A simple method is given to compare the relative fitnesses of various populations of one species when they compete with another species. Populations of Drosophila psevdoobscura polymorphic for the chromosomal inversions CH and AR are superior to monomorphic populations CH or AR when they compete for the available resources with D. serrata.

Blood glucose levels of mice from five strains were measured. The results showed that each strain was different from the others for genes affecting blood glucose level, although the strain differences were small.

Within strains, the chief sources of variation were sex differences, differences between mice from different litters of the same mother, and between mice assayed in different months. Repeatability of measurements on individual mice was low. Genetic analysis, even by biometrical techniques, was not possible because the small genetic effects were obscured by variation due to other causes.

A matrix notation is developed to facilitate study of natural selection in large populations. The processes of mating (taking into account differences between genotypes in fertility in both sexes), segregation, and differential viabilities are each expressed in matrix notation. Assortative mating and non-random segregation can also be described by the method. The separate processes can then be combined to give simple equations relating the genic and genotypic frequencies in one generation to those in the previous generation. This will facilitate computer treatment of natural selection processes.

The method can also be used to study equilibria and the conditions of their stability by examining the latents roots of the matrix. Several special cases of selection at an autosomal locus are examined. The method can be extended to sex-linked loci and two special cases are discussed.

N-methyl-N′-nitro-N-nitrosoguanidine treatment of a donor strain of Pseudomonas aeruginosa resulted in the isolation of strains which were presumably cured of their sex factors, strains which possibly contained mutant defective sex factors and one strain which showed enhanced donor activity. The latter was apparently due to a change in the bacterium and the sex factors of OT 302 and OT 15 were both shown to be similar to the original in OT 1 when the three were transferred into the same strain.

Thirteen markers in Aspergillus amstelodami were mapped through meiotic and mitotic recombination, resulting in good correlation of the two linkage groups established by each method. Mapping via mitotic analysis proved the more efficient of the methods because of the long distances between some of the markers established in this organism.

Visual selection of haploid segregants from mitotic recombination was aided by the ability to recover conidia which differed in colour from the haploid parents. These contained pairs of conidial colour markers which were not epistatic to each other.

The advantage in using membrane filters for crosses between P+ (donor) and P− (recipient) strains of Vibrio cholerae (Bhaskaran, 1964; Bhaskaran, Sinha & Iyer, 1967) is probably due to more frequent and effective contacts between the cells on the surface of the membrane than is possible in fluid media with actively motile cells of this species. The transfer of the fertility (P) factor itself, as well as the yield of genetic recombinants, was considerably improved by this procedure.

The effect of nalidixic acid, a specific inhibitor of DNA synthesis, on Escherichia coli strain B (lon) and its u.v.-sensitive derivatives is examined. Strain B itself is sensitive to nalidixic acid, whereas its u.v.-resistant derivative B/r is resistant.

It is shown that in all exr A strains, in which u.v.-induced filamentation is suppressed, resistance to nalidixic acid is increased. Among exr A strains, Bs4 is exceptionally resistant to nalidixic acid. This is because nalidixic acid kills only growing cells and strain Bs4, a try auxotroph, may grow poorly under the conditions used to test nalidixic acid.

The uvr genes of the HCR strains Bs1, Bs8 and Bs12 do not suppress u.v.-induced filamentation nor do they affect the response to nalidixic acid. The uvr gene of strain Bs3 is unusual in increasing the tendency to filament and also sensitivity to nalidixic acid.

Strains Bs1, Bs3 and Bs8 are all doubly mutated from strain B, the second mutation (not uvr) being responsible for their increased resistance to nalidixic acid as well as partially or completely suppressing filamentation.

It is concluded that the cell division mechanism of (lon) strain B is sensitive to inhibition of DNA synthesis. Mutations which suppress the tendency of strain B to filament reduce its sensitivity to inhibition of DNA synthesis.

Experiments were designed to investigate the effect of homozygous deletions upon the frequency and the average length of heterozygous regions in bacteriophage T4D. A long deletion, rdf41, which covers at least the whole rII region, was found to increase the heterozygosity for r48, while no increase was observed when a short deletion was employed. The long deletion was found to increase the average length of amber-HETs by a length approximately the size of the rII region.

A drastic reduction in average HET length was found in FUDR crosses homozygous for the long deletion rdf41, indicating that the type of HET that does increase in FUDR is very short.

In the cross with no deletion in either parent, premature lysis HETs were found to be much longer than normal lysis HETs. Assuming that redundancy HETs are long compared to heteroduplex HETs this result indicates that redundancy HETs are made earlier in the latent period than heteroduplex HETs. A fluctuation in HET frequencies was found for different markers, especially in FUDR.

About half of all HETs, both in normal crosses and in FUDR crosses, was found to be parental for outside markers.

