A Statistical analysis of the genome of D. melanogaster indicates that functionally related genes tend to be found on a particular chromosome and, when their locations within a given chromosome are measured in terms of map units, show a tendency to cluster; this clustering within chromosomes, however, is completely accounted for by the known clustering of all genes within chromosomes. Thus the analysis does not reveal any obvious functional significance for the arrangement of the genes within the chromosomes of this organism.

Analysis of the inheritance of the three bar-properties of bacteriophage T4: exclusion of T2 from the progeny of crosses, glucosylation of the hydroxymethylcytosine (HMC)moiety of the DNA according to T4, and plating with large plaques on E. coli K strains, was carried out by means of marker rescue from T4 by T2 on E. coli K (λh) as a selective indicator. Five of the strains isolated plated with large plaques on K (λh), but did not exclude T2 and showed T2 glucosylation; plating on E. coli K (λh) was found to segregate from the other two bar-properties. The sixth isolate showed, in addition to plating with large plaques on K, partial non-excludability by the parental T4 and T4 glucosylation of HMC. If partial non-excludability is the result of T4 glucosylation, the role of the additional glucose substitutions might be a protective effect on the DNA against the exclusion factor of T4. This proposal is supported by the analysis of the progeny from a single burst from a cross of T4 and T2. The following T2 genes were partially excluded: host-range, no exclusion of parental T2, sensitivity to ultraviolet, and limited plating efficiency on E. coli K (λh). The exclusion factor of T4 is not transmitted to all progeny and does not behave like a bar-property. Only resistance to exclusion and T4 glucosylation were transmitted to all twenty-seven progeny of the single burst. The elimination of sensitivity to exclusion and T2 glucosylation is explained by assuming that the recombinant class with the exclusion factor of T4 and T2 α-glucosylation will exclude itself and be suicidal upon infection of a new host. Exclusion and differential glucosylation are discussed with regard to restriction and modification, respectively.

Mutants of the Y locus differed appreciably in their basic conversion frequencies (frequencies of conversion in one-point crosses) to wild type. The differences in the basic conversion frequencies in the opposite direction, i.e. from corresponding wild-type allele to mutant, were in general not pronounced. For some alleles frequencies of conversion in both directions were similar, but for the others they differed markedly. No evident correlation between the position of mutants on the map and their basic conversion frequencies was observed.

In two-point crosses in repulsion, the great majority of recombinant octads were of conversion type. In these crosses symmetry or asymmetry of conversion depended mainly on similarity or differences in basic conversion frequencies of mutants crossed. In crosses between mutants from different clusters the recombination frequencies were near to the sums of their basic conversion frequencies. Such ‘mutant specificity’ makes it impossible to establish the linear order of mutants on the basis of recombination frequencies in two-point crosses.

The results of two-point crosses in repulsion between mutants within clusters pointed to the influence of one allele on the frequency of conversion of another one. This ‘marker effect’ was also evident in some three-point crosses.

The frequencies of simultaneous conversions in two-point crosses in coupling did not show negative correlation with the distances between the mutants involved.

It seems that many of the data presented here are most easily explained by recently developed hybrid DNA models.

1. A number of stable nystatin-resistant mutants of the yeast Saccharomyces cerevisiae have been isolated from platings of a sensitive wild-type strain on low concentrations of the antibiotic.

2. These mutants were found to be resistant to 10, 15 or 60 units of drug/ml.

3. Analysis of meiotic segregants from crosses of these mutants to wild-type indicate that resistance is determined by two types of genes; resistance genes and modifiers.

4. Functional analysis of the mutants demonstrated the existence of three recessive resistance genes, nys-l, nys-2 and nys-3 and that nys-1 and nys-2 were linked.

5. Genetic analysis showed that nys-1 was affected by two modifiers, Mnys-1 and Mnys-2, but that only Mnys-2 affected nys-2 and nys-3.

6. The modifiers Mnys-1 and Mnys-2 are dominant.

7. An investigation of the effects of temperature and medium on resistance demonstrated marked interactions between genotype and environment for both the resistance genes and the modifiers.

8. Second-step mutants have been isolated by plating first-step mutants on higher concentrations of the drug. Some of these are resistant to 800 units/ml.

9. Some possible mechanisms of nystatin resistance are discussed.

Crossing-over between the α and the β loci constituting the A incompatibility factor gives rise to two new specificities which are compatible with both parental As and with each other. The frequency of monokaryotic mycelia carrying re-combinant A factors is shown to be under genotypic control in a multiple crosses programme (6 × 6), selection for high and low recombination frequencies (two generations), and crosses between the selection lines. The recombination values based on samples of 100 monokaryons, range from 0 to 21%; however, the more accurately estimated values of the ‘High’ and ‘Low’ selections are 14% and 4%, each being based on approximately 2,000 mycelia.

