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.

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.

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.

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.

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. 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.

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.

Thirteen selected distal markers have been mapped by time of entry studies and linkage analysis. They appear to fall on two distinct linkage groups and with the ten previously described there are nineteen markers on one group and four on the other. There is as yet no clear evidence that the two groups are linked, but this possibility has not been excluded.

Of the thirteen markers described, six can be mapped by measuring their time of entry into suitable recipients. It should now be possible to establish the order of any further marker and to provide more accurate information for any marker transferred within 35 min from the time of mixing the cells.

The karyotype of Spalax leucodon (2n = 56) from Moldavia (Rumania) shows 27 pairs of autosomes: two metacentrics, three submetacentrics, eight suhtelocentrics and 14 acrocentrics. The X chromosomes are the longest subtelocentrics and the Y chromosome is a submetacentric of medium size. In metaphase I we have found a partial homology between the short arm of the X and the Y chromosome.

The population of S. leucodon from Moldavia shows considerable differences of karyotype as compared with other populations of the same species from the south of Europe. This species presents a high degree of chromosomal polymorphism in different populations.

The basic recurrence equations given in our previous paper have to be modified when males and females are differentiated. The modifications we gave are incorrect because we ignored the deviations from Hardy–Weinberg equilibrium when sampling and selection are applied separately to the two sexes. Appropriate corrections are presented in this paper.

Phenethyl alcohol is shown to induce cytoplasmic respiratory deficiency (petite mutation) in various strains of Saccharomyces cerevisiae. Apart from petite induction, phenethyl alcohol also affects cell growth both in respiratile and non-respirable media. This sensitivity to growth inhibition appears to be complex, and does not correlate with the degree of induction of respiratory deficiency.

I + × I + crosses were studied, using various Coll plasmids. Exclusion due to presence of Coll in recipients was removed by starvation, with production of I – phenocopies. Superinfection immunity prevented normal replication of transferred genes: thus, Itrp + donated to an I + trp– recipient led to formation of abortive recombinants.