1. It has been shown that the lines dp 1, dp 2, vg 4 and vg 6 of Thoday & Boam (1961) each have two high sternopleural chaeta number genes or ‘effective factors’ between h and eyg in chromosome III. Their line dp 6 does not contain these two genes.

2. Lines derived from ancestors of dp 2 and vg 4 before the latter produced their accelerated responses have third chromosomes affecting chaeta number as if they had only one or other of these genes.

3. Of the three stocks from which all the lines derived, one, Inbred Oregon, lacks these genes. The second, vg/vg, has third chromosomes similar in effect to Oregon. The third, dp/dp, was heterogeneous, having a class of third chromosomes similar in effect to those of Oregon and a class similar to those having one high gene.

4. It is suggested that the history of the accelerated response in dp 1, dp 2 and vg 4 was as follows. Initially most of these third chromosomes were − − at the two loci, but a minority (derived from the dp/dp stock) were + − and − + (where + indicates the allele increasing chaeta number. Selection would reduce the frequency of − −, and hence increase the proportion of + −/− + heterozygotes and the probability of recombination to produce + +. Origin and multiplication of + + would account for the accelerated response.

Genetic consequences of non-random segregation of chromosomes are discussed. In the experimental part preferential segregation of two linkage groups in Ascobolus immersus is described. This was established in several crosses involving mutants 164, XXVI and 231 (all in the same linkage group) and mutant 726.

1. Chromosomal polymorphism in two populations of D. subobscura from Barcelona has been analysed and compared with the polymorphism which has been reported from other populations.

2. In spite of microclimatic differences between the Barcelona sites, there is no evidence of differences in the frequency of inversion types nor of seasonal differences within one of these sites.

3. Regularities in the array of frequencies of chromosomal types in Barcelona and other populations suggest that the polymorphism is mainly adaptive although there is also evidence of differences due to distance which are not obviously associated with differences in habitat.

4. In general the index of free recombination gives higher values in the marginal populations of D. subobscura, but further data are needed to arrive at a definite conclusion on this point since the present evidence is rather ambiguous.

The genetics of two polymorphic loci in the fowl was studied. Each affects the electrophoretic mobility of a different egg albumen protein, and at each locus two alleles were shown to segregate in a Mendelian manner.

The polymorphic proteins are not any of the well-characterized egg albumen proteins, viz. ovalbumin, ovomucoid, ovomucin, flavoprotein, avidin and lysozyme.

In contrast to all others, two hens were found to have an anomalous, variable, phenotype. A tentative explanation of this was offered.

1. The problem of whether or not hybrids are more efficient, less variable and have less stringent nutritional needs than their parents is examined by finding the dose-responses of four inbred Drosophila lines and their crosses to casein, choline, RNA, riboflavine, nicotinic acid, pyridoxine and folic acid, under germ-free conditions.

2. Under more or less optimal conditions, survival, development rate and weight of the hybrids are generally superior to those of the inbreds; the 4-way cross is not inferior to the 2-way crosses. Variability of the crosses is not necessarily lower than that of the inbreds, and the 4-way cross is no more variable than the 2-way crosses.

3. As measured by growth rate, the hybrids use casein more effectively than the pure lines, but their relative efficiency declines as the casein supply is decreased. There is a positive correlation between casein requirements for optimal growth and minimum requirements of pyridoxine. Hybrids also tend to be more efficient in their use of choline, but not of the other nutrients examined.

4. Deficiencies of particular nutrients (and also of excess provision of the non-vitamins) affect the lines and crosses differently, so that their relationships to one another are altered. The hybrids show no special advantage in resisting departures from the optimum. Variability is also changed significantly under sub-optimal conditions and, in some situations, the hybrids may then be more variable than the inbreds.

5. Each line and each cross is found to have its own optimal nutritional environment, and its particular reactions to departure from this. The full potential of the genotypes cannot be manifest, therefore, by tests in a single, standard environment.

A computer has been used to investigate the effect of an initial period of reverse selection on the subsequent response of a population to renewed forward selection with the same population size and selection intensity. As the computer was used to derive gene frequency distributions, there was no random element in the results obtained. A theoretical solution to the problem was obtained for genes with small effects.

The process can be adequately described by the duration of the reverse selection (expressed in terms of the population size N), the product of population size and gene effect, Ns, and the initial gene frequency. If the duration of reverse selection, t, is less than N/2, the loss in selection advance due to the reverse selection is roughly t/N, though slightly greater than this for genes with low frequency. The ‘point of no return’ after which it is impossible, with the same population size and selection intensity, to return even to the starting frequency is 1·4N generations for genes with small effect and this declines as the gene effect increases.

Some extension of results to recessive genes is also given.

On the basis of chromosome behaviour in three strains of Drosophila melanogaster males—a wild-type strain, a translocation heterozygote strain and a double translocation strain—it is concluded that the cytologically detected apparent chiasmata in the autosomes are only surface associations of homologous chromosomes and do not involve exchange of chromosome material. Such chiasma-like associations occur also between non-homologous chromosomes. The stainable, Feulgen-positive material of the chromosomes is thought to be responsible for the surface associations.

1. It has been shown that metagons, gene-controlled units necessary for maintenance of mu particles in Paramecium aurelia, can be extracted from cells containing one or both of the dominant genes M1 and M2, and reinfected into other paramecia. Provided the latter contain mu particles before infection, the introduced metagons resume their normal function of maintaining the mu particles.

