for punctatum were 0·9% and 0·0% as female and male respectively; the corresponding estimates for barbadense were 5·0% and 20·1%.

Selective elimination of the donor gene H2 on transference to punctatum was observed in all backcrosses, causing disturbance to the expected 1:1 backcross ratio. Disturbance of a different order was recorded for the segment H2Lc2 transferred to barbadense. When the heterozygote H2Lc2/h2Lc2 was male and barbadense (h2lc2/h2lc2) female 50% of the H2Lc2 gametes were lost and a 1:2 backcross ratio was observed instead of the expected 1:1. The 1:1 ratio was restored when the heterozygote was the female parent. The lethal interaction of the H2Lc2tomentosum segment with an inhibitor IB from barbadense was postulated.

Differences in the reaction of the genotypes of G. barbadense and G. hirsutum race punctatum to the introgression of H2Lc2 from G. tomentosum were discussed in terms of evolutionary divergence of the species.

1. The DNA contents of twenty-eight different species and forms of Chrysanthemum have been measured by photometry. It is shown that there are large differences in DNA content between some species with identical chromosome numbers.

2. The DNA contents of natural polyploids are frequently not those expected when comparison is made with diploid forms of the same species. The DNA contents of induced polyploids are those expected.

3. Chromosome length and volume are positively correlated with DNA content.

4. The relationship between chromosome number, chromosome size, DNA content and gene number is considered, and it is suggested that the differences in DNA content may result from the presence of differing amounts of genetically inactive DNA in the chromosomes.

A method is described for quantifying the phenotypic expression of eyeless using gauged estimates of the mean and standard deviation of the distribution of facet numbers.

Gene-environment interactions of four eyeless alleles ey, ey2ey4 and eyK are compared in different genetic backgrounds and on a standardized Pacific background. The original strains differ in mean phenotypic value and in direction of response to variations in the ribonucleic acid content of the larval culture medium, whereas the four Pacific strains are more concordant. Consequently these differences are in part attributable to modifier genes present in the different genetic backgrounds of the original strains. Modifier genes also influence the periods in development at which eyeless is sensitive to nutritional treatments.

On the standardized Pacific background deficiencies of cholesterol cause an increase, whereas deficiencies of RNA or thiamine cause a decrease in the expressvity of all four eyeless alleles, but non-identity of their interaction profiles over an array of different suboptimal environments suggest that they may form a group of heteroalleles.

Using models which describe the change, by natural selection, of the actual numbers of genes rather than their relative frequencies, it is demonstrated that the equation familiar to geneticists, i.e. dp/dt = sp(1 − p), is appropriate under a wide range of circumstances. It was pointed out that, for realistic treatment of the evolutionary process through which gene substitutions are repeated, the models must have the property such that the total population number remains constant or nearly constant throughout the process, and is not appreciably influenced by the genes being substituted.

The load or cost for a gene substitution was studied assuming a haploid population and the effects on the load of such factors as epistatic gene interaction in fitness, finite population number and slow change of environment were investigated. The load may become very large under a strong ‘reinforcing’ type epistasis between advantageous genes. In a finite population, the load for one gene substitution may be inflated by about unity if the product of the effective population number (Ne) and the selection coefficient (s) is large but Nesp0 is much smaller than unity, where p0 is the initial gene frequency. On the other hand, slow change of environment may decrease the load somewhat. It was concluded that despite these and other complicating factors, Haldane's original formula, –logep0, for a haploid population (−2 logep0 for the case of a diploid without dominance) is still useful for assessing the approximate amount of selective elimination that accompanies the process of gene substitution in evolution.

Two inbred strains of mice differing in the mean percentage of spermatozoa with abnormal heads were used: KE (16·1%) and CBA (5·9%). The F1 resulting from the crosses exhibited a heterosis effect, while in the backcrosses an obvious segregation of genotypes was observed; both generations showed a reciprocal difference, depending on the source of the Y chromosome. The character of sperm head abnormality seems to be polygenically determined, one of the genes being located on chromosome Y.

