Under suitable experimental conditions, short denatured DNA fragments may be bound by high molecular weight denatured DNA immobilized in agar (‘long DNA’). The proportion of fragments bound depends in part upon the degree of taxonomic relationship between the organisms from which the two DNA components are taken. Before the method can be used as a measure of genetic relationship between closely related species or between intra-specific groups, its resolving power requires to be increased. The present study explores some modifications of the method from this point of view, using the degree of binding between DNA from laboratory mice and rats as a model.

Long rat DNA was less effective than long mouse DNA in binding short DNA fragments from either source. It was therefore necessary to test reciprocally, using both short mouse and short rat DNA on long DNA from each species. Combining reciprocals gives a measure of the proportion of fragments bound in within-species and in between-species combinations. With rats and mice, between-species binding was at about 70% of the within-species level. The discrepancy could be increased, and hence the discriminating power of the method improved, if the binding reaction was partially or totally ‘blocked’ with an excess of unlabelled DNA fragments in an appropriate species combination.

Even when short and long DNA came from the same source, only about a third of the fragments were bound. The bound and unbound fractions were recovered, and each reincubated with a fresh sample of long DNA. The two fractions then behaved very differently, suggesting that the original DNA preparation is heterogeneous. The initially unbound DNA showed a very low binding ability in subsequent incubations and may be considered virtually ‘unbindable’.

When the first incubation is a between-species one, the unbound fraction should include not only this ‘unbindable’ DNA, but also any DNA characteristic of that species alone, and not held in common by both species. If it could be isolated, this DNA fraction might be expected to discriminate maximally between the two species. By using three successive cycles of incubation, a fraction was obtained which showed considerably improved discriminating power, in that it showed only about 30% as much binding in between-species as in within-species combinations.

When a selectively favourable gene substitution occurs in a population, changes in gene frequencies will occur at closely linked loci. In the case of a neutral polymorphism, average heterozygosity will be reduced to an extent which varies with distance from the substituted locus. The aggregate effect of substitution on neutral polymorphism is estimated; in populations of total size 106 or more (and perhaps of 104 or more), this effect will be more important than that of random fixation. This may explain why the extent of polymorphism in natural populations does not vary as much as one would expect from a consideration of the equilibrium between mutation and random fixation in populations of different sizes. For a selectively maintained polymorphism at a linked locus, this process will only be important in the long run if it leads to complete fixation. If the selective coefficients at the linked locus are small compared to those at the substituted locus, it is shown that the probability of complete fixation at the linked locus is approximately exp (− Nc), where c is the recombinant fraction and N the population size. It follows that in a large population a selective substitution can occur in a cistron without eliminating a selectively maintained polymorphism in the same cistron.

A computer model of a two-locus genetic system with epistatic selection was used to investigate factors influencing the probability of the origin of reproductive isolation, due to a genetic revolution following a founder event (Mayr, 1954; Carson, 1975). Restricted population size can sometimes cause such a system to drift from one equilibrium to another, which can result in loss of fitness to hybrids with the ancestral population. The chance of such an event was found to be low unless the bottleneck in population size associated with the founder event was preceded by many generations of relaxed selection. It is highest when the bottleneck is not prolonged and when the population size during the bottleneck is not too small. It seems to be difficult to achieve a high degree of reproductive isolation in one step by this method, and it is concluded that it is unlikely to be a major cause of rapid speciation, although it could be a contributory factor.

Mutation is modelled in two quantitative characters under separate genetic control in a large population. A bivariate pattern of selection acts to correlate the characters and, without pleiotropy, their genetic correlation is due entirely to linkage disequilibrium. Data on spontaneous mutation, the effective number of genes, and the intensity of natural selection on quantitative characters are used to evaluate the models. It is concluded that, even when selection favors a high correlation between the characters, with random mating and no linkage between loci influencing different traits the genetic correlation between characters is likely to be small in magnitude. A genetic correlation of large magnitude can be maintained only if the loci influencing different characters are tightly linked, or there is a high level of inbreeding in the population created by frequent mating between closely related individuals.

I describe here a new screening procedure to isolate ribosomal suppressors in Podospora anserina. I have used the sporulation defect displayed by an antisuppressor mutation AS7–2. The revertants able to sporulate are due to either true reversions or external mutations. The mutations which restore most efficiently the sporulation show all the properties of ribosomal suppressors and are localized in two new suppressor loci su11 and su12.

