This paper presents evidence that the alcohol dehydrogenases (ADHs) in barley are specified by three loci. Six distinct ADH isozymes are observed following native slab polyacrylamide gel electrophoresis of crude extracts from flooded wild-type roots. Three of these isozymes are missing in flooded roots of plants homozygous for the Adhl-M9 mutation. The results also indicate that a simple binomial model (incorporating random dimerization and no inhibitive interaction of the two subunit species within heterodimers) is unable to account for the distribution of the total ADH activity between the ADH isozymes observed. Finally, the level and distribution of ADH activity in heterozygous (Adhl+ / Adhl-M9) flooded roots is not what would be expected if these contain only one-half of the available ADH1 protomers and the same frequency of available ADH2 and ADH3 protomers as is contained in the flooded roots of wild-type homozygotes (Adhl + / Adhl + ).

The accumulation of selfish DNA in eukaryotic genomes was studied from the standpoint of population genetics. Selfish DNA is assumed to replicate itself within a haploid set. For the selectively neutral case, the fate of a single self-replicating DNA segment (unit) within a population was investigated by the method of the probability generating function, and by Monte Carlo simulation, with special reference to the probability of survival and average number of units per haploid set. For the selectively deleterious case at the organismal level, the equilibrium between new occurrence and selective elimination was studied, and the average and variance of the number of units per haploid set in the population was examined by Monte Carlo simulation. It is shown that the process of self-replication (duplication–deletion) plays an essential role for the maintenance and elimination of selfish DNA.

Detectable allelic variation at the Gpi-1 loci on the short arms of the homoeologous group 1 chromsomes in wheat is not common. However, a variant null allele at the Gpi-D1 locus is present in some stocks of Chinese Spring. This has allowed the locus to be mapped between the ω-gliadin locus carried distally on the short arm of chromosome 1D, Gli-D1 (34·5%) and the high-molecular-weight glutenin subunit locus carried near the centromere on the long arm, Glu-D1 (36·2%). The origin of this isoenzyme polymorphism in Chinese Spring stocks is described and its potential significance is discussed in relation to quantitative analysis of aneuploids, alien chromosome addition and substitution lines and intervarietal chromosome substitution lines involving Chinese Spring.

Perculiarities of preimplantation and early postimplantation development were studied in embryos with partial deletions and duplications of chromosomes 7, 9, 14 and 17, in the progeny of mice heterozygous for the unequal reciprocal translocations T(7; 14)2Iem, T(16; 17)43H and T(9; 17)138Ca. Deficiencies for any part of autosomes 9 or 14 combined with duplications of the corresponding segments of autosomes 7 or 17 do not affect preimplantation development, though they are lethal soon after implantation. Deficiency for the distal part of chromosome 7 (Df7F4) induces embryonic death by the early blastocyst stage. Deficiencies for the distal part of chromosome 17 (Df17El–E5), as well as for its proximal region (Df17AB) carrying all genes of the T-t complex, have no detrimental effects on cleavage, blastulation and implantation, but are lethal after implantation, mostly during early neurulation. Deficiency for the middle part of chromosome 17 (Df17CD) is expressed just after a few cleavage divisions, and these embryos all die by the morula stage. It is suggested that the genes of the CD region of chromosome 17 and of the F4 region of chromosome 7 are of major significance for genetic control of early development in the laboratory mouse.

Two homologous autosomes of Drosophila that were attached to form a single entire compound autosome II were found to affect the segregation of the sex chromosomes in both males and females. The compound segregated nearly always from an attached X . Y chromosome in males with no other sex chromosome. When two sex chromosomes were present together with the compound they differed in their tendency to segregate from the compound. In males the X . Y chromosome segregated more often from the compound than did the Y chromosome, and the Y chromosome segregated more often from the compound than did the regular X chromosome. In females the X . Y segregated more often from the compound than did the regular X chromosome. This preferential segregation in females was observed for exchange X chromosomes as well as for the non-exchange chromosomes.

In the presence of the compound the frequency of primary non-disjunction of the sex chromosomes was elevated in both females and males; usually both sex chromosomes segregated from the compound and only rarely with it.

Flies devoid of most of the proximal heterochromatin of the sex chromosomes die. However, when the compound autosome was present some such flies survived. This indicates that a segment of the proximal heterochromatin of the sex chromosomes was intercalated into the compound when it was constructed. It was concluded that the segment intercalated into the compound carries specific sites for sex chromosome disjunction. Specific sites determine sex chromosome disjunction in males. In females they determine the disjunction of the sex chromosomes in cooperation with exchange pairing.

