A pedigreed control strain was divided into three lines which were maintained genetically distinct over nine generations. Mean coefficients of inbreeding of about 0·1 were attained. Results suggest that discernible genetic drift had occurred in several traits. Separate estimates of genetic variance were obtained from observed variation between and within lines. These estimates differed significantly in the case of only one of the eight traits observed. Observed variation between lines agreed reasonably well with that predicted.

The response to long-term selection for increased abdominal bristle number was studied in six replicate lines of Drosophila melanogaster derived from the sc Canberra outbred strain. Each line was continued for 86–89 generations with 50 pairs of parents selected at an intensity of 20%, and subsequently for 32–35 generations without selection. Response continued for at least 75 generations and average total response was in excess of 36 additive genetic standard deviations of the base population (σA) or 51 times the response in the first generation. The pattern of longterm response was diverse and unpredictable typically with one or more accelerated responses in later generations. At termination of the selection, most of the replicate lines were extremely unstable with high phenotypic variability, and lost much of their genetic gains rapidly upon relaxation of selection.

The variation in response among replicates rose in the early phase of selection to level off at approximately 7·6 around generation 25. As some lines plateaued, it increased further to a level higher than would be accommodated by most genetic models. The replicate variation was even higher after many generations of relaxed selection. The genetic diversity among replicates, as revealed in total response, the individuality of response patterns and variation of the sex-dimorphism ratio, suggests that abdominal bristle number is influenced potentially by a large number of genes, but a smaller subset of them was responsible for selection response in any one line.

In the housefly, mosaics appear spontaneously but rarely. Sexual mosaics or gynandromorphs also appear in strains in which sex determination is based on autosomal sex factors. Rare cases of recombination in the male have been reported by some authors. In field and laboratory populations, mitotic plates with figures indicating exchange of chromatid segments are regularly observed in tissues of individuals of both sexes and at all stages of development. All these anomalies are interpreted as outward manifestation of the same phenomenon: mitotic recombination. The cytological basis of mitotic recombination, its relative frequency, its influence on linkage and genetic variability are discussed.

A mathematical model for the kinetics of transfer of non-selftransmissible (nonconjugative) plasmids by mobilizing conjugative factors is presented and methods to estimate its parameters described. Using batch and chemostat cultures of Escherichia coli K-12 with the nonconjugative plasmid pCR1 and an F′ mobilizing factor, the parameters of this model were estimated. The observed changes in concentrations of the different parental and transconjugant cell types and the changes in these concentrations anticipated from the model are presented for two different ‘mating’ combinations in both exponentially growing and equilibrium chemostat populations of E. coli. The results of these experiments are interpreted to suggest that for bacterial populations dividing at a constant rate in liquid culture, the kinetics of mobilization transfer of nonconjugative plasmids can be reasonably well described by a simple set of mass action differential equations. These results also suggest that the carriage of the nonconjugative plasmid pCR1 has little, if any, effect on the capacity of a host bacterium to donate or receive conjugative F′ plasmids.

Lethal frequencies on the second and third chromosomes were estimated three times in six replicate lines of Drosophila melanogaster selected for increased abdominal bristle number, at G 14–16, G 37–44 and G 79. Ten lethals were detected at a frequency of about 5% or higher at G 14–16, of which only one recurred in subsequent tests. Another ten lethals which had not been detected previously were found at G 37–44, and the 5 most frequent ones recurred at G 79. In the last test, 15 presumably new lethals were detected, of which at least 4 appeared well established. In addition, six reversions (from sc to sc+), a new mutant at the scute locus and sca were discovered. The effects on the selected character of some lethals and visible mutants were large and variable, but not always sufficient to explain the observed frequencies. The major lethals detected at G 37–44 and G 79 for the first time were most probably ‘mutations’ (in the broad sense) which occurred during selection. The likely origins of such ‘mutations’ were discussed, with a suggestion that the known mutation rate for recessive lethals would not be incompatible with the observed frequency of occurrence of the ‘mutations’.

