Salmonella typhimurium strains of biotype 25x have been shown in transductional cross experiments to be clonal in the Nad character. The ancestral bacterium, probably of biotype 25a, mutated to a requirement for nicotinamide and subsequently diversified in phage type and secondary biotype characters. Such a sequence of events indicates interconversion among phage types 6, 16, 46, 49, 73, 76 and 135. Strains in biotypes 1x, 9ix, 17x, 17dx, 19dx and 25hix yielded Nad+ recombinants in interbiotype crosses, suggesting that each originated as an independent mutant line.

A natural population of the tetrapolar fungus Coriolus versicolor (L. ex Fr.) Quel, in a small woodland was observed over 3 years. Dikaryotic members of the population were identified using intraspecific antagonism and mating type markers. Records were kept of the distribution, longevity and fruiting behaviour of 44 individuals. Zone lines between individuals were found to remain constant in position throughout the study, but rates of decay and fruiting times varied between dikaryons and did not seem related to resource size. Detailed maps of the genetic structure of the populations in two stumps are presented. In general the results indicated thousands of alleles at both mating type loci. Repeated occurrences of alleles were very rare, when the data were interpreted rigorously in terms of the probable origins of the nuclei found in individuals. This approach was also applied critically to some published data. Comparisons were made between alleles found in local and more widely gathered samples, and led to estimation of worldwide frequencies but it is concluded that such exercises are of limited value.

The genetic basis of slow growth rate and aberrations in the ciliary pattern was studied in the multi-left-marginal variant of Paraurostyla weissei. The 3:1 segregation in F2 sibling crosses and 1:1 segregation in test crosses indicate that the aberrant phenotype is controlled by a recessive allele at a single gene locus termed mlm. The phenotypic change from wild type to mlm takes place about 5–8 cell cycles after conjugation. The study established that total conjugation in P. weissei is a true sexual process in which meiosis, fertilization and Mendelian segregation occur.

Crossover frequencies within three overlapping heterozygous paracentric inversions of differing length in the long arm of chromosome 1 of maize were estimated from cytological data. These frequencies were found to be close to normal for the regions involved and directly proportional to the respective physical lengths of these inversions. It is suggested that if pairs of centres which are specialized to regulate homologous pairing of the region between them exist in this part of the maize genome, they are most probably numerous and widely distributed.

The male mating ability and male fertility of 40 third chromosome homozygote lines has been measured. There was significant between-line differentiation for both characters, and comparison with a heterozygous stock indicated inbreeding depression and hence dominance variation for them. The characters showed significant positive correlation both with each other and with other fitness components and total fitness, as measured by Mackay (1985). This pattern of large positive correlations between fitness components is not expected to occur in outbred populations.

A tricomplex heterozygote has a synthetic chromosome complement consisting of four pairs of arms of chromosomes 2 and 3 in the form of a compound of two homologous arms (a homocompound) and of three compounds of two nonhomologous arms (heterocompounds), each being homologous to an arm of different compounds. In meiosis, pairing of homologous arms results in the formation of a single and a multiple configuration that are structural equivalents of a univalent and a trivalent. Data are presented indicating that, in a given complement, the pattern of the distribution of three heterocompounds at division I is the same in males and in females. The distribution depends on the arrangement of the 2nd and the 3rd chromosome centromeres in the trivalent. In configurations presumed to be homocentric (all three chromosomes having homologous centromeres), the distribution was random or nearly random while, in configurations presumed to be heterocentric, the distribution appears non-random, with one of the segregation alternatives being roughly twice as frequent as either one of the two other alternatives that were more or less equal in frequency. The results could be explained in terms of the 3rd chromosome centromere being ‘strong’ in directing the two 2nd chromosome centromeres to the opposite pole at division I, an explanation implying a functional differentiation of the two autosomal centromeres or adjacent sequences. Data are also presented showing that in females the distribution of the homocompound is non-random with respect to the distribution of the heterocompounds; the homocompound was recovered preferentially together with the single one of the three heterocompounds. This is inconsistent with the prediction based on the theory assuming an existence of two independent pairing pools.

A new allele of microphthalmia (mi) in the mouse was discovered among the progeny of a male that had been treated with the potent mutagen ethylnitrosourea. Homozygotes have white coats, mildly defective bone resorption and small eyes (about 60% of the normal size) with very little pigmentation. The iris and retina are abnormal, there is no vitreous body and iris pigmentation is restricted to a rim around the pupil. No haematopoietic defect was detected. Genetic studies showed that the mutation is linked to lurcher (Lc) on chromosome 6 and crosses with Miwh/ + and mi/ + mice indicate that the mutation is allelic with these two alleles of the microphthalmia (mi) locus. We designate the new allele microphthalmia-defective iris (midi). Some midi/+ heterozygotes (including the original mutant animal showed a bright ‘red-reflex’ when light was shone directly into the eye and this may have been caused by reduced choroidal pigmentation. Otherwise midi/ + mice appeared normal. The midi/mi compound heterozygotes had white coats, small eyes, and small teeth; bone resorption was more severely defective than in midi/mi homozygotes. The osteopetrosis was corrected by treatment of midi/mi mice with parental mi/ + bone marrow which suggests that the defect is intrinsic to midi/mi marrow cells. The coats of midi/Miwh compound heterozygotes were white; the irises were more symmetrical and iris pigmentation was less severely reduced than in midi/midi homozygotes but pupil dilation appeared to be restricted. Partial complementation occurred in the midi/Miwh compound heterozygotes with respect to eye size, which was usually near normal.

Ontogenetic series of phenotypic, additive genetic, maternal and environmental correlation matrices are presented and interpreted in the light of recent models for the Ontogenetic origin and variation in correlation between traits. A total of 432 mice from 108 full-sib families raised in a cross-fostering design were used to estimate the various components of phenotypic correlation for five live-body traits at eight ages. The level of genetic and phenotypic correlation decreased with age, while levels of maternal and environmental correlation remained more or less constant. Genetic correlations probably decreased due to compensatory growth. Phenotypic correlations decreased primarily due to the relative decrease in importance of highly correlated maternal effects and consequent increase in poorly correlated environmental effects as portions of phenotypic variation. The effect of compensatory growth on genetic correlation was also responsible for a portion of the decline in phenotypic correlation. Phenotypic correlation patterns remained constant over the ages studied here. It also seems likely the genetic, maternal and environmental correlation patterns do not change with age for the characters analysed.

The average effect and average excess of a gene substitution are formulated in terms of gene frequencies and inbreeding coefficient. This clarifies their meanings and shows how each is affected by non-random mating. The meanings of various definitions are examined, and one commonly used definition of average effect is found to be invalid with non-random mating. The concept of breeding value is shown to have no useful meaning when mating is not random.

Genetic polymorphisms revealed two distinct substrains of the inbred strain 101. One group included substrains 101*/Rl, 101/H and 101/HOxe; the other group comprised 101/El and 101/Sl. The two groups differed at 5 of the 8 genetic loci tested. The accompanying paper (Evans, Burtenshaw & Adler, 1985) shows that the two groups also differ for several chromosome polymorphisms. We suggest that genetic contamination occurred during the derivation of 101/El from 101/Rl and was already present in 101/El when 101/Sl was produced from this substrain. We further propose that these substrains be renamed 102/El and 102/Sl respectively.

A centromeric heterochromatin (Hc) comparison of two substrains of the mouse inbred strain 101 with another two, revealed differences in at least two and possibly in up to six chromosome pairs. The differences could have arisen by the inclusion of chromosomes from the inbred strain C3H/He.