2. Homogenates containing metagons have been fractionated in various ways. The metagon activity is found in the sediment after centrifuging at 105,000 g. for 90 min., and is retained after deoxycholate treatment removing the microsomal membranes, and after phenol extraction removing the ribosomal protein. By lowering the magnesium concentration, some metagon activity passes from the microsomes into the supernatant.

3. Metagon activity in RNA fractions is destroyed by ribonuclease at a concentration of 5 μg. per ml.

4. The metagons both in extracts and inside the cells appear often to be aggregated in clumps, the individual units of which are each functionally active in maintaining mu particles in living cells.

5. It is concluded that the metagon may be considered as a very stable and repeatedly functioning form of ‘messenger’ RNA.

1. Similar changes in the body-size of Drosophila melanogaster have been achieved by different developmental pathways, especially either by altering the duration of the early exponential phase of larval growth or by influencing the growth rate in the phase which is independent of time.

2. Such changes have been effected by selecting in the same population for larger or smaller size or shorter development time on chemically defined media, deficient in alternative nutrients. Selection for larger size on media deficient in protein or choline does not involve correlated changes in the larval period, whereas selection on media deficient in RNA does. The evidence suggests that shortage of this nutrient may be uniquely favourable for promoting a correlated change between body-size and duration of the larval period.

3. Strains which differ in presence or absence of such correlation are characteristically different with respect to gene-environment interaction. In the former, the differences due to selection are generally more fully or completely expressed when the diet is changed whereas in the latter this is not so, and different, especially competitive condition, leads to a drastic reduction of the difference.

4. How far the expression of the differences due to selection are affected, when the diet is altered, is also influenced by how long selection has been carried out. In early generations, the difference is only or best expressed in the special conditions provided during selection, but later on the changes due to selection are either fully expressed or partly so, as noted above.

5. Many of the differences in gene-environment interaction between selected strains can be accounted for in terms of variation in the duration of the exponential phase. Thus two lines selected for small body-size on low RNA or low protein diets responded in different ways to the same nutritional change—one became relatively larger and took porportionately longer to develop, the other became relatively smaller and developed in a shorter time.

6. There is clear evidence from various tests in which the amino-acid composition of the diet has been altered, that the nutritional requirements in the two stages of growth are not identical and this is consistent with the evidence for considerable genetic independence as well.

7. It is proposed that the first stage of larval growth, which principally determines the duration of the larval period and may also influence body-size, is canalized. Genetic variation which can influence this stage is present in the population but contributes little to the phenotypic variation of adult size, except under special nutritional conditions as when ribonucleic acid is the sole limiting nutrient. But, at the same time, such canalization is dynamic in the sense that the absolute amount of growth which is completed in the first stage may vary with respect to diet and thereby lead to correlated variation in the duration of larval life and adult size. But individuals of an adapted population behave alike in this respect so that gene-environment interaction which leads to correlated variation in the two characters is of a very low order.

8. The canalized phase sets a limit to the potential growth in the later stage and thereby influences greatly the mean value about which such growth is equilibrated. This canalization plays a major role in the general stability of growth relations and body-size although this is normally concealed by the high level of phenotypic variation. This interpretation can account for a great variety of data and provides a rational guide to further analysis.

Several stocks of Paramecium aurelia that had never expressed an E serotype were examined for E-reacting material. The regular presence of the cross-reacting antigen G in these stocks ruled out the use of hapten inhibition and other standard immunological techniques. Consequently, a genetic approach was attempted in which hybrids of the E-deficient strains and non-E stocks were bred, and antigen isolated and tested for the existence of molecules with a specificity differing from that of the E-containing parent.

Gel diffusion analysis suggested that E antigen from a hybrid indeed could differ from the ‘normal’ E. One of these hybrid clones which behaved as though its E antigen may have possessed a determinant contributed by the E-deficient strain was used as an immunizing antigen. The antibodies elicited possessed specificity unique for the hybrid. Thus the possession of E-like determinants by E-deficient stocks was confirmed.

A paramecium generally bears on its surface but one immobilization antigen from among the many it may have the potentiality for expressing. By assaying soluble extracts of whole cells, it had been demonstrated previously that animals of the E serotype may possess the cross-reacting G antigen. Now the existence of these secondary antigens—immobilization antigens undetected by in vivo tests—has been extended to include unrelated antigens, e.g. G antigen in animals of C serotype.

The amount of these cryptic immobilization antigens varies from stock to stock; also within a stock cultures harvested at different times may have quite different quantities.

The secondary and primary G antigens appear to be indistinguishable by the criteria of antigenic specificities, immunoelectrophoretic mobilities sedimentation rates and ammonium sulfate solubilities. As to their distribution within the cell, both secondary and primary antigens are located primarily on or within the cilia.

Weak but statistically significant sexual isolation has been demonstrated among Vetukhiv's six experimental populations of Drosophila pseudoobscura, all originally descended from founders taken from cultures of the same hybrids from four geographic localities. These six populations were maintained separately for almost 4½ years and then tested for the existence of sexual isolation. The sexual isolation has arisen in the absence of any selection for isolation, evidently as a by-product of genetic divergence.