The probability distribution of the heterogenic (non-identical by descent) fraction of the genome in a finite monoecious random mating population has been derived. It was assumed that in any generation the length of both heterogenic and homogenic segments are exponentially distributed. An explicit expression is given for the expected number of ‘external junctions’ (sites that mark the end of a heterogenic segment) per unit map length in any generation. The latter necessitates the introduction of two higher-order identity relations between three genes, and their recurrence relations. Theoretical results were compared with the outcome of a series of simulation runs (showing a very good fit), as well as with the results predicted by Fisher's ‘theory of junctions’. In contrast to Fisher's approach, which only applies when the average heterogeneity is relatively small, the present model applies to any generation.

When the technique of saturation mapping is employed to estimate the number of loci in a distinct chromosomal region, there is always the possibility that some loci will not be detected. If the number of mutants per locus follows a Poisson distribution, the number of mutationally silent loci can be estimated. This paper describes a method for fitting such data to a Poisson distribution truncated at the zero class and a method for estimating the number of mutationally silent loci. The use of these methods is demonstrated by their application to some published data.

An important question in evolutionary biology concerns the manner and tempo in which organismal and/or genetic changes that promote evolutionary divergence occur. One recent hypothesis, termed rectangular evolution, holds that most significant evolutionary change occurs during occasional or periodic speciation episodes, with long periods of evolutionary stability in the interim. An alternative view, termed phyletic gradualism, holds that evolutionary divergences proceed by the slow and even accumulation of genetic differences within populations of established species. Two brief tests of rectangular evolution are presented using chromosomal data from North American cyprinid fishes (minnows), a group known to have experienced heterogeneous rates of splitting. Within the rapidly speciated genus Notropis, rates of chromosomal evolution appear slower relative to other, less rapidly speciated confamilial genera. Species of Notropis also are less divergent chromosomally, on the average, than are species from other cyprinid genera. These results are in compatible with a rectangular mode of chromosomal divergence these fishes. The results also reveal inconsistencies with a gradual mode chromosomal divergence, but at present this hypothesis cannot be falsified. Consideration of these and other data suggests that different levels of the cyprinid genome may follow independent evolutionary paths.

A study of crosses between CBA/FaCam and C57B1/6Fa mice revealed an effect of the origin of the Y-chromosome on testis weight and aggressive behaviour, but failed to reveal any effect on sexual behaviour and androgen metabolism. There is therefore no evidence that androgens mediate the Y-linked variation in aggressive behaviour and testis weight. On behavioural grounds, it is difficult to compare measures of sexual and aggressive behaviour, but it appears from these results that there are major genetic components on the Y-chromosome controlling the development of sexual and aggressive behaviour which are distinct.

Normalizing selection with a moving optimum can result in strong selection for increased recombination. This conclusion is based on computer simulation of an infinite, random-mating diploid population with additive, polygenic determination of the selected phenotype. There can also be selection for increased recombination with a fluctuating optimum, and with directional selection. These effects which arise in an infinite population appear to be large compared to the additional effects arising in a finite population because of the random generation of linkage disequilibrium.

Linkage relationships of three gene markers of chromosome 17, namely Brachyury (T), tufted (tf), and Histocompatibility-2 (H-2), to the break-point of T(16; 17)43H male sterile translocation were established. The following order was found: T−tf−T43H−H-2. In all cases the translocation break was found in cis to H-2k, haplotype, no recombinant being found among 218 backcross individuals examined. More than 60 viable and fertile animals trisomic for the proximal part of chromosome 17 (including T-t genetic complex) have been recovered among progeny of T43H/+ female translocation heterozygotes as a result of adjacent −2 disjunction at first meiotic division. Mutation tf has been assigned to band 17B in chromosome 17 by comparing the location of T190Ca and T43H genetic and cytological breakpoints. Recombination between centromere 17 and T43H break was reduced almost to zero in the presence of Rb(16.17)7Bnr translocation. The unexpected restoration of male fertility was observed in T43H/Rb7Bnr hybrids (T43H/+ males being completely sterile) which made it possible to prepare the first homozygotes for T43H male–sterile translocation. Direct estimation of chiasma frequencies in centromere 17−T43H region indicated an 11 cM distance between the centromere 17 and the proximal end of t12 haplotype. The significance of centromere −t (or H-2) distance on the predictable restrictions of the possible haploid manifestation of T-t or H-2 gene products on sperm membrane is discussed.

