An effective breeding policy is described for detecting linkage in the mouse, for an ante-natal lethal from crosses of heterozygotes with stocks homozygous for several linked recessive markers. A method of analysing the ensuing data is described, together with a method of estimating the map distance to close markers. A practical illustration is described, involving an ante-natal lethal obtained from a colony of wild mice and a four fold recessive chromosome 2 marker stock.

Numbers of bristles are reduced in the dorsocentral regions of achaete Drosophila melanogaster. In achaete tissue of mosaics the effect is not uniform, and near the clone boundaries bristle numbers are significantly higher than they are elsewhere in the clone. It is argued that the cause of this non-autonomy stems from ‘factors’ that spread into the achaete clone from surrounding non-achaete cells.

Selection was practised in two replicates for both high and low testis weight in the mouse. Typically 7 males were selected out of 30 recorded for a total of 5 generations. From an initial average of 191 mg the mean divergence between high and low lines reached 112 mg, with a realized heritability of 0·52. The ovulation rate of the lines changed in the same direction as that of selection, the mean divergence was 2·0 eggs in primiparous females in generation 4 and 1·6 in nulliparous females in generation 5. Correlated changes in the body weight of both sexes also occurred but were inadequate to account for the observed change in ovulation rate. The genetic regressions of ovulation rate on testis weight were estimated to be 2·9 and 14 eggs/100 mg in primiparous and nulliparous females, respectively, which, along with data from other experiments, correspond to genetic correlations between testis weight and ovulation rate of 0·50 and 0·25 respectively. There were no correlated changes in litter size. The possibility of using male testis size in breeding programmes to improve female reproductive performance is discussed.

A collection of Gasterosteus aculeatus from a single locality (Friant) in Madera County, California, contains individuals with low and high lateral plate morphology, and very few intermediates. Electrophoretic evidence on protein similarities at 15 genetic loci is compatible with the thesis that members of these two morphs belong to a single interbreeding population. This thesis is also supported by broods from laboratory crosses between morphs, which segregate for low and high plate counts. Laboratory crosses between Friant fish and those from geographically isolated populations often yield some progeny with intermediate plate counts. The demonstration of significantly different patterns of plate development in intralocality versus interlocality crosses evidences a contrasting genetic basis for plate determination in different populations of sticklebacks.

We have developed a recombination test in Bacillus subtilis that provides a tool for rapid genetic classification of mutants of identical phenotype. The test has been used to classify 25 ts mutants in nine recombination classes that have been proved by independent evidence to correspond to nine genetic loci.

Mutations have been detected which affect the production of protoperithecia in Sordaria brevicollis. These mutations have been called perithecial-1 and perithecial-2. Ascus analysis revealed that the former was a centromere unlinked gene, whilst the latter was very close to the mating-type locus. The per 1−per 2− genotype was female sterile. Recovery of fertility by a sterile culture occurred and perithecial analysis revealed that this recovery was due to the accumulation of revertant per 1+ nuclei in the mycelium. Per l−per 2+ colonies were fertile and gave a cross of characteristic appearance.

Lines of Triticum aestivum Chinese Spring (2n = 6x = 42) which were ditelocentric or doubly ditelocentric, in turn, for the 14 chromosomes of the A and B genomes were pollinated by Triticum urartu (2n = 14). The behaviour of the marked telocentric chromosomes was scored in the 14 distinct hybrids obtained from these pollinations. In 6 of the hybrids in which different A genome chromosomes were marked by telocentrics there were from 50 to 80% of the pollen mother cells in which the telocentrics were paired. In the seven hybrids in which different B genome chromosomes were marked the telocentrics were never paired. It was concluded that the genome of T. urartu matched very closely the A genome of hexaploid wheat and that it did not correspond, as had been proposed by Johnson, to the B genome. The pairing behaviour of the 14 T. aestivum × T. urartu hybrids was compared with earlier results obtained from hybrids between T. aestivum and T. boeoticum. It was proposed that the higher trivalent frequencies seen in the T. boeoticum hybrids could be due to homoeologous pairing and that the genotype of T. boeoticum has the capacity partly to suppress the activity of the Ph locus of chromosome 5B of wheat, as a result of which homoeologous pairing is normally prevented.

When T6/+ female mice were mated to non-translocation-bearing males, the relative viability of the embryos at 13·5–14·5 days gestation was about 39%. About 36% of the oocytes ovulated by T6/+ females were aneuploid, as a result of non-disjunction at meiosis, the majority having either 19 or 21 chromosomes. However, aneuploidy only accounts for a proportion of the embryonic loss in T6/+ × +/+ matings, as many of the embryos with 41 chromosomes survive postnatally. The present findings indicate that approximately 50% of the oocytes ovulated with the normal haploid number of chromosomes (n = 20) were genetically unbalanced as a result of adjacent segregation, and that a high proportion of the resultant embryos die in the early postimplantation period. In the present study non-translocation-bearing mice which were genotypically similar to the T6/+ females acted as controls.

We have no serious disagreement with the mathematical arguments presented by Ohta & Kimura (1975). We do, however, have reservations about their conclusions that ‘the hitch-hiking effect is generally unimportant as a mechanism for reducing heterozygosity’, and still prefer our own approach (Maynard Smith & Haigh, 1974).

Cytoplasmic petite mutants, spontaneous and induced, show various patterns of ability υ. inability to utilize the sugars galactose, α-methylglucoside and maltose, depending on the strain from which they were isolated. Petites recombine and segregate their different sugar deficiencies among vegetative diploid progeny when crossed, indicating mitochondrial control. Crosses between petites and wild-type further indicate that mitochondrial factors may be interacting with nuclear factors in a complex regulatory system.