In non-FUDR crosses, polarized segregation was shown by 12 out of 27 multi-marker HETs after normal lysis and 5 out of 22 multi-marker HETs after premature lysis. In FUDR crosses, 24 out of 77 multi-marker HETs showed polarity.

Forty-two mutants of the anther smut fungus Ustilago violacea were mapped by means of complementation tests, mitotic haploidization, and meiotic segregation. Spontaneous mitotic haploidization was very rare, but haploids were induced at a high frequency using p–fluorophenylalanine (PFP). Haploid segregants appeared as fast-growing, spherical colonies (papillae) which grew away from the diploid growth on PFP medium. Thirty-three markers, classified by complementation tests into 21 genes, were mapped by mitotic haploidization in 10–12 linkage groups. There were no discrepancies in the linkage data, and all the markers could be assigned unequivocally to linkage groups. Although about 250 diploids were analysed, there were no segregants in which mitotic crossing-over and mitotic haploidization appeared to have occurred simultaneously.

Thirteen of the 33 markers, in six or seven genes, were expressed infrequently (0–5%) in the papillae produced on PFP medium. These markers, which behaved unusually and were designated missing-markers, were found to be on two chromosomes which tended to remain disomic on PFP medium. Thus 8–10 chromosomes haploidize readily on PFP medium, whereas two other chromosomes are resistant to the effects of PFP and remain disomic. Meiotic segregation was investigated in crosses of genetically marked haploid stocks and also hi diploids, using the host plant. Some of the results enabled preliminary maps to be made of three linkage groups. The results from meiotic segregation were fully compatible with those from mitotic haploidization and the complementation tests.

The genetical evidence for a haploid chromosome number of at least 10–12 is in conflict with the observations of several cytologists that n = 2 in this species.

Evidence is presented which indicates that the phenylalanine analog 2-amino-3-phenylbutanoic acid (APBA) induces mutation to APBA, streptomycin, and neamine resistance in the green alga Chlamydomonas eugametos. Apparent chromosome abnormalities (reciprocal translocations and inversions) were also found among some clones recovered after APBA treatment. The two, mutation and chromosome aberration, are separable.

The mouse X-chromosome controlling elements, detected by their influence on the position effect variegation caused by the X-autosome translocation T (1; X) Ct, have been found to modify the heterozygous phenotypes of two X-linked genes. It is proposed that X-inactivation can be incomplete, the level of inactivation or the frequency of cells in which inactivation is incomplete being dependent upon the ‘state’ of the controlling element located in the X. The data suggest that this is a consequence of a reversal, or partial reversal, of inactivation of the X as a whole in some cells rather than a vairable spread of inactivation along the length of the X.

Four white-spored allelic mutants of Ascobolus immersus were used to study the effect of mutant and cross specificity on the recombination pattern in intragenic crosses.

In the locus studied no correlation was found between the position of mutants on the map and their basic conversion frequencies. One of the mutants evidently caused an increase of conversion frequency of the two others. Crossing-over in intragenic recombination may be mutant-specific as revealed by using two mutants which give no recombinants when crossed with each other. The frequency of crossing-over was higher in crosses in repulsion than in coupling involving the same pairs of mutants. Polarization observed in two-point crosses was due in some instances predominantly to differences in basic conversion frequencies of the mutants used, and in others to the relative position of the mutated sites. Mutants located in the central part of the studied region were those which converted most frequently in two-point crosses. No reciprocal conversions were observed.

A number of recombinant asci resulting from two or more separate but highly correlated recombinational events within a gene were found.

The frequency of cotransduction of motC and H1 in Salmonella has been investigated, using four Salmonella serotypes and many serotypic recombinants as donors and S. typhimurium motC mutant as recipient. The frequency varied with the four serotypes from 7% to 52%. It is suggested that the difference in frequency arises from not only differences in genetic homology between the chromosome of the recipient and the fragment from the donor, but also from differences in genetic composition of the chromosome fragments carried by the phage. The frequency of serotypic recombinants selected for motC+ and H1 gene of the donor is generally higher than with recombinants selected for flaK+ and H1. The difference in genetic homology between S. typhimurium and other species is more marked in the region between H1 aud flaK than between motC and H1.

Strains of Aspergillus nidulans with a chromosome segment additional to the normal complement are vegetatively unstable. Previous work suggested that the deletions occurring at mitosis were confined to the unbalanced segments. It has been shown now that deletions, while probably always involving a duplicate segment, may extend beyond it to produce hypohaploids and hypodiploids, respectively, from unbalanced haploid and unbalanced diploid parents.

Hypoploids have been proposed tentatively as an explanation for some cases of phenotypic variegation; on this basis it is possible to account for some of the diverse phenomena shown by, for example, position-effect variegation.

