The 150 independently isolated thy− mutants of E. coli K12 Hfr 3.OSO were studied genetically and phenotypically. Variants were found among the mutants in respect to the lag period of thymineless death, and temperature sensitivity. The latter correlates with mutations located at a specific site on the genetical map.

The thy locus is located between the cys and ser/gly genes, and is a linear structure where 134 thy mutants are distributed over more than 17 sites. The site distribution of the mutants is not regular: about a half of them (62) are localized within one site and all these are temperature-sensitive.

Two further genes involving utilization of thymine—tlr and td—were found. Mutations of tlr lead to a reduced thymine requirement (0·5 μg./ml. instead of 20 μg./ml.). A mutation of td results in thymidine sensitivity.

This latter character is expressed when the td-s allele is transferred into E. coli K12, prototrophic for thymine, by conjugation. Thymidine inhibition can be reversed by the addition of any riboside to the growth medium. Both genes map at the proximal end of the Hfr 3.OSO chromosome and are linked with the thr gene. The most probable gene order is: tlr-td-thr.

The following results have been obtained from 14C-thymine incorporation experiments with wild-type cells, as well as with thy−tlr+ and thy−tlr− cells: (1) Wild-type cells incorporate exogenous thymine extremely poorly, but incorporate thymidine better. (2) The thy−tlr+ mutants are able to incorporate thymine only when high concentration are used, but can utilize a low concentration of thymidine. (3) The thy− mutants are able to incorporate exogenous thymine as well as thymidine at low concentration. (4) The tlr mutation is a thymine-specific one.

Mutants of Hfr strains of Escherichia coli K12 defective in conjugation owing to failure to produce sex pili were isolated by resistance to F-specific phage. Tests of the ability of six de-repressed F-like R factors and four de-repressed I-like R factors to restore Hfr donor behaviour to these mutants indicated that there were several ways in which such restoration could occur. Of a total of 26 defective mutants, 12 were restored by F-like R factors. In these twelve, the function of the integrated F factor was evidently restored, because the sex pili contained F pilin subunits, distinguishable from the R pilin by serological tests. In contrast, among the four I-like R factors, only two were effective, butin all 26 defective mutants; the restored Hfr bacteria produced only I-like pili. The I-like sex factors, in restoring Hfr donor behaviour, did not therefore act by complementing the defective F.

The basic recurrence equations given in our previous paper have to be modified when males and females are differentiated. The modifications we gave are incorrect because we ignored the deviations from Hardy–Weinberg equilibrium when sampling and selection are applied separately to the two sexes. Appropriate corrections are presented in this paper.

Genes or sequences of DNA present in multiple copies per cell include entire genomes of mitochondria and chloroplasts, nuclear ribosomal RNA genes, and highly repetitive sequences in heterochromatin. All copies are nearly identical, in spite of mutational pressure and weak selection. A zygote containing mitochondrial or chloroplast genophores of two different genotypes quickly produces progeny pure for one genotype or another (vegetative segregation). Evidence from yeast and Chlamy-domonas suggests that organelle genophores undergo repeated rounds of random mating and recombination. When two molecules carrying different alleles at a locus recombine, gene conversion can result in the cell becoming pure for one allele. Stochastic matching and conversion (SMAC) has been studied by computer simulations which suggest that it will tend to eliminate new mutations in yeast mitochondrial DNA and speed up vegetative segregation. We have verified previous suggestions that gene conversion, occurring during unequal mitotic sister-strand crossing-over, provides an efficient mechanism for maintaining the homogeneity of repeated sequences in eukaryotic chromosomes.

A new metric character in mice, ‘footpad bristle number’ is described. The character is easily measured under a stereo-microscope, is very highly inherited, and is independent of age, sex and the environment common to littermates. Such a character might prove useful in a number of quantitative genetic studies in mice.

Body size in Drosophila pseudoobscura is a continuously varying character with a high heritability; it is almost certainly related to fitness. Natural populations of D. pseudoobscura from Canada to Mexico have been sampled and found to vary geographically in body size. The geographic variation for the genes determining size is to some extent correlated with the physiographic division of the West. The populations from the Pacific coast have genetically smaller flies than do those from the interior provinces. Experimental populations derived from the samples of seven widely separated natural populations were crossed to yield F1 and F2 hybrid generations. Body size in the F1's varied irregularly, while the F2's showed a consistent ‘breakdown’, the F2's being significantly smaller than their F1 parents. The F1's were significantly less variable than their parents, while the F2's were significantly more variable than their parents of the F1 generation. The natural populations possess coadapted genetic systems, with genes mutually adjusted by selection for favorable interactions. Recombination disrupted the balanced genic complexes to give the F2 breakdown and the increased F2 variability. D. pseudoobscura differs from D. subobscura in showing the effects expected in crosses between coadapted systems. This species difference lends additional support to the hypothesis that the gene pools of these two successful species respond in different ways to environmental variation. The gene pool of D. pseudoobscura is flexible and changes readily, while that of D. subobscura is relatively rigid.