Seven generations of backcrosses were performed in which the Y chromosome from CBA was introduced to the genetical background of the KE strain. In the seventh generation 10·2% of abnormal spermatozoa were found, which is significantly lower than in the KE strain. The difference shows the net effect of the Y-linked locus. A correlated difference was found in the fertilization rate, indicating that a factor influencing male fertility is located on chromosome Y. It does not seem to influence the shape of normal spermatozoan heads.

Karyotype analysis did not reveal gross abnormalities in the KE strain.

The effect of selection on individual performance for a quantitative trait is studied theoretically for populations of finite size. The trait is assumed to be affected by environmental error and by segregation at a single locus. Exact formulae are derived to predict the change in gene frequency at this locus, initially by finding the probability distribution of the numbers of each genotype selected from a finite population of specified genotypic composition. Assuming that there is random mating and no natural selection the results are extended to describe repeated cycles of artificial selection for a monecious population. The formulae are evaluated numerically for the case of normally distributed environmental errors.

Using numerical examples comparisons are made between the exact values for the predicted change in gene frequency with values obtained using approximate, but simpler, methods. Unless the gene has a large effect (α) on the quantitative trait, relative to the standard deviation of the environmental errors, the agreement between exact and approximate methods is satisfactory for most predictive purposes. The chance of fixation after repeated generations of selection is also evaluated using the exact method, and by means of a diffusion approximation and simple transition probability matrix methods. Except for very small values of population size (N) and large α the results from the diffusion equation agree closely with those from the exact method. Similar results are found from tests made of the prediction from the diffusion equation that the limit is only a function of Nα for a given intensity of selection and initial frequency, and that the rate of advance in gene frequency is proportional to 1/N for the same set of parameters.

It is already well established in Escherichia coli that certain genes, or groups of genes, can behave as episomes—that is, they can undergo a free transition between the fully extrachromosomal state and one in which they are integrated in the bacterial chromosome (Jacob & Wollman, 1961; Broda, Beckwith & Scaife, 1964). Although this type of behaviour has been detected commonly among members of the enteric bacteria, it has never hitherto been demonstrated clearly in a Gram-positive species. This communication shows that a reversible transition of certain genes between the extrachromosomal and the integrated state can occur in some strains of Staphylococcus aureus at least.

Genetic and physiological studies of ovulation rate, both natural and induced, in a random bred population of laboratory mice led to the following results.

(1) The heritability of natural ovulation rate in nulliparous females was 22±19%, estimated from the correlation between paternal half-sibs.

(2) Selection applied to natural ovulation in primiparous females led to a response in both directions, with a realized heritability of 31% (h2 within litters = 0·18 ± 0·013). The number of eggs shed by the High, Control and Low lines after 12 generations were respectively 21, 16 and 14.

(3) Selection applied to ovulation induced by 4 i.u. of PMS led also to a response in both directions, with a realized heritability of 22% (h2 within litters = 0·11 ± 0·038). The induced ovulation rates of the High, Control and Low lines after 12 generations were respectively 29, 19 and 14 eggs.

(4) PMS-equivalents were calculated from dose-response curves, and were found to be nearly the same in all lines except the line selected for high natural ovulation rate, which had a PMS-equivalent nearly double that of the Control. From this it was inferred that the increase of ovulation rate resulting from selection for high natural ovulation was due to an increased FSH activity, whereas the changes of ovulation rate resulting from selection for low natural ovulation and for both high and low induced ovulation were due to changes of ovarian sensitivity.

(5) Genetic correlations, estimated from correlated responses to selection, were: (i) between natural and induced ovulation in primiparous females, 0·33; (ii) between primiparous and nulliparous females in natural ovulation, 0·46; (iii) between primiparous and nulliparous females in induced ovulation, 0·82.

(6) The selection for induced ovulation produced changes in litter size following natural ovulation that were nearly equal to the changes in natural ovulation rate. The selection for natural ovulation, however, produced no clear changes of litter size.

Expected changes in the gene frequency and the population mean for a quantitative trait are described for selection in a population of size N at a single locus where the favoured allele has initial frequency q0 and selective value s. Models of additive and completely dominant gene action are compared. Results are generally expressed as the half-life of the total change relative to N.

