1. The effect of genetic sampling, when this sampling is without replacement, on variation in gene frequency is studied, and equations describing the genetic drift are derived. The effective size turns out to be about one greater than under sampling with replacement.

2. The relation between ‘spread of genes’ and genetic drift is worked out.

3. The University of Queensland control poultry flock is analysed by these methods.

4. The design of control populations is discussed with particular reference to the relative importance of genetic drift and phenotypic sampling.

1. Techniques were developed to facilitate genetic analysis of a Myxomycete, Physarum polycephalum. The uninucleate cells that form part of the life-cycle (the ‘amoebae’), in two-membered culture with Pseudomonas fluorescens, were found to be suitable for handling by some of the standard techniques of microbial genetics, including plating.

2. As reported previously, a single-factor (+/−) mating-type system was discovered in P. polycephalum controlling the formation of plasmodia by the amoebae. Plasmodia are formed only when amoebae of different mating-type are mixed together. A single spore gives a culture of amoebae of one mating-type only, which may be assumed to be a clone.

3. A strain of amoebae resistant to emetine hydrochloride was isolated and was found to maintain its resistance through many subcultures in the absence of the drug.

4. A cross was made between the emetine-resistant strain (mating-type −) and an emetine-sensitive strain (mating-type +). Spores obtained from the hybrid plasmodium were plated and four sets of isolates of amoebae were made from these spores, and separately tested for mating-type and for emetine resistance.

5. The results of testing these isolates agree with the hypothesis that mating-type and emetine resistance are determined by pairs of alleles at two unlinked loci provided that factors disturbing the segregation for emetine resistance are assumed to be present.

6. Consideration of factors that could have disturbed the segregation for emetine resistance shows that selfing did not occur in the cross but that some misclassification for emetine resistance probably did occur.

1. Replacement of the dominant genes M1 and M2 in Paramecium aurelia, stock 540 (syngen/variety 1), results in loss of ability to maintain mu particles and manifestation of mate-killing after a delay of eight to fifteen fissions in most cells. The change, when it does occur, is relatively abrupt, extending over less than the space of one inter-fission period.

2. The delay between change of genotype and loss of mu particles is interpreted as being due to presence in the initial cytoplasm of some thousand ‘metagons’, which are non-replicating gene derivatives having the physiological activity of the corresponding genes. During successive fissions of paramecia deprived of M1 and M2 the metagons are passively distributed amongst the progeny, until virtually all animals lack them.

3. On reaching a stage at which some individuals of genotype m1m1m2m2 contain only a single metagon, the paramecia still contain large numbers of mu particles and are mate-killers. Fission of such animals gives rise to one daughter again with mu particles, and another in which the latter are destroyed during the next inter-fission period.

4. By induced cytoplasmic exchange between conjugants, metagons can be transferred from one animal to another via the cytoplasm. Where such transference is into an animal not originally containing mu particles, that animal is converted into a condition in which it favours the maintenance of mu particles and transmits the latter to one or more of its offspring.

5. Distribution of metagons amongst progeny of dividing paramecia is not random, due possibly to clumping of the metagons. Induced cytoplasmic exchange seems to break up the clumps.

6. Reintroduction of a dominant gene (M2) into a cell recently deprived of the same gene, succeeds—even after fifteen fissions—in re-establishing the ability to support growth of mu particles, provided that the recipient cell contains at least one metagon and one or more mu particles. There is a regular lag of only one fission between introduction of such a dominant gene and its phenotypic manifestation.

7. Mathematical formulae are developed for calculating the expected initial number of metagons, the proportions of animals lacking mu particles at each fission following loss of the dominant genes, and the proportions of cells containing 0, 1, 2 …, etc. metagons per cell at any stage. The consequences of one of the possible types of irregular distribution of metagons in dividing paramecia are also considered mathematically.

The efficiency of food utilization, the digestibility of foodstuffs, energy metabolism, and body activity have been studied in three lines of mice, one selected for large, another for small body size, and a third, control, line.

The gross efficiency of food utilization was highest in the large line, intermediate in the control line and lowest in the small line between 21 and approximately 35 days of age. During this period, gross efficiency declined in the large and control lines with increasing size and decreasing growth-rate, presumably due to an increase in maintenance costs in comparison with the weight gained. In the small line, the efficiency of food utilization increased up to 35 days of age but declined thereafter. The energetic efficiency (measured in Calories) was higher in the large than in the small line up to 4 weeks of age, i.e. when the growth-rate was high, and after 6 weeks of age, when fat was being deposited at an increased rate.

