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.

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.

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.

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).

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.

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.

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.

The shape of the mandible of the mouse can be described by a series of discriminant functions which have been used to discriminate between and investigate relationships among strains of mice. The effect of sex and variation in age of the animal on the results obtained from these functions has been investigated. Significant sex differences in mandible shape were detected, but these were considerably smaller than the differences found between two inbred strains. A simple correction for expressing each measure as a proportion of the sum of the measures on each bone removes the effects of overall size. Significant age effects were found, but these were only large in animals under seven weeks of age where considerable changes are taking place in the relative lengths of bone measurements. Routine testing for genetic authenticity using the shape of the mandible is possible over a wide age range and may be an efficient method for monitoring genotypes at the end of long-term experiments.

Mice were selected for one of three criteria: appetite (A), measured as 4- to 6-week food intake, adjusted by phenotypic regression to minimize change in 4-week body weight, fat percentage (F), using the ratio of gonadal fat pad weight to body weight in 10-week-old males, and total lean mass (protein, P), using the index, body weight in 10-week-old males − (8 × gonadal fat pad weight). For each selection criterion, there were 3 high, 3 low and 3 unselected control lines. At generation 11, the high and low A lines diverged by 17% of the control mean and the realized heritability from within family selection of adjusted food intake was 15%. Selection for this character produced changes in body weight, gross efficiency from 4 to 6 weeks, and percentage of fat, the high lines being heavier, more efficient and less fat than the lows. The high and low F lines diverged by 80% of the control mean and the realized heritability of the ratio of gonadal fat pad weight to body weight was 44%. Selection for this character produced changes in total fat per cent, but little change in percentage protein, body weight, food intake or gross efficiency. The high and low P lines diverged by 40% of the control mean and realized heritability of the lean mass index (10-week weight − [8 × gonadal fat pad weight]) was 51%. Selection for an increase in the index increased body weight at all ages, food intake and 4- to 6-week gross efficiency. There was no change in percentage fat. Responses in the selected traits were not highly correlated, and the different lines provide an opportunity for investigating responses in physiology, metabolism and gene products.

The Ybb− chromosome has been previously shown to induce reduction of X chromosome ribosomal genes in Xbb / Ybb− or Xbb+ / Ybb− flies. These reduction events are presumed to arise as one of the two products of unequal sister chromatid exchanges, which result in both magnified and reduced products. Bobbed reduced chromosomes may also arise as products of other recombinative events such as intrachromatid deletions. In this report we use the Ybb− chromosome to reduce the number of ribosomal genes present on X chromosomes from two wild-type stocks under ‘non-magnifying’ conditions. We then show that the bobbed reduced X chromosomes show no detectable difference in their Southern blot rDNA patterns when compared with the parental wild-type X chromosome. This indicates that reduction events do not preferentially delete certain repeat classes, and supports previous observations that the repeat types present in the D. melanogaster X chromosome nucleolus organizer are not significantly clustered.