If the favoured allele is additive or recessive the half-life of the gene frequency and mean of the trait are usually reduced when q0 or Ns is increased. However, if the dominant allele is favoured the half-life of gene frequency is still generally reduced as Ns is increased, but has a minimum at low or intermediate values of q0. Since inbreeding depression and selection oppose each other when the dominant allele is favoured the response in the mean of the quantitative trait may change in direction during selection.

Brachyphalangy (Xt/bph) is an allele of extra-toes (Xt) in linkage group XIV of the mouse. The phenotypes of both heterozygote and homozygote are distinguishable from, but similar to, those of extra-toes.

The behaviour of linkage disequilibrium between two segregating loci in finite populations has been studied as a continuous stochastic process for different intensity of linkage, assuming no selection. By the method of the Kolmogorov backward equation, the expected values of the square of linkage disequilibrium z2, and other two quantities, xy(1 − x) (1 − y) and z(1 − 2x) (1 − 2y), were obtained in terms of T, the time measured in Ne as unit, and R, the product of recombination fraction (c) and effective population number (Ne). The rate of decrease of the simultaneous heterozygosity at two loci and also the asymptotic rate of decrease of the probability for the coexistence of four gamete types within a population were determined. The eigenvalues λ1, λ2 and λ3 related to the stochastic process are tabulated for various values of R = Nec.

Selection for large and small 13-day larval weight in Tribolium castaneum was studied for sixteen generations in a replicated experiment to evaluate the effectiveness of various selection methods and the importance of genotype by environment interactions under two levels of nutrition.

Direct selection responses generally were larger than correlated ones under both Good and Poor nutritional environments. However, contrary to theoretical expectations, those populations selected on average performance in both environments were not superior for this attribute.

Asymmetrical responses were observed to be dependent on the environment of selection. When selection was based on performance in the optimal environment, the asymmetry was observed toward small size. This situation in the sub – optimal environment was completely reversed. This phenomenon was discussed in terms of physiological limits rather than gene frequency and directional dominance.

It was proposed that selection of compound traits such as body weight at a fixed age may affect the component characters quite differently. As the latter are differentiated by direction and environment of selection, the compound trait may reflect parametric changes and enhanced genotype by environment interactions. Such changes hamper the precision of current selection theory for predicting response even in the short run.

A small fraction of mutagen treated X chromosomes which would appear nonlethal in the Muller-5 test carried out on a yeast-containing medium act as lethals when tested on one or other of the defined Drosophila media. Such mutants have been termed ‘nutritional conditionals’. The three mutants obtained in this way have been characterized genetically, but have not yet been shown to act as simple auxotrophs.

Mutants of Pseudomonas aeruginosa male strains with altered mating properties have been obtained through treatment with nitrogen half mustards. The response varied with the type of male strain. A mutant of the infectious male PTO 13 was obtained which acts as a female strain and has apparently lost the FP sex factor. A mutant of the non-infectious strain 2 male mates not only with strain 1 females, but shows a thousand-fold increase in its ability to mate with strain 2 males, when it can act as either donor or recipient in conjugation. Derivatives of strain 1 males were obtained which had reduced recombinant forming ability.

While acriflavine is ineffective in producing such mutants, and has not been shown to cure male strains of their sex factor in P. aeruginosa, it is very effective in inhibiting infectious transfer of FP from FP+ to FP− strains. Furthermore, it markedly inhibits recombinant formation in 1 FP− × 1 FP+ crosses, and this latter effect is thought to be due to the inhibition of chromosome transfer by the male parent. In view of the almost complete lack of effect of AF on the normal growth of P. aeruginosa it is likely that the control of chromosome replication during vegetative cell division is different from the occurring during conjugation and that the FP factor is involved in this control during conjugation.

