The t-haplotype t6 interacts with brachyury, T, to produce taillessness, and exhibits lethality, male sterility and high male segregation ratio. Mutant haplotypes derived from t6 which have lost the lethality also lose the sterility. We here report that two mutants which have retained the lethality have also retained the sterility and suggest that the two properties of lethality and sterility are due to a factor or region of the t-haplotype located close to the locus of tf. Although t6 has a high male segregation ratio, mutants derived from it may have low or normal ratios. In trans heterozygotes of two mutant haplotypes the ratios tended to be equalized, whereas in cis heterozygotes a high ratio like t6 was obtained, at least in one case. On this evidence the low-ratio gene Low is reinterpreted as a mutant t-haplotype and designated tlow. It is suggested that abnormal ratio is due to an A-factor or region located between the loci of T and tf, so that t6 carries T-, A- and LS-factors which can exist separately and retain their properties. t6 may extend less proximally than other natural t-haplotypes, which in general may involve changes in interstitial heterochromatin.

Systems for the detection of postmeiotic segregation in unordered octads are described for 4:4(2), 6:2(2), 2:6(2), 5:3(3), 3:5(3) and 4:4(4) segregations, where the number in parentheses is the number of pairs of non-identical sister-spores. Linkage relations, centromere distances and phenotypic detectability of suitable combinations of markers for initial selective identification of such segregations, and subsequent confirmation tests, are discussed; correction formulae are given to allow for non-detection of certain classes in particular circumstances. Using one such system in Ascobolus immersus, we detected 4:4(2), 6:2(2) and 2:6(2) segregations at the w-78 site: 2:6(2)s were previously unknown. Initial visual detection in this system was from octad phenotype ratios for w-78 (white ascospore) and gr-3 (granular ascospore pigmentation), with mating type and parafluorophenylalanine resistance as confirmatory markers after germination. Corresponding-site interference for wider ratio and – for the first time – narrower ratio unique classes was determined: it was moderately positive or moderately negative, depending on the cross and segregation class involved.

The frequency of adjacent-2 disjunction in mice carrying the reciprocal translocation T(9; 17)138Ca was studied by mating together animals heterozygous for the translocation and carrying different recessive marker genes, using Tt for chromosome 17 and cwcw for chromosome 9. The proportion of marked young arising from adjacent-2 disjunction varied according to the markers carried in the two parents. When the female carried Tt the frequencies of marked young were always higher than when non-T females were used, and when Tt and cwcw were carried in the same parent there was a shortage of marked young obtaining both copies of the proximal region of chromosome 17 from the father. Both these effects were regarded as probably another example of the phenomenon discovered by Johnson, of inviability of young lacking a maternal homologue of a certain region of chromosome 17. There were other variations in frequency of marked young, among crosses using non-T females, which may have been due to differences in transmission ratio of male gametes carrying various t-haplotypes or to true variations in frequency of adjacent-2 disjunction.

Evidence is presented for the existence of a new allele at the patch locus called patch-extended (symbol Phe). Crosses of Phe with Ph, Wv and Rw produce double heterozygotes that are different from those resulting from the original patch allele. Besides the different coat colour in these mice, all the Wv + / + Phe animals that were tested resembled Wv + / Wv + and proved to be sterile. However, the two patch alleles interact to produce a split-face phenotype which resembles the presumed Ph/Ph.

The recessive mutant gene downless (dl) causes abnormal texture of the coat and absence of hair on the tail. The dl locus had previously been shown to act in the epidermis and not in the dermis. To obtain evidence on the pattern of proliferation of epidermis, downless ↔ normal chimaeras were produced by embryo aggregation, and the pattern of normal and mutant hair in the coat was examined. The chimaeras showed a pattern of narrow transverse stripes of normal and abnormal hair. This pattern was similar to that found in mice chimaeric for alleles at the agouti locus known to act in the dermis. This evidence supports the conclusion that the pattern of cell proliferation is similar in dermis and epidermis, and is compatible with the hypothesis that both tissues proliferate by lateral coherent clonal growth from a randomly mixed array of longitudinally arranged cells.

This paper describes a method for selecting for conditional lethal mutations in the filamentous fungus Schizophyllum commune. The method is, to our knowledge, the only successful application of filtration enrichment to Basidiomycetes. In addition, it is far more powerful than filtration enrichment methods reported previously for any fungus; it permits recovery of up to 80% auxotrophs starting with material untreated with any mutagen.

The inheritance of reaction to halothane anaesthesia (so-called malignant hyperthermia syndrome) in pigs was studied in a series of 61 litters by 18 sires from an F3–F4 Pietrain–Hampshire crossbred population. A single locus two allele model allowing for reactors from all three genotypes, was fitted by maximum likelihood. It was concluded that the reaction is due to a single autosomal recessive gene with a frequency of 0·46 (0·44–0·48) and penetrance of 0·95 (0·92–0·98) in this stock.