The inactive-X theory of dosage compensation postulates that in all somatic cells of adult female mammals one or other of the two X chromosomes is genetically inactive. This means that in a female heterozygous for two non-allelic genes acting through the same cells, and carried one on each X chromosome, one or other gene should act in all cells. Conversely, if the two genes are carried on the same X, then both genes should act in some cells and neither gene in the remainder. This point has been tested by breeding experiments with mice, using pairs of genes affecting coat colour and coat texture. In female mice carrying the colour mutant dappled, Modp, on one X and a translocation including the wild-type alleles of pink-eye, p, and albino, c, on the other, either Modp or the translocation acted in all cells. With the genes tabby, Ta and striated, Str, affecting coat texture, in Str + / + Ta females tabby acted only in the non-Str patches, while in StrTa/ + + it acted only in the Str ones. Thus these experiments confirm that only one of the two X chromosomes is active in the somatic cells of female mammals.

It has been suggested that isochores are maintained by mutation biases, and that this leads to variation in the rate of mutation across the genome. A model of DNA replication is presented in which the probabilities of misincorporation and proofreading are affected by the composition and concentration of the free nucleotide pools. The relationship between sequence G + C content and the mutation rate is investigated. It is found that there is very little variation in the mutation rate between sequences of different G + C contents if the total concentration of the free nucleotides remains constant. However, variation in the mutation rate can be arbitrarily large if some mismatches are proofread and the total concentration of free nucleotides varies. Hence the model suggests that the maintenance of isochores by the replication of DNA in free nucleotide pools of biased composition does not lead per se to mutation rate variance. However, it is possible that changes in composition could be accompanied by changes in concentration, thus generating mutation rate variance. Furthermore, there is the possibility that germ-line selection could lead to alterations in the overall free nucleotide concentration through the cell cycle. These findings are discussed with reference to the variance in mammalian silent substitution rates.

The rDNA of five Y chromosome mutants was examined with respect to their insert free (In−) repeat type multiplicity. The In− repeat number of each mutant was correlated with its hemizygous bobbed phenotype and additivity with an X NO bobbed (bb) mutant. Four of these mutants showed a direct relationship between their In− frequency, hemizygous bb phenotype and additivity tests. A fifth mutant, bb1–4, had a sufficient number of In− repeats to ensure viability to the late pupal stage and show additivity; however, the In− repeats genetically behaved as a complete rDNA deletion. Possible mechanisms resulting in the suppression of the bb1–4 In− repeats are discussed.

Cultivated plants of Nicotiana alata are self-incompatible and are of two kinds: normal (N); and exceptional (M). N plants are reciprocally compatible with N. langsdorffii; M plants are compatible only as males. M plants contain an unusual allele, SFI, which has a dual action in the style: it rejects both self-pollen, and Sf pollen from N. langsdorffii. The overall results agree with the assumption that the SFI gene produces two kinds of specificity in the style: primary specificity, which is responsible for the rejection of Sf pollen; and secondary specificity, which is responsible for the rejection of self-pollen as in SI alleles generally. The genetic sub-units concerned must be closely linked; there was no evidence for their dissociation in the 599 plants studied.

In both compatible and incompatible pollinations, SFI pollen grows more slowly than SI and, in addition, appears to depress the normal rate of growth of SI pollen. In consequence, crosses SfSf × SISFI ♂ yielded significantly fewer S.I. plants than the 50% expected. The two kinds of pollen grew at comparable rates, however, when F1 (M × M) plants involving parents from different original sources were backcrossed to SfSf ♀. Progenies then showed the expected 1:1 ratio of S.I. to S.C. plants. These results are assumed to be due to differential behaviour of the SFI allele according to its genetic background. The change in background would be from a degree of homozygosity, in plants from the same source, to a degree of heterozygosity, in crosses between plants from different sources.

The high incidence of the SFI gene in N. alata is considered to be due to the advantage it confers on a self-incompatible population when it is overlapping with a related self-compatible population (having the Sf gene). Plants carrying an SFI allele, by rejecting the Sf pollen, will restrict the spread of inbreeding and so be favoured by selection.

The origin of the SFI and Sf alleles are discussed in relation to the author's hypothesis of S-gene structure.

1. The first known effect of the gene for shaker with syndactylism (sy) in the mouse is a reduction, both preaxial and postaxial, of the foot plates at the 12½-day stage. This reduction forces the blastemata of digits 2, 3 and 4 to be laid down more nearly parallel to each other, with a subsequent tendency for adjacent blastemata to coalesce with each other. The possibility is considered that this essentially quantitative reduction of the foot plate material may itself be the consequence of some more specific and qualitative (but hitherto undiscovered) change in the limb buds.

2. The physiological connexion between syndactylism and the systemic changes in the cartilaginous and osseous skeleton which arise later in sy/sy mice remains obscure.

The extent of accumulation of mouse Y chromosomal repetitive sequences generally correlates with the known phylogenetic relationships in the genus Mus. However, we describe here a M. musculus Y chromosomal repetitive sequence, designated as ACClfl, whose accumulation patterns among eight Mus species do not correspond to their phylogenetic relationships. Although male-specific hybridization bands were present in all the species examined, significant accumulation (> 200 copies) in the Y chromosomes was found in M. minutoides (subgenus Nannomys), M. pahari (subgenus Coelomys) and M. saxicola (subgenus Pyromys) as well as in the three closely related species M. hortulanus, M. musculus and M. spretus that belong to the subgenus Mus. Unexpectedly, the Y chromosomes of M. caroli and M. cookii (both subgenus Mus) had considerably reduced amounts of ACClfl-related sequences. Furthermore, in rats (Rattus norvegicus) the major accumulation sites appear to be autosomal. These observations suggest that caution must be taken in the interpretation of data obtained with repetitive sequences that have evolved quickly.