The occurrence of induced recombination in the mitotic cell cycle in yeast has been analysed using conditional cell-cycle mutants held at the restrictive temperature. The strains used were heteroallelic at gall and assaying for functional galactokinase shortly after irradiation (Johnston, 1982) allowed an unambiguous determination of the cell cycle stages in which recombination could occur. Recombination was observed in most strains, including those with the cdc36 mutation, defective in ‘start’; the cdc4, 7 and dbf4 mutations which arrest cells in G1; the dbf1, 2 and cdc6 mutations affecting S phase; cdc16 and cdc17 which block cells in G2 and also cdc14 and 15 which arrest cells in ‘late nuclear division’. Recombination can therefore occur within each of the major phases of the yeast cell cycle. This analysis has also revealed that the cdc8 mutation results in a defect in induced mitotic recombination.

The loci eda, edd and zwf form a tightly linked cluster in E. coli that functions in the metabolism of galacturonate, gluconate and glucose. This cluster has been transferred from six natural isolates into the genetic background of E. coli K12 and examined with regard to effects on growth rate in chemostats. Although the naturally occurring eda and zwf alleles are selectively neutral, the edd alleles are not. The edd alleles fall into three functional classes distinguished by their effects on growth rate in gluconate medium, the most common classes differing in fitness by approximately 1% per hour. This extensive non-neutral genetic variation of edd is discussed in light of the evident rarity of gluconate as a natural substrate. We propose that gluconate selection is intermittent in space or time, providing the population an opportunity to accumulate non-neutral genetic variants during periods of relaxed selection. Such genetic variants will eventually be sorted out by the intermittent periods of gluconate selection, and during these periods the linked eda and zwf alleles will experience pronounced hitchhiking effects.

Two new conversion control factors (ccfs), ccf-5 and ccf-6, have been characterized in the Pasadena strains of Ascobolus immersus. Both are monogenic, with two known allelic forms (called A and B) of each factor, and affect the frequency of meiotic gene conversion at a white (w) ascospore locus closely linked to it, ccf-5 affecting w-9 and ccf-6 affecting w-BHj. The ccfs appear to be specific to their own target site, with no effect on at least nine unlinked w mutations. Conversion of the w locus affected was studied in + × w crosses with all four possible ccf arrangements: for example, for + × w-9, with ccf-5(A) in both parents, with ccf-5(B) in both parents, with ccf-5(A) in +, B in w-9, and with ccf-5(B) in +, A in w-9. For both ccfs, there were slight differences between crosses homozygous for A and those homozygous for B, and also slight differences between the two forms of heterozygous cross, A/B and B/A, but the major effect was for heterozygosity for the control factor to depress conversion frequency of the w locus, compared with either homozygous state. These two ccfs are compared with other sites affecting recombination in fungi and higher eukaryotes. Two possible modes of action of ccfs 5 and 6 are (i) on pairing closeness before hybrid DNA initiation, and (ii) on later stages such as the spread of hybrid DNA from an initiation point.

The brlA12 variegated position effect mutant is particularly suited for tests of environmental and genetic influences on variegation, but out of a large number of substances added to the medium, only salts at high concentrations and methylamine significantly increased expression of this gene. Medium shifting experiments showed that brlA12 activity could be switched on late, but once active, was rarely switched off again during conidiation. Separate brlA12 clones in heterokaryons were activated independently. Some brlA12-specific suppressor mutants, including those at loci giving almost complete suppression, have been studied. One class of suppressors also confers inability to utilize galactose as carbon source and comparison with other, pre-existing mutants showed that the brlA12 phenotype was either suppressed or enhanced by mutants with complex phenotypes involving galactose utilization, molybdate resistance, acid phosphatase production and sulphur metabolism. Tests for the involvement of DNA methylation in brlA12 expression gave negative results.

Two mutations at the niaD locus (structural gene for nitrate reductase apo-protein) are genotypically suppressible. Both mutations result in loss of nitrate reductase enzyme activity and cross reacting material and are non complementing, nonleaky and highly revertible. They have the properties of nonsense mutations. This implies that some of the allele specific suppressors, which act on these and alleles at several other loci, are nonsense suppressors.

Six axonemal mutants of Paramecium tetraurelia have been isolated that are unable to respond properly to calcium. The mutants, designated atalantas, cannot swim backward when stimulated by ions or heat. Genetic analyses reveal that all six mutants are recessive and fall into four complementation groups. Three of the mutants in one complementation group are phenotypically non-leaky, one is leaky and two are extremely leaky, only displaying their phenotypes at elevated temperatures. The complete mutants, ataA, are also abnormal in their forward swimming. This abnormality co-segregates with the inability to swim backward. ataA1 is not allelic to several membrane mutants of P. tetraurelia.

