A pedigreed control strain was divided into three lines which were maintained genetically distinct over nine generations. Mean coefficients of inbreeding of about 0·1 were attained. Results suggest that discernible genetic drift had occurred in several traits. Separate estimates of genetic variance were obtained from observed variation between and within lines. These estimates differed significantly in the case of only one of the eight traits observed. Observed variation between lines agreed reasonably well with that predicted.

A mathematical model for the kinetics of transfer of non-selftransmissible (nonconjugative) plasmids by mobilizing conjugative factors is presented and methods to estimate its parameters described. Using batch and chemostat cultures of Escherichia coli K-12 with the nonconjugative plasmid pCR1 and an F′ mobilizing factor, the parameters of this model were estimated. The observed changes in concentrations of the different parental and transconjugant cell types and the changes in these concentrations anticipated from the model are presented for two different ‘mating’ combinations in both exponentially growing and equilibrium chemostat populations of E. coli. The results of these experiments are interpreted to suggest that for bacterial populations dividing at a constant rate in liquid culture, the kinetics of mobilization transfer of nonconjugative plasmids can be reasonably well described by a simple set of mass action differential equations. These results also suggest that the carriage of the nonconjugative plasmid pCR1 has little, if any, effect on the capacity of a host bacterium to donate or receive conjugative F′ plasmids.

A study of crosses between CBA/FaCam and C57B1/6Fa mice revealed an effect of the origin of the Y-chromosome on testis weight and aggressive behaviour, but failed to reveal any effect on sexual behaviour and androgen metabolism. There is therefore no evidence that androgens mediate the Y-linked variation in aggressive behaviour and testis weight. On behavioural grounds, it is difficult to compare measures of sexual and aggressive behaviour, but it appears from these results that there are major genetic components on the Y-chromosome controlling the development of sexual and aggressive behaviour which are distinct.

Normalizing selection with a moving optimum can result in strong selection for increased recombination. This conclusion is based on computer simulation of an infinite, random-mating diploid population with additive, polygenic determination of the selected phenotype. There can also be selection for increased recombination with a fluctuating optimum, and with directional selection. These effects which arise in an infinite population appear to be large compared to the additional effects arising in a finite population because of the random generation of linkage disequilibrium.

Chromosomal E. coli DNA appears to be sensitive towards in vivo DNA restriction when transformed to a restrictive E. coli recipient. It is therefore concluded that transforming chromosomal donor DNA is present in a double-stranded form immediately after uptake.

Genetic analysis of E. coli transformants, obtained with UV-irradiated donor DNA under conditions that exclude photorepair, show, especially in a uvrB recipient, loss of donor DNA information compared with the situation where DNA was not subjected to UV-irradiation. Similar conclusions were arrived at after genetic analysis of transductants obtained with UV-irradiated particles of the generalized transducing phage P1. The processing in E. coli of DNA after P1 transduction is thus similar to that of transforming DNA. The observations are discussed and a possible explanation based on single-stranded DNA integration is presented in detail.

The size of nuclear DNA from wild-type Ustilago maydis was determined to be approximately 6·09 ± 0·3 × 108 daltons from neutral sucrose gradient sedimentation analysis. Following exposure to ionizing radiation the nuclear DNA size was reduced due to the production of double-strand breaks in the DNA. These breaks were repaired when the irradiated cells were incubated in medium for at least one hour after irradiation. The repair was seen as a shift in the DNA profile from a low molecular weight region where the control DNA sedimented. Inhibition of protein synthesis by cycloheximide prevented this type of repair. Blocking protein synthesis also decreased the survival of irradiated wild-type cells but not radiation-sensitive mutants. Protein synthesis was necessary within the first one and a half hours after irradiation for the survival of wild-type cells to be unaffected. The results provide additional evidence for an inducible repair process in U. maydis.

The 6-phosphogluconate dehydrogenase (6-PGD) E.C. no. 1.1.1.44, zymogram phenotypes of wheat–rye addition lines and triticale were determined. Genes involved in the production of 6-PGD were located on the long arms of C and F chromosomes of Imperial Rye and the long arms of II and IV chromosomes of King II rye. The results indicate that this enzyme is a dimer.