In order to detect regulatory genetic sites in the autosomes of Drosophila melanogaster, the levels of X-linked glucose-6-phosphate dehydro-genase and autosomally linked α-glycerophosphate and isocitrate dehydrogenases have been monitored in extracts of flies aneuploid for regions of chromosomes II and III. In addition to expected structural gene dosage responses of α-GPDH and IDH, flies hyperploid for several autosome regions were found to display altered levels of one or more of the enzymes studied. While IDH activity was increased in flies hyperploid for segments of both chromosomes II and III, α-GPDH activity was decreased in specific hyperploids for chromosome II regions only. The latter group of segmental aneuploids were normal with respect to levels of chromosome II-linked alcohol dehydrogenase. To test if the observed responses were due to dosage changes of discrete genes lying within the larger effective segments, flies aneuploid for subdivisions of the chromosome segments 21A-25CD, 35A–40, and 70CD–71B were assayed. For two of these large segments so analysed, the apparent effects were attributable to specific small subdivisions, suggesting the presence of discrete regulatory sites within the latter. For the 35A–40 region the α-GPDH effect observed for subdivisions was not sufficient to account for the large α-GPDH decrease seen in flies hyperploid for the large, inclusive region. These observations are discussed with respect to the possible bases of effect of regulatory elements on enzyme activity.

The mutagenic effects of feeding calf-thymus DNA to larvae of Drosophila melanogaster have been studied on second chromosome recessive lethals, sex-linked dominant and recessive visibles and dominant autosomal visible mutations. While a positional specificity of DNA-induced lethals has been confirmed, no evidence has been obtained for a preponderance of visible mutations or for a specificity of phenotype among the visibles.

A genetic analysis following the initial detection of a female sterile variant resulted in the finding of seven or eight different female sterile mutants, most or all of which are on linkage groups I and II. They were present in heterokaryotic condition in already existing strains, except one which originated spontaneously during the study. All mutants fail to produce functional protoperithecia. Most of them, however, are able to function as female parents in heterokaryons. All mutants differ morphologically from the wild type, most being subtly different, but two being appreciably different. The apparently high frequency of occurrence of female sterile mutants suggests that protoperithecial development is under an elaborate genetic control. Differences in vegetative morphology appear to be a common property of mutants affecting the early stages of the sexual development.

A system has been developed for the study of reversion of an amber mutation responsible for a deficiency in DNA synthesis in T4 phage E51. When complexed with bacteria able to suppress the amber mutation the induced mutation rate per phage genome per rad is

When complexed with bacteria unable to suppress the amber mutation (and being thus unable to synthesize phage DNA) the induced mutation rate is at least 14 times lower indicating that DNA synthesis is necessary for the production of the majority of functional reversions at the amber site. The induced mutation rate in suppressor-containing bacteria is independent of multiplicity of infection between 0·2 and 5, suggesting that recombination immediately after irradiation between phage genomes is unlikely to be a requirement for the mutation process.

From studies involving inhibition of DNA synthesis in Hfr strains of Escherichia coli K12, either by thymine starvation (Pritchard, 1963) or amino-acid starvation (Suit, Matney, Doudney & Billen, 1964), during mating with F− strains, it has been concluded that transfer of DNA from males to females can occur in the absence of DNA synthesis. This conclusion is at variance with the hypothesis (Jacob, Brenner & Cuzin, 1963) that the energy required for transfer is derived from the process of DNA replication. On the other hand, a second prediction from this hypothesis, that one polynucleotide chain of the DNA transferred during mating will have been synthesized during transfer, is strongly supported by recent experiments (Ptashne, 1965; Blinkova, Bresler & Lanzov, 1965; Gross & Caro, 1965).

A method for analysing probabilistic models representing an extension of path analysis, and a model of mixed homogamy based on this method were introduced recently (Rao, Morton & Cloninger, 1979; Cloninger, 1980). However, constraints imposed by the method on linear models, or a rationale for and implications of the mixed homogamy model have not been clearly stated.

A mathematical treatment of the extension of path analysis and of the mixed homogamy model is presented in this paper. Constraints on linear models imposed by the extension are obtained. It is demonstrated that the mixed homogamy model, when applied to analysing nuclear families in a population, implies the following mating system in the population: some individuals choose their mates strictly on the basis of group membership, others choose their mates strictly on the basis of phenotype, and no individual chooses a mate on the basis of both group membership and phenotype.

The resolution of antibiotic-CsCl gradients enabled an examination of the satellite DNAs in the nuclear DNA of Drosophila simulans. Of the eight distinct satellite DNAs which were detected, four band at almost the same buoyant density in CsCl but can be resolved in netropsin sulphate-CsCl gradients. Each consists of a repeated sequence which, in five of the satellites, is shown to be arranged in tandem for long regions of the chromosomal DNA. One satellite (1·697 g/ml in CsCl) contains repeated sequences interspersed with other sequences. The satellite DNAs were compared with the satellite DNAs known to be present in the sibling species, D. melanogaster. The two species have different overall complements of satellite DNAs, but one satellite (1·672 g/ml) may be identical.