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.

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.

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.

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.

The esterase 6 (Est-6) locus in Drosophila melanogaster is located on the third chromosome and is the structural gene for a carboxylesterase (E.C.3.1.1.1) and is polymorphic for two major electromorphs (slow and fast). Isogenic lines containing X chromosomes extracted from natural populations and substituted into a common genetic background were used to detect unlinked factors that affect the activity of the Est-6 locus. Twofold activity differences of esterase 6 (EST 6) were found among males from these derived lines, which differ only in their X chromosome. These unlinked activity modifiers identify possible regulatory elements. Immunoelectrophoresis was used to estimate quantitatively the levels of specific cross-reacting material in the derived lines. The results show that the variation in activity is due to differences in the amount of EST 6 present. The data are consistent with the hypothesis that there is at least one locus on the X chromosome that regulates the synthesis of EST 6 and that this regulatory locus may be polymorphic in natural populations.

Two mouse populations, randombred albino mice and a cross of four inbred strains, were divergently selected for high (H8) and low (L8) 8-week body weight over 18 generations using within-family and individual selection. The crossbreds showed asymmetry of selection response and realized heritabilities (H8 0·29 ± 0·01; L8 0·17 ± 0·01). In the randombred population realized heritabilities were symmetrical (H8 0·23 ± 0·01; L8 0·22 ± 0·02). Over the first nine generations individual selection was nearly 40 per cent better than within-family selection, as was expected from the full sib correlation in both populations. As selection progressed, within-family selection reached 82% and 61% of the responses obtained with individual selection in the crossbreds and randombred respectively. Correlated responses for 3-week (weaning) and 5-week body weights agreed with observations made on direct responses, but selection for L8 did not reduce weaning weight. Selection for L8 decreased and selection for H8 increased first litter size at birth. However, mass-selected L8-pairs had a higher life-reproduction and life-span than H8-pairs.