The increased efficiency of large mice was not entirely associated with a greater proportion of the ingested food being absorbed from the gut. Large mice absorbed a greater proportion of protein, though the difference was not sufficient to account for the large weight difference between the large and small lines.

The energy expenditure of mice of the large line was greater than that of the small line at all ages and similar for the same body weights. The reduced growth-rate of small mice was not due to abnormally high or low energy costs. There was no evidence that body activity determined or restricted the rate of growth in either line.

Mice selected for small size were phenotypically unlike pituitary dwarf mice, although the low nitrogen retention during the growing-period indicated a deficiency of some growth stimulus.

The linear relationship between fourteen allelic p-aminobenzoic-acid-requiring mutants of Aspergillus nidulans was investigated. The mutants were assigned to twelve different mutational sites within one functional region. No case of intra-cistron complementation was found. The order of sites as determined by the distribution of the outside markers in different crosses between paba alleles is consistent and the recombination fractions between the sites are approximately additive. Crossing-over between alleles is associated with strong localized negative interference. The topography of the pabal region is characterized by a marked concentration of the mutational sites in a small part of the cistron.

1. The problem of improving rate of larval development of Drosophila by selecting for this ‘character’ on deficient diets is examined by culturing larvae axenically on low-casein and low-pyridoxine media. Under these conditions it is possible to develop strains which grow faster than the parent population.

2. Selection for fast- and slow-growing larvae on a low-pyridoxine diet proceeds with a realized heritability of about 20%, but progress ceases after eight to nine generations. The selected larvae show no alteration of pyridoxine requirements up to the tenth generation, but the lines develop at different rates under optimal conditions. This difference is exaggerated when the diets are low in pyridoxine. By the fourteenth generation, requirements of the two lines for optimal growth have become distinct, the fast line requiring less pyridoxine than the control. Casein requirements show about the same optimum for the two lines but this is lower than that of the control, foundation population.

3. Selection for fast- and slow-development lines on a low-casein diet continues to be effective throughout the fifteen generations of the test. The realized heritability in this case is about 10%. The optimal requirements of the two lines are the same, and there is little difference in their development rates when reared on this optimal diet. The response is found only under sub-optimal conditions, both of deficiency and of excess casein. Pyridoxine requirements do not seem to be altered in the two lines.

4. Crosses between the selected lines show that each genotype has its own optimal environment, as judged by pyridoxine and casein requirements. Crosses among the lines after fourteen generations show that all the hybrids are superior to the mid-parent and three grew faster than the better parent. Other environments would have given different results.

The competitive-index method of measurement of over all fitness in Drosophila has been used to measure the effect of inbreeding and of artificial selection for metric characters in a large population of Drosophila melanogaster. The technique itself was examined in detail with particular reference to its repeatability and to the effect on it of the modification of various environmental variables.

With continued full-sib mating the decline in the competitive index was very rapid (it was reduced to a half by a single generation of full-sib mating) and there were no indications that interactions between deleterious genes at different loci were important in determining the rate of decline of fitness as inbreeding increased. Other unselected lines with ten pairs of parents in each generation were carried to serve as a control for the lines under artificial selection. At the same theoretical degree of inbreeding the control lines had a much higher average fitness than the lines produced by continued full-sib mating.

From the base population lines were selected in both directions for abdominal bristles, sternopleural bristles and for wing length, there being two replicates in all cases. Four control lines were kept with the same number of parents as the selected lines. In all cases the selected lines declined in fitness below the value for the base population. However, in three of the lines the fitness was not significantly below the value for the control lines. The effect of artificial selection on fitness was asymmetrical, the decline being greater with down selection for all characters.

The relevance of these results to various theoretical models is discussed. If the variation in these characters is actively maintained in the base population by the selection of heterozygotes then the results are consistent with an average selection disadvantage of homozygotes relative to heterozygotes of about 0·5%.

1. Changes in muscles and tendons in sm/sm are confined to hands and feet. They are closely correlated with the changes in the osseous skeleton.

2. In Os/+, muscular changes are much more complex and not confined to hands and feet, but include the muscles of the forearms and of the lower legs. Some of the muscular changes, especially on the postaxial side, are not correlated with the changes in the osseous skeleton.

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.

Until 1960 all the chromosome counts performed on cells of this species were done on histologically prepared specimens of testicular tissue. In January 1960, Chiarelli et al. published the results of their experiments to grow renal tissue in culture. Their specimens were obtained from male and female calves. They confirmed that 2n = 60 and described the chromosomes as being all acrocentric except the X chromosome, which was submetacentric.

The purpose of the work described below was to try to apply the peripheral-blood culture technique of Moorhead to bovine blood, and to attempt to grow bovine muscle cells in a tissue culture.