1. The loss of mu 540, mu 138, and kappa 51 following the loss of the dominant genes required for their maintenance was studied. With mu 540 and mu 138 the loss usually occurred within 4–7 cell generations, but occasional cells with mu were found as late as generation 12 with mu 540 and generation 17 with mu 138. Kappa 51 was lost between 5 and 60 fissions in different clones. The variability in time of loss remains unexplained.

2. No trail of inheritance of resistance to specific kappa killing comparable to the trail of inheritance reported for mu 540 by Gibson & Beale (1962) was found.

3. No loss of kappa 51, mu 540, mu 138, or mu 130 was found following treatment of paramecia with RNase.

4. Didinium nasutum was shown to be able to acquire and maintain kappa 51 when it could not possibly have acquired any RNase-sensitive metagons from paramecia.

5. These results show that the metagon hypothesis cannot be accepted without a number of ad hoc and at present untestable assumptions.

1. First- and second-generation crosses between killer, neutral and sensitive strains of yeast have been carried out in all combinations.

2. The results of this analysis indicated that the killer character is under the control of two types of cytoplasmic genetic determinant. One type, (k), determines killing, and the other, (n), neutrality. The absence, (o), of both types of determinants confers the sensitive phenotype.

3. That both types of cytoplasmic determinant require the same dominant nuclear allele, M, for their maintenance has been indicated in two ways. First, both types are lost when the nuclear genotype is changed from M to m. Secondly, cells of genotype m(k) or m(n), which have been shown to occur among the first formed cells arising from spores of Mm(k) and Mm(n) diploids respectively, are unable to maintain their cytoplasmic determinants. On the other hand, spore cultures of M(k) and M(n) genotype derived from these same diploide continue to maintain the determinants.

4. Thus genotype of killer cells is M(k), of neutrals M(n), and of sensitivities either M(o) or m(o).

5. Cells maintaining both types of cytoplasmic determinant (i.e. of genotype M(k)(n) or M−(k)(n)) have been obtained by appropriate crosses, and shown to be of killer phenotype.

6. Alternative hypotheses to account for the results of this genetic analysis have been discussed.

An attempt is described to produce chromosome translocations in Glossina austeni which, it is hoped, might ultimately be used for tsetse fly control. The method consisted of selecting among the progeny of irradiated males for cases of inherited partial sterility. Thirty-four per cent of the testable sons of males which received doses of 5–7 krads showed these properties. In two cases the inheritance was patrilineal which suggests that the Y chromosome was involved. In most of the other cases a segregation among both male and female progeny of the partially sterile and normal types occurred and in these cases it appears that autosomes only were involved. Among males in these stocks the segregation ratio was close to 1:1, but in females there was a deficit of the partially sterile type. This may be partly associated with the fact that a large proportion of totally sterile and in-viable females were produced by these stocks. The proportion of zygotes which died at the embryonic, larval and pupal stages as a result of the action of the partial sterility factors varied between factors of different mutational origin. In view of the pattern of inheritance of these factors and their high frequency of induction it is argued that in all probability they are translocations in the heterozygous state.

In Paramecium aurelia, syngen 4, gene ts21m, which is shown to segregate normally and independently of two other genes: ts111 and m1; is closely linked to gene ts401. The frequency of then recombination is of the order of a few per cent. Genetic analysis was carried out to confirm the genotypes of both double mutants and wild-type recombinants among F2 clones from crosses ts21m × ts401. This is the first case of linkage so far reported in P. aurelia.

Two mouse-adapted scrapie agents of different sheep origin were compared. The titre, reached in the brains of mice in the terminal stage of scrapie, is of the same order for both agents. There is a threefold difference between the incubation periods of the two agents in some mouse strains, of which C57 is one, and in this strain incubation of the 22A agent, given as a large dose by a peripheral route, occupies almost the whole life-span.

The most fundamental difference between the agents concerns the reversal of the ranking of incubation periods, typically in the VM and C57 mouse strams: incubation of ME7 in VM takes almost twice as long as in C57, whereas most sub-lines of 22A take half as long in VM as in C57. The implications of this type of host-genotype, agent-strain interaction are discussed in terms of the possible nature of agent differences, the possibility of latent infection and the consequences for scrapie eradication programmes.