A selection of single and double mutant strains was backcrossed repetitively at least four times to each of the wild-types Abbott 4A, Abbott 12a and Lindegren 1A, and then re-isolated. Twelve crisp progeny isolated from the fifth backcross to Abbott 4A were heterogeneous for recombination frequency in the interval between cr and me-6 while thirteen crisp progeny from the fifth backcross to Lindegren 1A were homogeneous for recombination frequency in this interval. Thirteen crisp strains from the fifth backcross to Abbott 12a were homogeneous for recombination frequency between cr and nic-1 loci. However, mutant strains which had been backcrossed independently to Lindegren 1A were heterogeneous for recombination frequency in the intervals cr-aur and aur-nic-1.

In general, recombination frequencies in crosses between strains of the same wild-type ancestry including Abbott 4 and Abbott 12 were significantly higher than those in crosses of Lindegren × Abbott 4 or Lindegren × Abbott 12 ancestry but exceptions were found. Recombination frequencies between the same markers usually, but not always, increased on inbreeding and the changes in frequency were non-uniform in the marked region in some crosses.

The limitations of backcrossing as a means of transferring a mutant marker into a wild-type genome were discussed. It was concluded that inducing marker mutations anew in a given wild strain, preferably Lindegren 1A, might be more successful in obtaining constancy of map distance.

The translocation break T(2;9)138Ca is located about three map units from the H-2 region on the non-centromeric side of the IXth linkage group. The recombination frequency in the T-H-2 interval is increased in the presence of the T138 translocation to about twice its value in the absence of the translocation.

Seventeen dominant cataract mutations of the mouse recovered in ethylnitrosourea mutagenesis experiments have been genetically characterized as to penetrance, fertility, and homozygous viability. Nine mutations were shown to be fully penetrant with no fertility effects, four mutations were classified as having reduced penetrance with no fertility effects, one mutation had reduced penetrance and reduced fertility, two mutations were shown to have a reduced frequency of mutant offspring due to penetrance and viability effects, and one mutation most likely has a reduced viability of carrier individuals. Of the eleven mutations for which definitive homozygous viability data were obtained, ten were shown to be homozygous viable and only one was shown to be homozygous lethal. In similar experiments in which dominant cataract, dominant skeletal or dominant visible mutations were recovered after radiation treatment, comparable frequencies of mutations with reduced penetrance were observed but there was a strikingly higher frequency of homozygous lethal mutations. These observations support the hypothesis of a qualitative difference in the mutations recovered after ethylnitrosourea as compared to radiation treatment. Finally, it is argued that a systematic comparison of the induced mutation rates to dominant and recessive alleles with subsequent genetic characterization of the recovered mutations provides a critical set of data necessary for an improvement in the indirect and direct procedures of genetic risk estimation.

A mathematical treatment has been presented of the ideas of de Haan & Gross concerning transfer delay and chromosomal withdrawal during conjugation In Escherichia coli. The calculations involve three parameters: (i) the maximum delay, λ, which can occur between the formation of effective contact in mating pairs and the initiation of chromosomal transfer, (ii) the probability, b, that mating pairs separate with the withdrawal of that segment of the Hfr chromosome which has entered the female cell, and (iii) the probability, c, that mating pairs separate with the breakage of the Hfr chromosome at the point where it enters the female cell, leaving the injected fragment in the female.

A comparison of the theory with the experimental results of de Haan & Gross obtained when chromosomal transfer occurs either in minimal medium or in broth shows good agreement under the following conditions:

(i) the value of λ is the same under both growth conditions,

(ii) the value of b is the same under both growth conditions,

(iii) the value of c is much greater during transfer in broth than it is in minimal medium.

Irradiation has been employed successfully to increase the reverse mutation rate at the agouti and dilute loci in the mouse. The dilute allele has previously been shown to be due to the insertion of an ecotropic-specific murine leukaemia virus in the vicinity of the dilute locus, and its instability to be due to the excision of the proviral sequence (Jenkins et al. 1981). Molecular analysis of the recovered radiation-induced revertant at the dilute locus indicated excision of all but approximately 500 bp of the proviral sequence. The proviral sequence remaining in the mouse genome hybridizes to a probe specific for the proviral long terminal repeat (LTR) sequence. Previous characterization of two spontaneous reverse dilute mutations indicated precise proviral excision of all but a single LTR, and suggests homologous recombination between the proviral LTR sequences as the mechanism of proviral excision (Hutchison, Copeland & Jenkins 1984). The present results indicate that radiation and increases the reverse mutation rate at the dilute locus acts by a similar mechanism, and suggest that mutagenic treatment may be employed to produce genetic variants of interest.