Some 400 wild mice (Mus domesticus) from southern Germany (the triangle formed by the cities Tübingen, Heidenheim and Friedrichshafen) were karyotyped and, in 243 of them, the chromosome compositions were determined by banding techniques. Virtually all mice tested carried at least one pair of metacentric chromosomes; some mice had up to ten metacentric chromosomes. Based on their chromosome composition, five mouse populations could be distinguished. Population I was characterized by the diploid chromosome number of 2n = 38 and the presence of two copies of metacentric chromosome Rb(4.12)1Tu. This translocation was also found in virtually all mice captured in southern Germany, almost always in a homozygous state. Mice of other populations had extra metacentric chromosomes Rb(5.15)15Tu (population II), Rb(13.14)17Tu (population III), Rb(5.14)18Tu (population IV) and Rb(11.13)6Tu (population V). In addition, rare variants (1 or 2 mice) were found in the different populations, which were heterozygous for additional metacentric chromosomes. Population V was quite heterogeneous in that it contained up to five metacentric chromosomes in addition to those mentioned. The number and the composition of these metacentric chromosomes varied from place to place. With the exception of population I, the individual populations occupied geographically distinct areas: Representatives of population I were found concentrated in one area, but, in addition, some were scattered over the entire studied region.

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.

A total of 207 wild mice trapped at different localities in southern Germany were tested for the presence of antigenic determinants controlled by class I genes (K and D) of the H-2 complex. The test was based on the complement-dependent killing of lymphocytes in the micro-cytotoxicity assay. Both private (allele-specific) and public (shared) determinants were tested using polyclonal and monoclonal antibodies. The results of the H-2 typing were in agreement with karyological typing which divided the sampled mice into 5 populations. Each population was characterized by a certain antigenic profile (occurrence of individual determinants at certain frequencies); the profiles of the individual populations were sufficiently unique to differentiate these populations but at the same time sufficiently similar to indicate common origin of the populations. The karyological typing of the same mice reveals that all 5 populations share 1 pair of metacentric chromosomes, Rb(4.12)1Tu, but that, in addition, each population has at least one metacentric chromosome differentiating it from other populations. We interpret these findings as evidence that all wild mice in southern Germany stem from a common stock in which the Rb(4.12)1Tu translocation became fixed and which subsequently differentiated into the individual populations. This differentiation is accompanied by the fixation of new Robertsonian translocations (different ones in different populations) and the acquisition of characteristic H-2 antigenic profiles.

Lethal and semilethal t-haplotypes in the mouse are polymorphic in wild populations, where they are presumably maintained by transmission distortion from males, which transmit the aberrant chromosome to over 95% of progeny. We report here measurements of transmission ratio that show an unexpectedly strong effect of background genotype, specifically genes on the homologous chromosome, on the degree of transmission distortion of t-haplotypes. We also provide additional evidence that within t-haplotypes two separable elements co-operate to control transmission distortion, and that the system is thus closely analogous to segregation distortion in Drosophila, as has been previously proposed by Gluecksohn-Waelsch & Erickson (1970), Demin, Safronova & Lapkina (1978), and Styrna & Klein (1982).

We studied the amount of Mu(G+) and Mu(G−) phages in different Mu lysates prepared either upon induction or upon infection of E. coli and Erwinia strains. We also looked at the level of expression of the modification function (mom) by Mu(G−) phages, both after induction and after infection of E. coli and Erwinia. The expression of mom seems to be regulated in the same manner in E. coli and in the strain of Erwinia carotovora tested. The proportion of both types of Mu(G+) and Mu(G−) phages in induced lysates is very variable and we found growth conditions favouring the production of Mu(G−) particles. This should extend the use of Mu as a genetic tool and as a generalized transducing phage to many enterobacteria.

Time until fixation of a mutant that occurs in one copy of a multigene family was investigated from the standpoint of population genetics. Because of the complexity of the double process of random drift (on the chromosome and in the population), an approximate method based on the rate of steady decay of genetic variability is applied. The simple model of gene conversion with constant gene family size is also used. The expectation based on the approximate method is shown to be valid by extensive Monte Carlo simulations, and the results are useful for understanding the mechanisms of turnover of multigene families, when comparison is available between closely related species.

The degree of heterozygosity as determined by electrophoretic analysis of three or four polymorphic loci correlates positively with survival in age groups of the American oyster (Crassostrea virginica) collected as spat from two different natural populations. The phenomenon is shown to operate in ages from 2 weeks (post-settlement) to 3 years and appears to be general in populations of marine molluscs. The most likely explanation for this result is that heterozygosity improves survival through its effect on growth (heterozygotes grow faster). The effects of individual loci on viability are independent of each other. A direct involvement of the enzyme polymorphisms is the most probable genetic interpretation of the data, but associative overdominance cannot be excluded.

