The model of random selection coefficients is considered in the context of a finite population of diploids. The selection coefficients of the homozygotes are allowed to vary with equal variance while the fitness of the heterozygote is kept fixed. Steady-state solutions are found in the case of equal two-way mutation rates with particular reference to the expected heterozygosity. Increasing the variance of the selection coefficients of the homozygotes is found to uniformly increase the heterozygosity for all values of the average selection coefficients and its effect is largest when the selection coefficients of the homozygotes are fully correlated. The fate of mutant genes is also considered in the case of random selection coefficients by looking at the probability of ultimate fixation and the mean times to fixation and extinction. The errors in previous calculations (e.g. Kimura, 1954; Ohta, 1972) are pointed out. It is found that a small average heterozygote advantage together with a reasonable degree of variance in the coefficients can cause an unexpectedly large amount of heterozygosity to be maintained. It is also seen that probabilities of fixation and mean times to boundaries are usually increased by increasing the variance showing that it in fact helps to keep the population heterozygous for much longer than the non-random case. This is in contradiction to some conclusions of Karlin & Levikson (1974) because their haploid results are not easily extendable to the consideration of this sort of diploid model.

A population with N monoecious individuals, and having two alleles, is considered. The problem of calculating the fixation probability of a particular allele under random fluctuation of selection intensities is re-examined, employing finite Markov chain methods. An approximate but general expression for this probability is obtained and the results obtained by previous workers are shown to be special cases of this result.

Females from an Indian wild population of Aedes aegypti were crossed to males carrying the sex ratio distorter factor MD which shows meiotic drive. Progenies from F1 males were tested for sex ratio distortion, i.e. the chromosomes from the wild females were screened for their resistance to the action of MD. The distribution of sex ratio in the progenies of different F1 males indicated a polymorphism in the wild population for resistant and sensitive variants of the X chromosome. Seven discrete categories of X appear to exist, associated with sex ratios ranging from 50% ♀ to less than 1·25% ♀. The overall level of resistance varied slightly but significantly in different parts of a town. The results are discussed in relation to the use of sex ratio distortion for genetic control of mosquitoes.

Mice were selected for high and low body weight at 5 and at 10 weeks of age. Selection was performed (1) separately for each trait, and (2) for various combinations of the two traits, using (a) independent culling levels and (b) restricted indices. Two-way selection for each trait separately gave large responses and correlated responses. Selection by independent culling levels intended to increase 5-week weight while restricting change in 10-week weight gave no demonstrable response; selection by culling levels intended to decrease 5-week weight while restricting change in 10-week weight resulted in decreases in body weights at both ages. Index selection, intended to change weight at one age while holding that at the other age constant, was generally successful. Observed responses did not conform very well with predicted responses for either index or culling levels selection. The significance of these observations in regard to the problem of selection involving restriction of traits is discussed.

Mutants with abnormalities which were the consequence of the formation of haematomas during gestation were designated as ‘foetal haematomata’ (fh). The fh homozygotes were found to be morphologically and embryologically very similar to myelencephalic bleb (my) mice. It was also found, however, that the fh and my mutants were separate genetic entities due to two independently segregating recessive genes.

Cell lineage analysis of the maroon-like mutation of Drosophila melanogaster revealed the most extensive degree of non-autonomy reported to date in Drosophila: all 1454 gynandromorphs in which X chromosome loss uncovered the ma-l mutation had ma-l+. eye colour. In contrast, among 331 gynandromorphs in which X chromosome loss simultaneously uncovered the vermilion and maroon-like mutations, approximately 16% had v phenotype but with one possible exception all gynandromorphs again had ma-l+ eye colour. These results suggest that very small amounts of the ma-l+ gene product are necessary for wild-type eye colour development and they are therefore compatible with the one cistron–allelic complementation model that has been proposed for the ma-l locus. They also provide the best estimate available to date of In(1)wvc-induced internal mosaicism: 7%. A preliminary attempt to detect DNA-induced transformants among 6 DNA-injected preblastoderm ma-l embryos and at least 80000 of their F1 to F4 descendants has yielded completely negative results. An investigation of the maternal effect which ma-l+ mothers exert on the eye colour of their genetically ma-l offspring revealed that, in contrast to earlier observations, this effect is not universal: some phenotypically ma-l and intermediate ma-l flies were observed in young cultures. The discrepancy between this and earner observations is probably attributable to as yet uncharacterized nutritional deficiencies in the diet of flies used in this experiment. Cytoplasm drawn from blastoderm ma-l+ embryos and injected into the posterior region of ma-l preblastoderm embryos failed to induce eye-colour alterations in all seven flies which survived the treatment. Injection of the contents of embryos of certain genotypes and developmental stages into ma-l pupae 24–48 h old did alter in some instances the eye colour of treated ma-l flies. Various tests strongly suggest that these alterations are not due to injection of a substance that has been stored in the egg during oogensis or that has been produced by the embryo itself prior to injection and they therefore preclude the possibility that a simple in vivo bioassay for the ma-l+ substance has been achieved. Rather, they indicate that the observed eye-colour alterations are due to transplantation of blastoderm-stage embryos which remain active long enough within ma-l hosts to produce and release a substance into the hosts' haemolymph and that this substance in turn induces phenotypic alterations in the hosts' eye colour. When v and ma-l eye colour changes are simultaneously monitored, it appears that injection of embryonic contents into pupae is equally or more effective in modifying the v phenotype than in modifying the ma-l phenotype. Based on these observations, a tentative hypothesis regarding the time of activation of the ma-l+ gene and the relationship between the immediate product of this gene, the maternal substance stored in the egg and the substance released by tissue transplants is proposed.

In order to test the feasibility of preservation, of genetic stocks of mice by storage in liquid nitrogen, mouse embryos at the 8-cell stage, were frozen and stored in liquid nitrogen at – 196 °C under increased radiation exposures of 1·8×, 9× and 84× background levels for periods of 6–8 months, 10–12 and 27–29 months, the 1·8 × level being regarded as a control. Their survival rates to the blastocyst stage, and after transfer to recipient females, to foetal or liveborn stages were then compared with those of unfrozen or short term frozen control embryos. The freezing process per se caused a marked loss of viability, in comparison with the unfrozen controls, but at the 1·8× radiation level there was then no further loss in viability even at the longest storage time (27–29 months). Similarly, at the 9× radiation level there was no loss of viability during storage up to 29 months, but at the 84× level the proportions of implanted embryos and live foetuses were slightly reduced. It was not clear if this was a true effect of radiation, since it was not related to time of storage. Considering all groups, about 20–30% of the embryos originally frozen were recovered as foetuses or liveborn young. It is concluded that the preservation of genetic stocks by storage in liquid nitrogen is a feasible proposition.

Evidence is presented for the existence of a new allele at the patch locus called patch-extended (symbol Phe). Crosses of Phe with Ph, Wv and Rw produce double heterozygotes that are different from those resulting from the original patch allele. Besides the different coat colour in these mice, all the Wv + / + Phe animals that were tested resembled Wv + / Wv + and proved to be sterile. However, the two patch alleles interact to produce a split-face phenotype which resembles the presumed Ph/Ph.