Genetic variation at 11 enzyme loci in Aedes aegypti populations collected from 63 localities around the world is presented. A UPGMA tree based on genetic distance values clusters populations of sylvan A. aegypti formosus from West Africa and East Africa together, along with Asian and south-eastern U.S. populations. Domestic A. aegypti aegypti from East Africa and all other New World populations form the other major cluster. Multivariate discriminant analysis allows recognition of seven major global ‘genetic–geographic’ groups, which are consistent with the genetic distance data. Populations from the south-eastern U.S. are clearly genetically distinct from other U.S. and Mexican populations. This distinct gene pool may be indicative of reinfestation of areas where A. aegypti appears to have been absent in the recent past. Other evolutionary and epidemiological implications of the genetic population structure of Aedes aegypti are discussed.

Several ribosomal RNA (rRNA) genes from D. busckii were cloned and characterized. The prominent repeat classes have lengths of 12·8 and 13·6 kb and lack 28S introns. rRNA genes were cloned containing 28S insertions which exhibit heterogeneity in size and sequence. The non-transcribed spacer (NTS) contains two regions composed of different repeated sequences that exhibit pronounced instability in HB 101. NTS region II, centrally located within the NTS, contains predominately 11 or 16 HincII generated 160 bp repeats. NTS region III, next to the 18S gene, contains repeats that are variable in number, and are either heterogeneous in length or are dispersed within unique sequences. The organization and composition of the rRNA gene NTS of D. busckii is different in comparison to the NTSs of other drosophilids. In addition, the pronounced instability of two different NTS regions is unique in comparison to all other cloned rRNA genes.

This paper describes a new recessive maternal lethal which disrupts normal nuclear division and migration during cleavage in Drosophila. We have named this gene locus supernova. Deletion mapping and in situ hybridization have located this gene to 88 F9/89 A1 on the polytene chromosome map. The terminal mutant phenotype is characterized by the presence of many variable-sized nuclei scattered throughout the cytoplasm of the unhatched egg. Following fertilization, the initial cleavage divisions appear delayed and are often accompanied by the formation of ring-like association of chromosomes and/or chromosome bridges. Although the polymerization of tubulin into spindles occurs during the initial cleavage divisions, there appears to be both a spatial and temporal uncoupling of DNA replication from the formation and proper functioning of spindles. Eventually no functional spindles are formed, but nuclei continue to increase in size and number with increasing age of the embryo following fertilization.

Starting from a geographically hybrid foundation population of Drosophila subobscura, selection for fast and for slow development has been practised without inbreeding on a diet with an unusually high level of protein. Realized heritabilities in the fast and slow lines were + 0·063 ± 0·029 and + 0·186 ± 0·031 respectively. A half-sib analysis of the foundation population and full-sib analyses of the first two and the last two selected generations were carried out. Hybrids between the two lines were approximately intermediate between their parents.

Two types of genetic explanation of the asymmetrical response are discussed. The first assumes directional dominance of alleles for fast development. Such an assumption can explain the asymmetrical response, but runs into difficulties in explaining the nature of the genetic variance in the selected populations and the intermediacy of the hybrids between the two lines.

A second assumption, which appears to fit the facts better, is that there exists a ‘developmental barrier’ preventing development at a rate appreciably faster than that of the foundation population. In physiological terms this implies that more rapid development requires a more profound modification of the population than could be achieved by a few generations of selection. In genetic terms, it implies epistatic interactions between genes at different loci: gene substitutions at a given locus which increase development rate on a genetic background causing slow development have little or no effect on a genetic background causing rapid development. In other words, there is a law of diminishing returns as more and more alleles for fast development are accumulated in the genotype. It is suggested that genetic situations of this kind may be common in populations which have been exposed to directional selection for a long time in reasonably large populations, either in nature or in domestication.