Isolation and characterization of five new nuclease (nuc) deficient mutants of Neurospora have been described. The new mutants are unable to utilize nucleic acids as the sole phosphorus source and possess growth characteristics similar to those nuc (nuc-1 and nuc-2) mutants described previously. Two new mutants (nuc-4 and nuc-5) were able to use RNA or predigested DNA (but not intact DNA) as phosphorus source and showed temperature sensitive growth at 37 °C. Based on the data from complementation and genetic analyses the five new nuc mutants (nuc-3, nuc-4, nuc-5, nuc-6 and nuc-7) were found nonallelic to each other and to previously described nuc (nuc-1 and nuc-2) mutants; the new nuc mutants mapped to the right of arg-12 on linkage group II. On biochemical analyses, these nuc mutants were found to possess a lower level of extracellular nucleases and alkaline phosphatase as compared to the wild type strain. The ds DNase activity of the new mutants was only about 2–12% of that of the wild type strain; thus, the low level of these extracellular enzymes in the nuc mutants causes their inability to utilize nucleic acids as the sole phosphorus source. Wild type levels of these enzymes were restored in the complementing heterokaryons capable of full growth on the DNA medium. Data from intercrosses, mutagen sensitivity and spontaneous mutation-frequency studies (as discussed in a subsequent paper) indicated the involvement of the nuc genes in DNA repair and recombination.

A mutant, YS17, at the buff spore colour locus in Sordaria brevicollis has previously been described. It shows conversion with high frequency, predominantly to wild type, and is believed to act as a recognition site for an endonuclease that initiates recombination at the YS17 site. The discovery is now reported of a gene that causes loss of the high-frequency conversion shown by the YS17 mutant. The gene was present in existing stocks of the fungus. It reduces the conversion frequency of YS17 to a level similar to that of other buff mutants, from which it is inferred that the YS17 mutant no longer acts as an initiation site for recombination. When the conversion frequency of YS17 is low the bias in conversion to wild type rather than to mutant is lost, suggesting that this bias may relate to the initiation of recombination at the site. The loss of high frequency conversion of YS17 appears to be determined by a single recessive gene linked to mating type and unlinked to buff. It is suggested that the dominant allele induces recombination at the site of YS17 by controlling either the synthesis or the activity of an endonuclease that is capable of recognising the nucleotide sequence at the YS17 site. Some anomalous results point to the existence of modifiers of the action of the gene.

Non-germinating gibberellin (GA) responsive mutants are a powerful tool to study genetic fine structure in higher plants. Nine alleles (EMS-and fast neutron-induced) of the ga-1 locus of Arabidopsis thaliana were tested in a complete half-diallel. No wild type ‘recombinants’ were found in the selfed progeny of 9 homoallelic combinations (in total 3 × 105 plants); in the progenies from the 36 selfed hetero allelics the wild type frequency ranged from zero to 6·6 × 10−4. These frequencies allowed the construction of an internally consistent map for five different sites representing eight alleles. The ninth allele covered three sites and thus behaved like an intragenic deletion. The estimate of the total genetic length of the ga-1 locus was 0·07 cM. The order of the sites was also clearly reflected by the association with proximal outside markers. On the assumption that wild type gametes predominantly arise from reciprocal events, it was shown that a cross-over within the ga-1 locus leads to positive interference in the adjacent region.

The results are discussed with respect to the mutagen used, the frequencies found in other plant and Drosophila genes, and the possible occurrence of gene conversion.

Five new nuc mutants of Neurospora crassa were characterized for their relative sensitivities to different mutagens (UV, MNG, MMS), to mitomycin-C and to histidine; latter has been shown to inhibit the growth of certain UV sensitive mutants. These mutants were also compared for their capabilities for spontaneous mutation as determined by resistance to p-fluoro-phenylalanine. Based on these characterization, the mutants seem to belong to two groups. The first group included nuc-3 and nuc-6 which showed sensitivity to all mutagen tested and possessed capability for a very high frequency of spontaneous mutation (i.e. mutator effect). The second group included nuc-4, nuc-5 and nuc-7; these were as resistant to different mutagens as the wild type strain, but possessed an antimutator effect (i.e. the frequency of spontaneous mutation by these three mutants were at least 0·5–100 × less than the wild type strains). There was some variation in these properties of mutants belonging to the two groups. Among all the five nuc mutants, nuc-3 was characterized by extreme sensitivity to all mutagens. None of the five nuc mutants were sensitive to histidine. The properties of nuc mutants are discussed in relation to their possible role in DNA repair and recombination.

