This paper examines the suggestion of Gale & Mackay (1979) that dominance modification will be slower in an environment with fluctuating selection coefficients than in a constant environment. The case of statistical independence between successive environments is studied. It is shown that there is an effect in the suggested direction, but that it is quantitatively slight with biologically plausible values for the parameters involved. It therefore seems unlikely that the phenomena noted by Charlesworth (1979) can be explained in these terms.

Using interchanges T(1, 3) and T(3, 6) of Pennisetum americanum orientation types of the interchange multiple at MI were studied in different genetic backgrounds. Orientation types alternate 1 and alternate 2, in addition to adjacent 1, adjacent 2 and indefinite could be identified for both the interchanges. The relative frequencies of various orientation types were influenced by a change in the genetic background.

For these interchanges, homologous centromeres do not seem to play a predominant role in the co-orientation of interchange multiple. The non-homologous co-orientation types were more subjected to genetic regulation than the homologous co-orientations.

The heritability (h2) and frequency of the neural tube closure defects, cranium bifidum (CB) and spina bifida (SB), have been estimated for a population of 9-day-old hamster embryos through half-sibling analysis. The average frequency of the total affected embryos per litter is approximately 17% while the pooled estimate for h2 based on between sires and between dams within sires components was 4%. This value points to the importance of environmental factors in contributing to the variance in defect frequencies observed within this population.

Two spontaneously formed R-plasmids (pFS401 and pFS402) originating from the multiple drug-resistant Escherichia coli strain UR12644 were found to carry transposable drug-resistance elements. Incompatibility between these two plasmids was used to select for transposition. An ampicillin transposon (Tn1781) residing on pFS401 and a tetracycline transposon (Tn1771) present on pFS402 were independently translocated to the endogenous RTF-plasmid pFS2. Molecular weight determinations of pFS2::Tn1781(Ap) and pFS2::Tn1771(Tc) revealed a value of 2·9 Mdal for Tn1781 and 7·1 Mdal for Tn1771. The arrangement of 3 PstI and 1 BamHI restriction endonuclease sites was found to be characteristic for the ampicillin transposon whereas the restriction map of Tn1771 features a nearly symmetrical location of 3 EcoRI cleavage sites, two of them close to the termini and one in the middle of the transposon. A model is presented suggesting the existence of repetitive DNA-segments at these positions which represent the structural preconditions for the genetic properties of Tn1771. The role of a cryptic plasmid involved in the generation of the endogenous R-plasmids pFS401 and pFS402 is discussed.

Population cages were set up containing an X-chromosome balancer, and either a single wild-type chromosome(homozygous cages) or a mixture of wild-type chromosomes(heterozygous cages). The balancer chromosome was eliminated more rapidly from the heterozygous cages, indicating that chromosome heterozygotes are at an advantage over chromosome homozygotes. The disadvantage of X-chromosome homozygosity in the female is estimated to be about 40%. From earlier studies it is known that the average disadvantage of homozygosity for either of the two major autosomes of D. melanogaster is approximately 80%. Since these autosomes are both about twice as long as the X chromosome, the disadvantage per unit length is similar for both chromosomal types.

Both X-chromosomal and autosomal heterosis can be explained by either dominance or overdominance at individual loci. However, a dominance model can only explain the similarity if many of the X-linked loci (about 50%) are limited in expression to the female.

A map of the seventeen R.SmaI sites within the 33·4 kb transfer region of plasmid F has been obtained. The number of R.SmaI sites contrasts sharply with the few sites for other restriction endonucleases with hexanucleotide specificity. Location of the R.SmaI sites in γtra transducing phages allowed physical location of lambda insertions into the traB, traF and traH genes.

Drosophila melanogaster adults were collected throughout the summers of 1976 and 1977 in Hartford, Connecticut. The frequencies of the Adh isoalleles were determined and seasonal variation demonstrated. Correlation coefficients were calculated between Adh frequencies and the seasonal variations in temperature, precipitation, relative humidity and barometric pressure. Although five correlations were significant it was concluded that these were due to chance.

Estimation of meiosis and sporuiation efficiencies in the fission yeast by ascus analysis

Genet. Res. (1979), 33, 109–119

Page 113, line 9: formula should read

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‘pi’ should read ‘Pi’