I report the results of an experimental study of the effects of sex ratio and density on the mean and variance in offspring numbers in both sexes of the flour beetle, Tribolium castaneum. The variance-effective number is estimated from the observed variance in offspring numbers using the methods of Crow & Morton (1955).

Both the mean and the variance in offspring numbers were found to vary with sex ratio and density; as was found in previous studies (Wade, 1980), males were generally more variable in offspring numbers than females. The ratio of variance to mean progeny numbers in each sex was approximately unity at all sex ratios.

A cryptic, lethal t haplotype (named tLmb) has been discovered in the non-inbred MF1 laboratory stock of mice. Detailed genetic and molecular studies demonstrate a close relationship between tLmb and a well-studied t haplotype, t6, previously extracted also from a laboratory stock. It appears likely that t6 is an aberrant t haplotype derived by recombination from a tLmb-like ancestral chromosome. The persistence of tLmb through 40 years of random breeding in a laboratory stock indicates that high transmission ratio distortion is sufficient to overcome the deleterious effect of a recessive lethal gene in a mouse population.

The position and orientation of an interchange quadrivalent in flattened, lateral views of metaphase I were studied in pollen-mother-cells of an interchange heterozygote of Allium triquetrum. The quadrivalent is most often located in marginal rather than central positions in the equator. Moreover, when positioned marginally the quadrivalent is more often than expected found in adjacent orientation, whilst when positioned centrally it is more often than expected found in alternate orientation. Consequently, the frequency of alternate (genetically balanced) orientation in the quadrivalent varies sharply according to whether data are obtained from more marginal or more central positions in the metaphase plate.

A second chromosome line of Drosophila melanogaster (S-90), isolated from a northern California natural population, is able to induce (1) an increased frequency of X-chromosome visible mutations, (2) male recombination activity subject to reciprocal cross suppression, and (3) strong meiotic drive from heterozygous males. Based upon several lines of evidence (including the response to suppressor chromosomes of both systems) we conclude that S-90 contains both SD (Segregation Distortion) and MR (P or I) chromosome activity. The two systems appear to behave independently and simultaneously, and a small centromeric region of the S-90 chromosome appears to contain the major genetic elements of both systems.

Complement testing of a Thp line revealed a failure of survival of Thp/t0 embryos. The genetic factor responsible for this lethality maps between Brachyury (T) and tufted (tf) on the murine seventeenth chromosome. This lethal factor permits recombination between T and tf and does not affect the transmission of its seventeenth chromosome. Its effect upon embryonic development is similar to that of the t0 haplotype. It would appear to represent a point mutation of a single gene, t-complex lethal zero (tcl0).

Minute (M) lesions exhibit a striking propensity for interacting with many different mutations. In the past, few attempts have been made to explain these diverse phenomena. This study describes a variety of temperature-sensitive (ts) interactions exhibited by the ts third chromosome Minute mutation M(3)LS4Q-III (Q-III). Most of these interactions (i.e. those involving vg, cp, Dl, Dfd or Ly) reflect Q-III-induced enhancement of the respective mutant phenotypes at the restrictive temperature. However, Q-III also suppresses the extra-sex-comb phenotypes of Pc and Msc at 29 °C and evokes lethal and bristle traits when combined with J34e at the restrictive temperature. All of these interactions are characteristic of non-ts Minute lesions and thus they appear to be correlated with general physiological perturbations associated with the M syndrome. In addition, our findings show that mutations that affect ribosome production and/or function, namely su(f)ts67g and bbts−1, exhibit interactions comparable to those elicited by Q-III. Hence, in accordance with previous findings, we argue that most of the Q-III interactions can be attributed to reduced translational capacity at the restrictive temperature. Finally, reciprocal temperature shift studies were used to delineate TSPs for interactions between Q-III and vg (mid to late second instar), cp (about mid-third instar), Dfd (early third instar) and Dl (late second to mid third instar). We believe that these TSPs represent developmental intervals during which the respective gene products are utilized.

Genetic divergences between the wild mouse populations with various Robertsonian translocations from the Poschiavo Valley, Yugoslavia, Milan and the Apenninies, were estimated based on the mitochondrial (mt) DNAs. The mtDNAs isolated from the liver were analysed by agarose slab-gel electrophoresis after digestion with eight kinds of restriction endonucleases: BamHI, EcoRI, HindII, HindIII, PstI, HpaI, HpaII and BgII. These preparations were further used to make restriction maps, from which sequence divergence between each Rb variation was calculated to be 0·2–2·2%. These rather larger values appear to be in conflict with the present concept that the Rb variations occurred during the last several thousand years. Both, however, might be reconciled by assuming genetic introgression of the founder with a small number of Rb translocations into other subspecies populations genetically remote and the subsequent rapid accumulation of Rb translocations unique to each population due to an unknown mechanism occurring specifically in the intersubspecies hybrids between M. m. domesticus and the other M. m. subspecies. This was the case also in a new Rb (9.15) translocation obtained from Ogasawara Islands in Japan which was the intersubspecies hybrid between M. m. molossinus and M. m. domesticus.

