Robertson (1960) used probability transition matrices to estimate changes in gene frequency when sampling and selection are applied to a finite population. Curnow & Baker (1968) used Kojima's (1961) approximate formulae for the mean and variance of the change in gene frequency from a single cycle of selection applied to a finite population to develop an iterative procedure for studying the effects of repeated cycles of selection and regeneration. To do this they assumed a beta distribution for the unfixed gene frequencies at each generation.

These two methods are discussed and a result used in Kojima's paper is proved. A number of sets of calculations are carried out using both methods and the results are compared to assess the accuracy of Curnow & Baker's method in relation to Robertson's approach.

It is found that the one real fault in the Curnow-Baker method is its tendency to fix too high a proportion of the genes, particularly when the initial gene frequency is near to a fixation point. This fault is largely overcome when more individuals are selected. For selection of eight or more individuals the Curnow-Baker method is very accurate and appreciably faster than the transition matrix method.

It is already well established in Escherichia coli that certain genes, or groups of genes, can behave as episomes—that is, they can undergo a free transition between the fully extrachromosomal state and one in which they are integrated in the bacterial chromosome (Jacob & Wollman, 1961; Broda, Beckwith & Scaife, 1964). Although this type of behaviour has been detected commonly among members of the enteric bacteria, it has never hitherto been demonstrated clearly in a Gram-positive species. This communication shows that a reversible transition of certain genes between the extrachromosomal and the integrated state can occur in some strains of Staphylococcus aureus at least.

A recessive autosomal mutation has been found in the CXBI/ByEss recombinant-inbred line but in neither of the parental strains, C57BL/6ByEss and BALB/cByEss. Its presence in the CXBI/ByJax and CXBI/ByLac sublines suggests an origin early in inbreeding. The locus, seminal vesicle shape (svs), appears to be linked to the albino locus on chromosome 7. The homozygote has seminal vesicles with a smooth tubular external appearance. In segregating crosses homozygotes had slightly lighter seminal vesicles but the weights of other androgen target organs were not reduced. Exogenous testosterone increased the size of the seminal vesicles but did not alter their shape. The mutation did not affect the pattern of proteins on SDS–acrylamide gel electrophoresis, which did differ between the parental strains. The locus affecting a 27000 Da protein has provisionally been assigned the symbol svp-4.

Mutants of Pseudomonas aeruginosa male strains with altered mating properties have been obtained through treatment with nitrogen half mustards. The response varied with the type of male strain. A mutant of the infectious male PTO 13 was obtained which acts as a female strain and has apparently lost the FP sex factor. A mutant of the non-infectious strain 2 male mates not only with strain 1 females, but shows a thousand-fold increase in its ability to mate with strain 2 males, when it can act as either donor or recipient in conjugation. Derivatives of strain 1 males were obtained which had reduced recombinant forming ability.

While acriflavine is ineffective in producing such mutants, and has not been shown to cure male strains of their sex factor in P. aeruginosa, it is very effective in inhibiting infectious transfer of FP from FP+ to FP− strains. Furthermore, it markedly inhibits recombinant formation in 1 FP− × 1 FP+ crosses, and this latter effect is thought to be due to the inhibition of chromosome transfer by the male parent. In view of the almost complete lack of effect of AF on the normal growth of P. aeruginosa it is likely that the control of chromosome replication during vegetative cell division is different from the occurring during conjugation and that the FP factor is involved in this control during conjugation.

The hairpin-tail allele (Thp) of brachyury (T) on chromosome 17 of the mouse is unique in that the phenotype of heterozygous offspring depends on which parent contributed the Thp.The present paper offers formal proof of this fact using the th2 allele of T as a marker, and discusses possible modes of action of the hairpin allele.

The amount and pattern of genetic variation in a population can be estimated from genes or DNA sequences sampled from the population. Although random sampling is assumed in almost all cases, we often do not know whether sampling is random or not. Using a simple non-random sampling model, the effects of non-random sampling on the estimation of parameters in population genetics were investigated. This non-random sampling model assumes that n genes are randomly sampled with replacement from m genes which were randomly sampled from a large random mating population, and various degrees of non-randomness can be generated by changing the value of m. The results obtained show that the effect of non-random sampling on the number of alleles and the number of segregating sites is substantially large whereas the effect of non-random sampling on heterozygosity and the average number of nucleotide differences is negligibly small unless non-randomness is extremely large. The effects of non-random sampling on the tests of neutrality were also investigated, and the results obtained indicate that the effect of non-random sampling is stronger on Fu and Li's tests than on Tajima's test.

1. An investigation of the serum esterases of the field-vole, M. agrestis, has been carried out by the method of starch-gel electrophoresis.

2. A total of 254 animals, mainly from Carron Valley, near Stirling, have been examined.

3. Evidence is presented for a system of four enzymes controlled by two genetic loci, which it is proposed to call Es-3C and Es-3R.

4. Each locus specifies one polypeptide sub-unit of the E3 enzyme, which is a dimer.

5. The two loci each have two alleles present at high frequencies in the population examined.

A class of rII mutants revertible by both base analogues and acridines is described. The members of this anomalous class are base-substitution mutants suppressed by a phase-shift mutation in a phage gene mapping outside the rII region.

5-azacytidine-treated human fibroblasts exhibit a significant decrease in the frequencies of Barr body+ cells as compared to nontreated cultures. This presumably indicates that 5-azacytidine can induce a change in the degree of condensation of the Barr body. It is suggested that the state of chromatin condensation of the Barr body may be related to the reactivation process by 5-azacytidine of gene loci in the inactive X.