1. Changes in muscles and tendons in sm/sm are confined to hands and feet. They are closely correlated with the changes in the osseous skeleton.

2. In Os/+, muscular changes are much more complex and not confined to hands and feet, but include the muscles of the forearms and of the lower legs. Some of the muscular changes, especially on the postaxial side, are not correlated with the changes in the osseous skeleton.

The genetic consequences of autogamy have been analysed in the autogamous strain A-25 of E. minuta. During autogamy as well as during conjugation four pronuclei (two genetically identical pairs) are formed in each individual. In conjugating pairs, any two of the four pronuclei of the autogamous conjugant may participate in synkaryon formation. Consequently, an 8:13 segregation ratio of clonal and synclonal mating-type inheritance has been found to occur in the progeny obtained by crossing A-25 animals with non-autogamous animals. During autogamy, synkaryon formation appears to be brought about by a preferential karyogamy of genetically dissimilar nuclei more often than is expected on a random basis. Therefore, heterozygosity is usually maintained after autogamy in spite of the extreme inbreeding characteristics of this sexual process.

To simplify the description of selection in two environments the terms ‘ antagonistic’ and ‘synergistic’ are used. Selection upwards in a bad environment or downwards in a good environment is antagonistic, the selection and the environment acting in opposite directions on the character. Synergistic selection is the reverse, upwards in a good environment or downwards in a bad, selection and environment acting in the same direction. Published experiments are reviewed to see how well they agree with two expectations. First, Jinks & Connolly (1973) showed that antagonistic selection reduces environmental sensitivity and synergistic selection increases it. The experiments reviewed showed many exceptions to this rule, but they all showed that sensitivity was less after antagonistic than after synergistic selection. This is shown to be simply the consequence of correlated responses being less than direct responses. Second, I suggested (Falconer, 1989) that antagonistic selection might be the best way to improve the mean performance in the two environments. In the experiments reviewed, antagonistic selection was significantly better than synergistic for changing the mean, but it is now shown that there is no theoretical justification for this expectation; if one type of selection is better in one direction the other ought to be better in the other direction.

Expressions are given for the changes of mean performance and of sensitivity resulting from selection in one or other environment; these changes can be predicted from the parameters of the base population. In the experiments reviewed, an increase of mean performance accounted for 49% or more of the upward response. Equations are presented which allow the variance of mean performance, the variance of sensitivity, and the covariance of mean with sensitivity to be derived from parameters estimated in an unselected population, namely the variances in the two environments and the corresponding covariance. The variance of sensitivity that might be ascribed to scale effects is deduced. Directional selection in a single macro-environment is synergistic with respect to the micro-environmental differences, and is expected to increase environmental sensitivity and consequently to increase environmental variance. Stabilizing selection is antagonistic selection in both directions at the same time, and so is expected to decrease environmental variance.

In Saccharomyces lactis, a class of mutants isolated for low β-glucosidase activity are reduced in activity for β-galactosidase as well. Genetic studies indicate that their properties are the result of a single mutation in a nuclear gene. In diploide containing a wild-type and mutant β-galactosidase allele, the mutant phenotype is partially dominant. The two enzymes can be separated physically and under appropriate conditions are induced independently in wild-type strains.

DNA fingerprint is a pattern of a variable number of bands (DNA fragments) with different sizes on a Southern gel for each individual, generated by one or many VNTR loci. Genetic divergence between individuals within and between populations can be studied in terms of number of shared bands between individuals. Using a population genetic model we show that the expectations of measures of genetic distance between populations based on band sharing data from DNA fingerprint patterns are functions of composite parameters M = 4Nv, and time of divergence (t) between populations, where N is the effective size of the populations, and ν, the mutation rate. The expected genetic distance remains linear with time of divergence at least up to N generations as long as the average heterozygosity at the DNA fingerprint loci remains at or below 90%. Neither incomplete knowledge of the allele frequencies at each locus, nor the unknown number of loci underlying DNA fingerprint pattern, compromise these evolutionary dynamics of DNA fingerprint patterns. Applications of this theory to data on three human populations, and review of literature indicate that co-migration of alleles, and the presence of syntenic loci underlying the fingerprint pattern have little impact of the reliability of evolutionary conclusions from DNA fingerprint studies.