Aneuploidy can result from a variety of defects at meiosis. Results are presented of crosses of Sordaria brevicollis in which aneuploid spores are detected through complementation of spore colour mutants at the buff or grey-6 loci on linkage groups II and IV respectively. By using ascus analysis, the underlying cause of the aneuploidy can be deduced in many cases. Thus non-conjuction (pairing failure) and non-disjunction at the first meiotic division, premature centromere division, non-disjunction at the second division, and pre-meiotic errors such as extra replication of the chromosomes can be distinguished. Both linkage groups were found to give a similar proportion of the different errors. Non-conjunction and first-division non-disjunction formed 60–70% of detectable cases, whilst premature centromere division and second-division non-disjunction comprised 10% and 5% of aneuploids respectively. However, only a small proportion of second-division errors are detected.

It is proposed that the systems described in this paper can form the basis of a valuable screening method for detecting agents which increase aneuploid frequency. The advantages and disadvantages of using lower eukaryotes in this way are discussed.

To investigate whether the preferential expression of genes on the translocated X chromosome in female mice carrying the X-autosome translocation T(X; 16)16H (Searle's translocation) is due to non-random inactivation or to cell selection, we examined tissues of mouse embryos heterozygous for the X-linked gene coding for phosphoglycerate kinase (Pgk-1). From the cross T16H Pgk-lb/+ Pgk-lb ♀ × +Pgk-la / Y ♂, embryos expressing both isozymic forms of PGK-1 in the epiblast, and only the maternally inherited Pgk-lb allele in extra-embryonic tissues, were assumed to be chromosomally balanced, heterozygous female embryos carrying the Searle's translocation (like the mother). The normal X chromosome in this cross carries a high-expression Xcec locus. At 6 days post-coitum (p.c.) both isozymes were equally expressed in the epiblast as expected if both X chromosomes are active, but by 7 days p.c. the PGK-1B contribution was significantly less than 50%, suggesting that X inactivation has occurred with a bias towards inactivation of the translocated X chromosome carrying the lower-expression Xce allele. By 8 days p.c. the situation was the reverse, with a Pgk-lb contribution of significantly more than 50%, and by 12½ days p.c. no Pgk-la expression could be detected. We interpret the dramatic change in isozy me expression between 7 and 8 days p.c. as indicating rapid selection against cells that had inactivated the translocated 16X chromosome. Two 7-day p.c. embryos unexpectedly showed equal expression of both Pgk-1 alleles in both embryonic and extra-embryonic tissues; these were presumably chromosomally unbalanced embryos which had inherited from the mother both an active translocated 16X chromosome carrying Pgk-lb and an active normal X chromosome carrying Pgk-la.

Cytogenetic studies have ascertained that the segregation of the X-chromosome, during the first meiotic division of the oocyte in XO mice, occurs at random, contrary to the finding of some earlier authors. The ratio of nullo-X to X-bearing oocytes at ovulation does not change with maternal age. The usefulness of the XO mouse as a model for aneuploidy production in women (Lyon & Hawker, 1973) is discussed.

The zymogram phenotypes of 11 enzymes were determined for 22 Triticum aestivum cv. Chinese Spring-Elytrigia elongata disomic and ditelosomic chromosome addition lines. Eleven isozyme structural genes were located in specific arms of six E. elongata chromosomes, as follows: Gpi-E1 in 1ES, Est-E1 in 3ES, Got-E3 in 3EL, Adh-E1 and Lpx-E1 in 4ES, Adh-E2 and Lpx-E2 in 5EL, Amp-E1 in 6Eα, Adh-E3 and Got-E2 in 6Eβ, and Ep-E1 in 7EL. The E. elongata chromosomes present in five disomic addition lines have previously been designated 1E, 2E, 4E, 6E, and 7E to indicate their homoeology with Chinese Spring chromosomes. The results of this study support these designations. The development of disomic putative 3E and 5E addition lines is reported. The added chromosomes designated IV, V, and VI that are present in three of the seven original disomic T. aestivum-E. elongata addition lines are translocated. Evidence that VL and VIL are opposite arms of 2E and that IV is partially homoeologous to 3E has been published. The results reported in this paper indicate that IVS = 3ES, IVL = 7EL, VS = 3ES, and VIS = 5ES and are consistent with VL and VIL being opposite arms of 2E. The synteny relationships of the 11 E. elongata isozyme genes identified in this study are fully consistent with those of homoeologous T. aestivum cv. Chinese Spring genes and thus provide evidence that the gene synteny groups which these two species inherited from their common ancestor are conserved. This study further documents the valuable role that studies of isozyme genes can play in the isolation, characterization, and maintenance of alien chromosomes, telosomes, and chromosomal segments in wheat strains.