A mutant (Mb-s) of Escherichia coli K12 which is more sensitive than wild-type (Mb-r) to such dyes as methylene blue or acridines was studied. The Mb-s mutant can also be disinfected of F factor with lower concentrations of acridine orange than Mb-r bacteria. The Mb locus was mapped by crosses between Hfr and F−, F-duction with F13, and transduction with phage P1. It was found to be situated between the pur and T6 loci on the chromosome. Moreover, it was found to be contained in the chromosomal fragment carried by the F-prime, F13. In a heterogenote obtained by F-duction, the Mb-r gene is dominant over the Mb-s gene. The Mb gene was transduced jointly with the pur gene by phage P1.

An approximate equation is derived, which predicts the effect on variability at a neutral locus of background selection due to a set of partly linked deleterious mutations. Random mating, multiplicative fitnesses, and sufficiently large population size that the selected loci are in mutation/selection equilibrium are assumed. Given these assumptions, the equation is valid for an arbitrary genetic map, and for an arbitrary distribution of selection coefficients across loci. Monte Carlo computer simulations show that the formula performs well for small population sizes under a wide range of conditions, and even seems to apply when there are epistatic fitness interactions among the selected loci. Failure occurred only with very weak selection and tight linkage. The formula is shown to imply that weakly selected mutations are more likely than strongly selected mutations to produce regional patterning of variability along a chromosome in response to local variation in recombination rates. Loci at the extreme tip of a chromosome experience a smaller effect of background selection than loci closer to the centre. It is shown that background selection can produce a considerable overall reduction in variation in organisms with small numbers of chromosomes and short maps, such as Drosophila. Large overall effects are less likely in species with higher levels of genetic recombination, such as mammals, although local reductions in regions of reduced recombination might be detectable.

P transposable elements of Drosophila melanogaster cloned from the strong P strain π2 have been analysed. The structures and chromosomal locations of 26 of the 30–50 elements estimated to be present in π2 have been determined. At one location two elements are inserted 100 base pairs (bp) apart, and in a second location two elements are only separated by the 8 bp duplicated upon P-element insertion. In addition to 2.9 kilobasepair (kbp) elements, elements with 14 different internal deletions from 1.3 to 2.3 kbp in size have been isolated. There are 7 copies of the 2–9 kbp element, 2 copies each of 5 internally deleted elements and a single copy of 9 internally deleted elements. One of the elements found twice is the KP element, which may play a role in the regulation of hybrid dysgenesis in strains which contain many copies of this element. Apart from internal deletions the elements are extremely homogeneous in DNA sequence, with only 2 single base polymorphisms detected twice each in over 16 kbp of P-element sequence. Although transpositions are infrequent in an inbred P cytotype strain such as π2, the distribution of these cloned elements indicates that when the genomic library was made, the strain was polymorphic with respect to element location. The distribution and structures of the element are discussed with respect to models for regulation of P-element transposition.

Following an experiment in which mice had been selected for the length of the mitochondrial section (midpiece) of their sperm tails, an attempt has been made to determine experimentally the adaptive significance of this character; this was by the artificial insemination of mixtures of selected and unselected (control) spermatozoa, and the subsequent disclosure of their competitive fertilizing ability in the paternity of the offspring. After the birth of nearly 500 offspring, there was no indication that the control cells were—by this criterion—functionally superior. From the discrepancy between the pairs of males sampled, however, it is possible that other biological or technical factors have been important.

A Monte Carlo simulation study to evaluate the benefits of marker assisted selection (MAS) in small populations with one marked bi-allelic quantitative trait locus (QTL) is described. In the base generation, linkage phase equilibrium between the markers, QTL and polygenes was assumed and frequencies of 0·5 for the two QTL alleles were used. Six discrete generations of selection for a single character measured on both sexes followed. An additive genetic model was used with the QTL positioned midway between two highly polymorphic markers. Schemes were simulated with a distance of 10 cM between the QTL and either of the two markers and with the QTL explaining 1/8 of the total genetic variance in the base generation. Values of 0·5, 0·25 or 0·1 were assumed for the heritability. Eight males and 16, 32 or 64 females were selected each generation with each dam producing four sons and four daughters as candidates for the next generation. Animals were evaluated with a conventional BLUP animal model or with a model using marker information. MAS resulted in substantially higher QTL responses (4–54%), especially with low heritabilities, than conventional BLUP but lower polygenic responses (up to 4%) so that the overall effect on the total genetic response, although in the majority of cases favourable, was relatively small. With QTLs of larger size (explaining 25% of the genetic variance) comparable results were found. When the distance between the QTL and the markers was reduced to 2 cM, genetic responses were increased very slightly with a heritability of 0·5 whereas with a heritability of 0·1 responses were increased by up to 10%, compared with conventional BLUP. Results emphasize that MAS should be most useful for lowly heritable traits and that once QTLs for such traits have been identified the search for closely linked polymorphic markers should be intensified.

By the use of pintail (Pt) and brown (b) as markers, the location of Mup-a, a locus controlling electrophoretic variation of one of the components of the major urinary protein (MUP) complex, on mouse linkage group VIII has been determined. The order and intervals determined from recombination frequencies in 121 offspring from a back-cross were Pt 4·1 b 6·6 Mup-a.