Mutants of Aspergillus flavus impaired in aflatoxin production were induced with N-methyl-N′-nitrosoguanidine and analysed by means of the parasexual cycle. The gene symbol afl was assigned to this type of mutation. Diploid complementation tests revealed that most of 14 afl mutants belonged to different complementation groups. One mutant (afl-1) failed to complement or only partially complemented all other mutants. Haploidization of one diploid revealed the independent segregation of two afl mutants. Heterozygous diploids were synthesized between two afl mutants and tester strains genetically labelled on eight linkage groups. Haploidization of the diploids led to the assignment of two aft mutants to linkage groups. Linkage of afl-4 to w+ and afl-1 to leu on linkage groups II and VII, respectively, was demonstrated.

A sex-linked recessive gene with visible effect will first be detected in the hemizygous sex (male). In lines with equal numbers of males and females, when the gene is initially present in a single female the probabifity of detection falls from 2/3 in single pair lines to 0·54 in large lines. The mean and standard deviation of time to detection are almost independent of population size, being about 4/3 and 2/3 respectively. About 98% of all detections occur within three generations, so a gene detected much later than this after the foundation of a selection line is likely to be a new mutant. Higher initial frequencies and selection favouring heterozygotes increase the chance of detection. The time taken is decreased with higher initial frequencies and increased slightly by selection favouring heterozygotes.

The genetic fine-structure of the niiA niaD gene region of Aspergillus nidulans has been studied using deletion mapping. Deletions were identified as niiA niaD double mutants and comprised 1% of spontaneous chlorate-resistant mutants. All such double mutants were shown to involve deletions and their frequency was not increased by mutagenic treatment with either N-methyl-N′-nitro-N-nitrosoguanidine or with ultraviolet light. Deletion maps of the niaD and niiA genes have been constructed. A further class of mutation was also mapped using the deletions. These crn mutations, which affect a gene whose function is as yet unknown, map on the centromere proximal side of niiA. This analysis of a eukaryote gene cluster will provide a framework upon which to base studies of the regulation of the nitrate assimilation pathway.

Over 100 temperature-sensitive mutants of mt-h (apogamic) strains of Physarum polycephalum were isolated either by testing clones of mutagenized amoebae (ATS mutants) or by the more laborious method of testing plasmodia derived from such clones (PTS mutants). When amoebae and plasmodia of each mutant were tested for growth temperature sensitivity on different media (to give optimum growth of each phase), only 21% of 73 ATS mutants and 32% of 31 PTS mutants appeared to be temperature-sensitive in both phases, suggesting that the majority of mutants are phase-specific, as concluded from several similar studies by previous authors. When the mutants were tested on a third medium which allows growth of both amoebae and plasmodia, many of the mutants no longer had a temperature-sensitive phenotype in either phase. Among the remainder, 51% of ATS mutants and 67% of PTS mutants were temperature-sensitive in both phases. It was suggested that certain media have a remedial effect on some temperature-sensitive mutants so that the phenotype is apparently normal. Thus, the proportion of phase-specific mutants may be over-estimated if tests of temperature-sensitivity are done on the different media commonly used for culture of amoebae and plasmodia respectively. It was concluded that the most efficient procedure for isolation of temperature-sensitive mutants expressed in plasmodia is to screen clones of amoebae on a medium resembling as closely as possible that which is to be used for testing plasmodia.

Alloimmunization in sheep revealed an antigen specificity B1 which is also to be found in cattle sera. The molecular weight of the protein carrying B1 was greater in sheep than in cattle, whilst in both cases its isoelectric point was at pH 6·2. The electrophoretic position of the antigen suggests that the molecule carrying B1 might be a β-globulin. Family studies indicated that in both species B1 was inherited in a simple Mendelian manner, as if it were a product of a dominant allele B1 at an autosomal locus B.

Macronuclei assorting simultaneously for H, Chx, Mpr, and co, and containing only one or two copies of the HD allele produced several combinations of phenotypes at the other loci, instead of only one or two such combinations. It follows that macronuclear subnuclei, if they exist at all, must frequently exchange parts. Models involving somatic recombination, transient subnuclei, or progressive chromosome fragmentation are discussed as possible explanations.

New experimental designs have detected unexpectedly large variations in the time at which macronuclear assortment begins, and in the ratios of the stabilized products. Variation is detected between strains, and, within strains, between conjugating pairs. The Chx locus gave results ranging from late assortment (40–60 fissions) to early assortment with skewed input, indicating the existence of some relation between the parameters of input ratio and time of determination.

Using interchanges T(1, 3) and T(3, 6) of Pennisetum americanum orientation types of the interchange multiple at MI were studied in different genetic backgrounds. Orientation types alternate 1 and alternate 2, in addition to adjacent 1, adjacent 2 and indefinite could be identified for both the interchanges. The relative frequencies of various orientation types were influenced by a change in the genetic background.

For these interchanges, homologous centromeres do not seem to play a predominant role in the co-orientation of interchange multiple. The non-homologous co-orientation types were more subjected to genetic regulation than the homologous co-orientations.