The data are compatible with a gene-system consisting of the postulated locus rec which has a major effect on recombination and which is linked to the A factor, and several minor effects by other loci. Alternative interpretations are presented and discussed. The apparent dominance of low frequencies of recombination on high frequencies can be related to the breeding behaviour of S. commune. Thus close linkage between α and β allows a high number of A specificities to be maintained in a population as well as high out-breeding potential, while the inbreeding potential (i.e. dikaryotic combinations between monokaryons originating from a single fruit-body) is kept low and near its minimum.

The significance of the two-locus structure of the incompatibility factors is examined theoretically in an Appendix at the end of the Discussion section.

Reports of sex differences in crossing-over in animals, published since Haldane in 1922 suggested that crossing-over should be less frequent in the heterogametic sex, have been reviewed and discussed. No general rule is discernible apart from the absence of crossing-over in males of the dipteran genera Drosophila and Phryne and in females of some lepidopteran species, due apparently to failure of chiasma formation in the heterogametic sex. In the majority of animal species examined crossing-over occurs in both sexes. While there is some tendency in mammals for crossover values in females to exceed those in males, it was of greater interest to find that marked sex-differences occur in the same species (data chiefly from the house mouse) in opposite directions in different chromosomes. The influence of factors acting locally in the chromosomes, such as those associated with hetero-chromatin, were indicated as promising subjects for the study of variations associated with sex.

1. Lines with ten pairs of parents and selected at an intensity of 20% were exposed to 1000 r. of X-rays for 0, 2, 10 or 30 generations.

Lines which received some irradiation generally gave greater response than the unirradiated controls. The phenotypic variance in the irradiated lines was much higher than in the controls. There was little difference in behaviour between lines receiving ten generations of irradiation and those irradiated every generation. Lines receiving only two generations of irradiation had lower variances than the other irradiated lines, but in one of three replicates the response was greater than the corresponding continuously irradiated line.

3. Lethal frequencies were much higher in irradiated than unirradiated lines. Particular chromosome II and III lethals were at high frequencies in most of the irradiated lines but in only two out of five controls.

4. On relaxation, the mean of the irradiated lines generally declined considerably, but in the unirradiated lines there was only a very small regression.

5. It appears that most of the extra response and increased variance in the irradiated lines were caused by a few genes with large effect on bristle number.

1. Repeated measurements on the growth of individual leaves in seedlings and young plants of ryegrass combined with dissections of the apex of the shoot and of very young leaves have shown that the basic pattern of leaf formation is very similar in Lolium multiflorum and L. perenne.

2. The initial growth rate of a leaf primordium is low but increases suddenly at a point when the primordium is somewhat longer than the apex and about 1 mm. in length. After this transition, which I have called the unfolding of the leaf, the rate of elongation is faster and more or less linear until the leaf is nearly mature.

3. The time of unfolding of a leaf is very closely associated with the time of maturity of the next older leaf on the same side of the apex. Thus a leaf ceases growth when the next younger leaf immediately above it starts elongating rapidly, though which is cause and which is effect is it not possible to say.

4. This close relationship between duration of leaf elongation and rate of unfolding of successive leaves holds for both species in two seasons. It leads one to predict that selection for increased leaf size, in so far as it is a result of greater duration of leaf elongation, is likely to be accompanied by a slower rate of leaf appearance, and conversely that selection for rate of leaf appearance is likely to result in smaller leaves.

Detailed measurements of leaf growth and leaf dimensions in the seedling stage were made on lines which had been selected either for large or small leaf size or for fast or slow rates of leaf appearance within one population of Lolium multiflorum (Italian ryegrass) and two populations of L. perenne (Irish perennial and Hunsballe perennial ryegrass).

Selection for either character had no effect on the rate of initiation of primordia at the apex, but did change the rate at which successive primordia became leaves. This rate of unfolding was very highly correlated with the rate of visible appearance of leaves and in all cases showed a parallel response to selection for the latter, as did also the rate of maturation of leaves. All three rates showed a negative correlated response to selection for leaf size.

Selection for increased leaf size in all cases led to a longer duration of the elongation of an individual leaf, but selection for faster rate of leaf appearance always reduced this duration. The rate of elongation of individual leaves increased under selection for larger leaf size but showed irregular changes under selection for faster leaf appearance, going down in Irish but up in Hunsballe.

Data for dimensions of cells from the lower epidermis showed that changes in leaf length under selection were sometimes associated with changes in cell length, some-times in cell number and sometimes with both.

Selection had in no case disrupted the close association between the maturation and cessation of growth of a leaf on the one hand, and, on the other, the unfolding from the apex and onset of rapid growth of the next younger leaf on the same side of the apex. Thus in all lines only two leaves (one on each side of the apex) were elongating rapidly at any one time, and an increase in the rate of unfolding was associated with a decrease in the duration of elongation and vice versa.

This association was the basis of the observed negative correlated responses between leaf size and rate of leaf appearance. But the fact that the rate of elongation could change independently of the duration opened up the possibility of setting up a selection criterion which would increase the total rate of leaf area formation.

The value of this kind of analysis of a character complex in a plant-breeding programme is suggested to lie in discovering physiologically or developmentally limiting processes rather than merely identifying morphological components.