Genetic traits associated with the hybrid dysgenesis syndrome were quantified in strains deficient in two major host-coded DNA repair pathways, post-replication and excision repair. A defect in either (or both) pathway(s) fails to influence the frequency of male recombination or sex-linked recessive lethal mutations associated with hybrid dysgenesis, suggesting that the DNA transposable elements associated with this syndrome act independently of these cellular functions. However, when the post-replication repair pathway is blocked, the recovery of second chromosomes containing factors associated with hybrid dysgenesis activity is reduced. The decrease in recovery is associated with zygotic lethality.

Mice carrying particular deletions of chromosomal material at and around the colour (C) locus on chromosome 7 (c3H/c6H) are viable but sterile. An insertional translocation of chromosome 7 into the X (Cattanach's) has been previously used to rescue females carrying deletions of chromosome 7 which are ordinarily lethal. We studied the ability of this translocation to correct the sterility found in the presence of the two partially complementing deletions. We predicted that the sterility would be corrected in females who would be mosaics because of X-inactivation. The result in males was uncertain since the sterility had been shown to be due to defective spermatogenesis, and the X chromosome is inactivated early in the course of spermatogenesis. The c3H/c6H male and female deletional sterility were rescued by Cattanach's translocation.

Changes in brain size, body size and their co variance are reported from a long-term replicated directional selection experiment on body weight gain in rats. Two strains had been selected for increased and two for decreased weight gain between 3 and 9 weeks of age, and there were two randomly selected control lines. Selection produced significant changes in body weight in all selected lines. Divergence from the controls occurred in brain size in those strains selected for increased weight gain; no significant divergence was found for the strains selected for decreased weight gain. Divergence among unselected control lines suggests that genetic drift occurred in expression of brain size. Sexual dimorphism in response to selection results from sex differences in heritabilities and genetic correlations in relevant traits. In spite of considerable change in body size and brain size, no significant change in their covariation occurred either between the selection lines or between sexes. The relevance of these results to a brain and body size ‘scaling effect’ during evolutionary divergence is discussed.

Knobbly, FuKb, a dominant mutation on chromosome 17 of the mouse, causes a kinked tail in heterozygotes and embryonic lethality at about nine days in homozygotes. Abnormal organization of the embryonic ectoderm is first apparent at about embryonic day 7, and the retarded and malformed embryos die by mid 9 days.

Kink, FuKi, a dominant gene that maps in the same region, is also an embryonic lethal in homozygotes; heterozygotes have kinked tails and often a circling gait. We have shown FuKb and FuKi are allelic; the FuKb / FuKi compound dies at the same time as FuKb / FuKb embryos with the same morphological syndrome.

The FuKb phenotype is not seen in offspring in expected ratios. The FuKb males may have a low transmission ratio or penetrance may be incomplete.

An incidental finding of our histological studies is a high incidence of complete twinning when FuKb heterozygotes mate inter se or when FuKb × FuKi matings are made.

The heterogametic sex in birds (ZW) is female for presence of the W or for lack of a second Z chromosome. These alternatives can be distinguished given ZO or ZZW aneuploids and segregation of a Z-linked marker in their progeny. Having discovered the Z-linked cinnamon locus with two alleles (B, black eyes; b, red eyes), Durham observed 21 black-eyed daughters (BO or BbW) of bb cocks and B hens. Using Durham's data on sex–colour phenotypes of 14 young of two of the 21 matroclinous females show that the sex chromosome constitution of the two exceptional hens were more likely ZO than ZZW. This case plus that of Crew's (1983) trisomic rooster (ZZW) shows that sex determination in birds follows the ‘genic balance’ scheme as in Drosophila, not the ‘dominant Y’ scheme as in mammals.

By means of grafting skin in which the epidermis of one genotype was combined with dermis of another to a nude host, we have shown that the effect of the mutant at the tabby (Ta) locus on follicle initiation in tail skin was not confined to either the epidermis or the dermis, that the effect of Ta on the morphology of body hairs was determined by the epidermis but that the close proximity of epidermis of a different genotype also affected body hair morphology.

We measured traits involved in physiological and behavioural thermoregulation in the 6 replicates of a selection experiment for large and small size (6-week weight) in mice, including control lines (18 lines in all). The observed genetic correlations between body size and thermoregulatory traits are consistent with a thermoregulatory advantage of large size, including decreased weight-specific food consumption and increased nest-building, with no change in body temperature. The differences in food consumption were closely paralleled by differences in amount of brown adipose tissue, strongly suggesting that much of the decreased efficiency of the small lines is due to heat production by brown fat. These results are consistent with available observations on natural populations, that selection for temperature adaptation probably has had some influence on body size in this species.