The expression of X-linked phosphoglycerate kinase (PGK-1) in germ cells from embryos heterozygous for both PGK-1 and Searle's translocation T(X; 16) 16H was examined to investigate X chromosome activity during oogenesis. The Pgk-lb allele on the translocated X chromosome was the only allele active in somatic cells of all embryos and in germ cells from 12·5 d.p.c. embryos. However, an additional faint band representing Pgk-la activity was observed in germ cells from older embryos (13·5–18·5 d.p.c.) and neonates (1–2 d.p.p.). It is concluded that there is a period when only one X chromosome is active in early female germ cells and that reactivation of the inactive X chromosome takes place just prior to meiotic prophase.

Partially dominant mutations to carboxin resistance occur in three, freely recombining, nuclear genes in Aspergillus nidulans. Mutations at all three loci reduce carboxin inhibition of succinate dehydrogenase (EC 1.3.99.1), succinate-cytochrome c reductase (EC 1.3.99.1) and succinate oxidase (EC 1.3.99.1) in mitochondrial preparations. It is therefore probable that the ability of carboxin to prevent growth of A. nidulans is a direct consequence of its ability to prevent succinate oxidation.

Curly-whiskers (cw) is a recessive gene which was found in 1958 by Mr C. J. W. Smith of the Chester Beatty Research Institute, London. It arose in a subline of the CBA/Cbi inbred strain. The first mutant animals were one male and one female in a litter of five. The two mutants were mated together and a sib-mated subline was continued from them in which 500 mice were bred, all of which were curly-whiskered. This established the mutant to be fully penetrant. Curly-whiskers resembles the hair-waving genes in causing waving of the vibrissae, but it has no obvious waving effect on the hairs of the coat. The coat texture is, however, slightly abnormal and Mr Smith noted also that on the CBA background there was an appreciable darkening of the coat colour. Homozygotes (cw/cw) are easily classifiable soon after birth by the curled vibrissae. Heterozygotes (+/cw) often have slightly curled vibrissae, and the gene is therefore not fully recessive; but the distinction between +/cw and +/+ could not be relied on, and in the linkage tests cw was treated as a recessive gene.

We found that la is located in linkage group XVIII. It is highly probable that la and tg are alleles, and closely, linked to Es-1. Mice of the genotype la/tg are abnormal, with clinical signs similar to tg, although more severe. They develop earliest signs at about 15 days of age, similar to la, are runted but fertile and can live for months. Clinical signs are ataixa, stiffness, retarded motor activity and intermittent focal seizures. The pathological basis for these symptoms is still elusive. The three types of mice, la/la, la/tg and tg/tg are thus distinct clinically, la/tg resembling in some respect either of the other two.

The frequency and distribution of P elements were investigated in the third chromosomes of two wild-type strains of Drosophila melanogaster using in situ hybridization of biotinylated probes to the polytene chromosomes. The relationship between these data and the extent of hybrid dysgenesis was determined through assays of egg production, egg hatchability (F2 embryo lethality), snw destabilization and male recombination along the third chromosome. The results suggest that P-element distribution, frequency and structure are all contributory factors in the regulation of hybrid dysgenesis. Texas 6 was shown consistently to be a stronger P strain than Texas 1, eliciting greater reductions in fertility, more extensive snw destabilization and higher frequencies of male recombination. Clustering of male recombination events, arising from pre-meiotic crossing over, was evident among the dysgenic progeny of each strain. Male recombination and snw destabilization were independently distributed among the dysgenic males studied, suggesting that these traits represent separate P-mediated functions. The third chromosome male recombination maps produced by the two strains differed significantly from each other and from the published female meiotic and polytene chromosome maps. Male recombination breakpoints were associated with the original distribution of P sequences in the two strains and the results suggest that this relationship may be closer for potentially complete P factors than for P sequences in general. An analysis of sub-lines derived from individual recombinant males revealed that chromosomal breakpoints could also be associated with novel insertions following P-element transposition.

An animal with low erythrocyte triose phosphate isomerase (TPI) activity was found amongst mice trapped on a farm in Leicestershire. The low TPI activity was caused by the segregation of a single co-dominant gene which also affected the Km for glyceraldehyde-3-phosphate and heat stability of the enzyme. We designate the gene Tpi-1, the structural locus for TPI, with the a allele in the common inbred strains and the b allele derived from the wild-caught mouse. Tpi-1 was known to be on chromosome 6 by somatic cell techniques and, as shown in a preliminary report (Peters & Bulfield, 1984), we have confirmed and extended this finding using three chromosome-6 marker genes giving the order: Sig-28-Lc-11-Miwh-16-Tpi-1.

Doublets of Oxytricha fallax possess two complete sets of ciliature. The doublet phenotype is inherited through sexual and asexual reproduction as a cortically determined trait. The trait is also inherited through cystment, independent of cyst size. Prior work shows an absence of all visible cortical organelles, except cell membranes, in the cyst; thus the visible structures are not themselves determinative.

Results from excision experiments performed on encysting doublets indicate that the determinative difference between doublets and singlets is the presence of one or two ‘determinative regions’ located on the ventral surfaces of the organisms which serve as sites for the initiation of ciliary primordium development. Doublets possess two such areas, but singlets possess only one.

Five allelic mutants of Neurospora which lack glutamic dehydrogenase (am mutants) were induced to revert with ultra-violet. The glutamic dehydrogenase produced by the revertants was compared to that of wild-type. Several distinct classes of revertants could be distinguished by these tests. However, genetic analysis showed that all the reversions resulted from events at or near the site of the original am mutation. The spectrum of reversion types depended on the nature of the am mutant employed. One mutant, which produces an am protein believed to be altered in its active site, yielded revertants which were all indistinguishable from wild-type. Another mutant, which produces a protein with a functional active site but altered folding properties, gave at least six classes of revertants which were different from wild-type.

A new mutant (Wct) has been identified at the W locus of the mouse. The homozygote is poorly viable. Whereas the heterozygote (Wct / +) is only mildly anaemic like Wυ / +, the double heterozygote Wct + / + Ph is considerably more anaemic than Wυ + / + Ph and it and Wsh + / + Ph have significantly raised leucocyte counts. Wct + / + Ph is also unduly radiosensitive to whole body X-irradiation, 50% dying from haematopoietic failure at a dose of 4·59 ± 0·14 Gy, whereas the median for Wct / + was 6·49 ± 0·28 Gy. Serial blood counts of mice after low- or sub-lethal doses of X-rays revealed significantly more profound depression of counts of both red cells and leucocytes in Wct +, and more notably in Wct + / + Ph, than in + / + or Wsh / + (haematologically normal) iso-dosed mice. We conclude that control of haematopoiesis by chromosome 5 is not confined to the W locus but is shared by the linked gene Ph (and perhaps Rw) and that expression of the change is not limited to the erythron but involves the pluripotent haematopoietic stem cell.