By physical and genetic analysis of deletion mutants of the narrow host range IncP-10 P. aeruginosa conjugative plasmid R91–5 it has been shown that the phenotypes related to its transfer, namely the inhibition of the replication of the phage G101, entry exclusion and the fertility inhibition of the wide host range plasmid R18 map at kilobase coordinates 19·7–20·7, 18·5–19·7, 28·8–30·15 and/or 34·9–36·15, respectively. These locations have been confirmed by cloning restriction enzyme generated DNA fragments of R91–5 into a small broad host range, multicopy non-conjugative plasmid.

The position and orientation of an interchange quadrivalent in flattened lateral views of metaphase I were studied in inflorescences of an interchange heterozygote of Allium triquetrum showing high, medium and low alternate orientation frequencies. As the overall frequency of alternate orientation changes in these different inflorescences so the positioning of the alternately oriented quadrivalents changes, in that it becomes more like that of the adjacent quadrivalents. The present data argue for the biological significance of the position/orientation data, and imply the presence of two components of alternately oriented quadrivalents with different positional characteristics.

Lactate dehydrogenase regulator gene Ldr-1 controls the amount of the lactate dehydrogenase A subunit in rat serum and the amount of lactate dehydrogenase B subunit in rat erythrocytes because no recombinant rats were found among 295 pups produced in the BDIV × (ACI × BDIV)F1 and (ACI × DI) Fl × DI backcrosses. Two alleles regulate the high (Ldr-1a) and low (Ldr-1b) activity enzyme. The Ldr-1 is not linked to hooded coat pattern or glyoxalase-1. Sex and Ldr-1 assorted independently; hence the gene is autosomal.

Chromosomal localization of a gene involved in sisomicin production of Micromonospora rosea n.sp. was suggested by the results of matings between antibiotic producer and non-producer auxotrophic mutants. The frequency of producers among prototrophic recombinants varied with the chromosomal markers used for selection. The sis allele of the parent which was able to overgrow its partner in mixed culture predominated amongst the recombinants.

The electrophoretic expression of the X-linked enzyme glucose-6-phosphate dehydrogenase was examined in heterozygous Macropus robustus embryos and their extra-embryonic membranes. The amnion and allantois, like the somatic tissues of the embryo proper, showed paternal X inactivation while the avascular and vascular yolk sac cells showed evidence of activity of both maternal and paternal alleles.

We report here the occurrence of a new spontaneous mutation at the tufted locus present within the crossover-suppressed region of the complete t haplotype fwLub1 This new tufted allele shows complete expressivity of the tufted phenotype without any apparent effect on viability or reproduction. The presence of a tufted mutation within a t haplotype that is cytologically marked by a Robertsonian translocation is particularly useful in genetic experiments aimed at understanding the structure and function of t haplotypes.