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.

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.

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.

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.

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.

An arg− conditionally Sm-dependent E. coli B strain became unable to grow on minimal agar containing low levels of Sm when the strain acquired an R factor which mediates Sm and Blu adenylation. The same R-containing strain, however, grew normally when Blu was substituted for Sm in the agar. The data strongly suggest that Sm-adenylate does not suppress the arg− phenotype in an in vivo system. The possibility that Blu-adenylate can cause phenotypic suppression without being bactericidal in vivo is discussed.

Nine methionine mutants of Aspergillus nidulans (six new mutants plus three isolated previously) were examined; five responded to homocysteine, none responded to cystathionine or cysteine.

Ten revertants of one of the mutants, methB3, blocked before homocysteine, were shown to be due to suppressor mutations. The suppressors were divided into six genes on the basis of complementation tests and recombination data. Mutants of two of the genes were semi-dominant in the heterokaryon but recessive in the diploid. Experiments in which conidial ratios in the heterokaryons were determined suggested that semi-dominance of these suppressors is due to a shift in nuclear ratios in the heterokaryon in favour of the suppressor nuclei.

Some of the suppressors were tested for suppression of two other methionine loci; they acted on methG1 blocked before homocysteine, but not on methH2 blocked after homocysteine, although four out of 13 crosses tested gave ambiguous results.

Cellular functions of a double mutant of Escherichia coli K 12 deficient in recombination (recA) and defective in excision of pyrimidine dimers (uvrB) have been compared to those of isogenic recA or uvrB single mutants and ‘wild type’ bacteria. A combined effect of the two mutations on cell survival both under normal conditions of growth and after exposure to ultraviolet light or mitomycin C was demonstrated. The ratio of optical density to the number of colony formers in growing cultures of the double mutant is three times greater than in similar cultures of the recA single mutant and 9 times greater than in either uvrB or in ‘wild type’ cultures. The doubling time in growing recA uvrB cultures is 90 min, compared to 60 min, for the recA single mutant and 40 min for the uvrB single mutant and ‘wild type’ bacteria. Growing cultures of recA uvrB (λcI857) bacteria contain a substantial fraction of cells which are unable to form colonies at 32 °C, but produce phage when heated to 42 °C. No such cells were found in cultures of the single mutants or the ‘wild type’ bacteria lysogenic for λc1857. The double mutant is 10 times more sensitive to ultraviolet light and twice more sensitive to mitomycin C than the recA single mutant. In contrast to recA bacteria, exposure of the double mutant to mitomycin C induces little additional breakdown of cellular DNA. Induction of the prophage by mitomycin C is, however, prevented in both recA uvrB (λ) and recA (λ) bacteria. Exposure to mitomycin C creates conditions which render the prophage inducible by a newly transduced ree A gene. This effect of mitomycin C persists and can be revealed in complete medium at 37 °C after 100 min of incubation. The decay of the prophage, in cells exposed to mitomycin C, proceeds at a similar rate in both the double mutant and the recA single mutant. The inability of recA lysogens to be induced to phage production is discussed in the light of the present findings.

Certain drd mutants of fi+ R factors, when carried by strain HfrC, allowed polarized transfer of the Hfr chromosome to occur at the normal rate. These mutants were independently shown to be repressor-sensitive and so owed their de-repression to failure to produce represser (i−). With other drd mutants, independently shown to be repressor-insensitive (0c), the rate of polarized chromosome transfer was as low as with the wild type and only R pili were produced by the HfrC+ bacteria. These R factors, therefore, continued to produce repressor and the donor behaviour of an Hfr strain depends on functioning of the integrated F.

F-like R factors are related to ColB-K77, but not to ColB-K98 or ColB-K166, by the criterion of superinfection immunity. Tests of de-repressed mutant R factors for susceptibility to repression by ColB-K98, known to produce represser for F, showed that these could be either repressor-sensitive or insensitive. The former were independently shown not to produce repressor, which was thus, presumably the reason for their de-repressed state.

An Arg− conditionally streptomycin (Sm)-dependent strain of E. coli B that carries an R factor mediating Sm adenylation cannot grow on minimal agar containing 20 μg Sm/ml unless the strain either becomes Arg+ or loses the ability to adenylate Sm (Harwood & Smith, 1969b). We have therefore studied the efficacy of this selection with respect to enriching for SmS mutants of such R factors. Following u.v. mutagenesis, cells were plated on the selective medium; of the clones which were examined, Arg− R-SmS colonies were found up to 3000 times more frequently than Arg+ R-SmR colonies.

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.

Chlosyne lacinia (Geyer) is a nymphalid butterfly ranging from the Southwestern United States and thence southward to Argentina (Ehrlich & Ehrlich, 1961). The insect is multibrooded and preferably feeds on sunflower (Helianthus annuus L.), although high populations have also been observed to be maintained on cocklebur (Xanthium saccharatum Wallr.).

A method has been devised for the selection of auxotrophic mutants in higher plants. The method depends upon the incorporation of BUdR into the DNA of non-auxotrophic cells and upon its lack of incorporation into the DNA of auxotrophic cells. A wide range of auxotrophic types were recovered.

In yeast it has been reported that some types of erythromycin-resistance are due to cytoplasmic genetic factors, localized in mitochondria (Thomas & Wilkie, 1968; Slonimski, 1969). In view of the current interest in genetical aspects of mitochondria, and the ease with which cytoplasmic heredity can be demonstrated in Paramecium aurelia, some tests have been made to determine whether erythromycin-resistant variants could be obtained in this ciliate and, if so, to determine their genetic basis.

Complementation tests have been performed on a group of closely linked genes affecting the formation of the central pair of microtubules in the flagella of Chlamydomonas reinhardii. The procedure adopted involved the selective isolation of diploid strains in which the mutant genes were recessive to then wild-type alleles. The mutations tested were found to be in the same cistron.