Comparisons of skeletal dimensions are made between adult females of four lines of fowl, selected respectively for relatively long and short shanks (HI and LI) and for high and low body wieght (HW and LW) at 10 weeks of age. It is shown that elongation of the leg in HI compared with LI is greatest in the tarsometatarsus, least in the femur. HW and LW are intermediate in this respect. There is no comparable proximo-distal gradient of elongation in the wing. HI differs from LI in a manner comparable to ectomorphy in humans: limb bones are all longer in HI but relatively (and in most instances absolutely) thicker in LI. HW is again intermediate in this respect. The nasal region of the skull is disproportionately elongated in HI compared with LI. In certain bones HI and LI differ widely in cross-sectional shape of the shaft: LI is flatter than HI in the tarsometatarsus, less flat in the coracoid and scapula. HW resembles in each case the line with the less flattened cross-section.

Two polymorphic esterase loci, EstA and EstB, of the olive-fruit fly Dacus oleae were studied in a natural population. The analysis of about 500 individuals revealed the presence of 15 alleles for EstA and 12 alleles for EstB. A ‘silent’ allele was found segregating at both loci. Segregation data for most of the alleles are presented. The allele frequency distribution follows the same pattern at both loci: one allele of each gene has a frequency of nearly 0·50, a few have frequencies between 0·05 and 0·15 and many are below 0·05. Two main hypotheses, those of overdominance and selective neutrality, were examined in order to explain these polymorphisms. We deduced that under both hypotheses a relatively high mutation rate is necessary to balance the result of random drift. This rate was estimated to be higher than 4 × 10−5 for the EstA locus. Since homozygotes for the ‘silent’ allele at the first or at the second locus were found in the population in expected frequencies, it was concluded that these alleles are not inferior to active ones under natural conditions.

The gene pn was found to be active already in larvae 30 h old, long before eye-pteridines accumulated. At this developmental stage pn interacts with the gene K-pn. From studies with induced somatic recombination, gynandromorphs, and induced pn mutations, in a K-pn background it was concluded that the interaction pn-K-pn is non-autonomous in the epidermis, but in some internal organs it is apparently autonomous and causes the death of pn–K-pn flies. It is proposed that pn mutants interfere with an early step of the conversion of guanosine to pteridine, so that a deficiency of drosopterines, but not of the competing sepiapterine and biopterine, is caused. The accumulated precursor is presumably the substrate for a poisonous reaction triggered by K-pn.

Morphology and the rate of RNA synthesis of the X-chromosome in XX/XO mosaic larval salivary glands of Drosophila melanogaster have been examined. For this purpose the unstable ring-X was utilized to produce XX and XO nuclei in the same pair of glands. The width of the X-chromosome and the left arm of the 3rd chromosome (3L) of larval salivary glands was measured and the rate of RNA synthesis by them was studied upon the use of [3H]uridine autoradiography in such XX (female) and XO (male) nuclei developing in a female background (i.e. otherwise genotypically XX). In such mosaic glands the width of the single X-chromosome of male nuclei is nearly as great as that of the paired two X's of female nuclei, as is also the case in normal male (X Y) and female (XX). The single X of male nuclei synthesizes RNA at a rate equal to that of the paired two X's of female nuclei and nearly twice that of an unpaired X of XX nuclei. Neither the developmental physiology of the sex nor the proportion of XO nuclei in a pair of mosaic salivary glands of an XX larva has any influence on these two characteristics of the male X-chromosome.

It is suggested that dosage compensation in Drosophila is achieved chiefly, if not fully, by a hyperactivity of the male X, in contrast to the single X inactivation in female mammals, that this hyperactivity of the male X is expressed visibly in the morphology and metabolic activity of the X-chromosome in the larval salivary glands of the male, and that this hyperactivity and therefore dosage compensation in Drosophila in general is not dependent on sex-differentiation, but is a function of the doses of the X-chromosome itself.

Previous studies had shown that haploid strains of Aspergillus nidulans which have a chromosome segment in duplicate are unstable at mitosis. Through the study of various haploid and diploid strains, with and without translocations and with balanced and unbalanced genomes, it has been shown: (1) that imbalance of chromosome segments is responsible for instability, and (2) that the chromosomal deletions produced are confined solely or largely to the segments which provoke instability.

The term ‘mitotic non-conformity’ has been proposed for this instability phenomenon. An explanation for it has been sought in terms of attachment sites, limited in number and specific for chromosome segments, at which replication is initiated.

Temperate phages φ 80 and λ grown in E. coli strain C differ markedly in the degree to which they are restricted by E. coli strain K. The basis of this difference is subject to genetic recombination and deletion. The loss of restriction sensitivity by deletion shows that sensitivity is determined by loci in the phage genome. These loci are interpreted to be targets of the restriction mechanism, in agreement with previous biochemical evidence.