This paper shows that a number of models of the maintenance of polymorphism in a heterogeneous environment, including those of Levene and Dempster, can be derived from a simple assumption about the way in which the numbers and kinds of individuals emerging from a niche depend on the number of eggs laid in it. It is shown that for such models, unless selective advantages per locus are large, protected polymorphism requires that the relative niche sizes lie in a narrow range. This lack of robustness applies also to models of stable polymorphism proposed by Clarke and by Stewart & Levin. Excluding models relaying on habitat selection or restricted migration, the only models which may escape this criticism are diploid models with partial dominance with respect to fitness, such as one proposed by Gillespie, in which in all niches the fitness of heterozygotes is higher than the arithmetic mean of the homozygotes.

Chromosomal E. coli DNA appears to be sensitive towards in vivo DNA restriction when transformed to a restrictive E. coli recipient. It is therefore concluded that transforming chromosomal donor DNA is present in a double-stranded form immediately after uptake.

Genetic analysis of E. coli transformants, obtained with UV-irradiated donor DNA under conditions that exclude photorepair, show, especially in a uvrB recipient, loss of donor DNA information compared with the situation where DNA was not subjected to UV-irradiation. Similar conclusions were arrived at after genetic analysis of transductants obtained with UV-irradiated particles of the generalized transducing phage P1. The processing in E. coli of DNA after P1 transduction is thus similar to that of transforming DNA. The observations are discussed and a possible explanation based on single-stranded DNA integration is presented in detail.

Currently favoured models postulate that gene conversion is due to the correction of mis-matches in heteroduplex DNA. If heteroduplex is formed reciprocally on both chromatids participating in recombination, the mis-matches due to a heterozygous site will be different on the two chromatids, and there will be four correction probabilities to be taken into account. It is shown that, given the frequencies of the five different kinds of aberrant ascus ratios, it is possible to calculate four alternative sets of values for the four correction probabilities and the total number of asci in which heteroduplex is formed. These four solutions reduce in effect to two when there are no other markers distinguishing the two chromatids. With the aid of flanking markers and the assumption that heteroduplex formation is chemically polarized, it is possible, in principle, to choose one best solution.

The method has been applied to the five one-point crosses in Sordaria fimicola from which most data are available. The data from four different mutants crossed to wild type are compatible with a restricted model in which the correction frequencies, from mutant to wild and from wild to mutant, are the same on both chromatids. In the case of the fifth mutant the data are not consistent with this restricted model, and indicate different correction frequencies in the two chromatids.

Experiments were conducted to look for minority male mating advantages among several Drosophila melanogaster genotypes. The strains of flies used were Canton-S, Oregon-R, a strain carrying the recessive mutation vermilion, and highly geo-positive and geo-negative populations which had been selected for over 200 generations in Hirsch-geotaxis mazes. Two series of experiments were carried out, one series using ten pairs of flies, and another series using 20 pairs. Regression analysis gave one significant slope out of seven, suggesting that in the present study, frequency-dependent advantages are not as common or as strong as reported for D. pseudoobscura.

The size of nuclear DNA from wild-type Ustilago maydis was determined to be approximately 6·09 ± 0·3 × 108 daltons from neutral sucrose gradient sedimentation analysis. Following exposure to ionizing radiation the nuclear DNA size was reduced due to the production of double-strand breaks in the DNA. These breaks were repaired when the irradiated cells were incubated in medium for at least one hour after irradiation. The repair was seen as a shift in the DNA profile from a low molecular weight region where the control DNA sedimented. Inhibition of protein synthesis by cycloheximide prevented this type of repair. Blocking protein synthesis also decreased the survival of irradiated wild-type cells but not radiation-sensitive mutants. Protein synthesis was necessary within the first one and a half hours after irradiation for the survival of wild-type cells to be unaffected. The results provide additional evidence for an inducible repair process in U. maydis.

We have examined mice doubly homozygous for both pale ear (ep/ep) and beige (bgJ/bgJ) mutations in order to detect genetic interactions between these 2 loci affecting pigmentation and lysosome physiology. The doubly homozygous mouse has a new pigmentation phenotype consistent with independent effects of ep and bg. The beige (Brandt, Elliott & Swank, 1975) and pale ear (Novak & Swank, 1979)genes have abnormal kidney lysosomal enzyme accumulation caused by defective secretion into urine. No cumulative effect on these functions was observed in the new double mutant phenotype. The new phenotype has giant lysosomes typical of the beige mutation. Unexpectedly, the beige gene corrects the effect of the pale ear on serum lysosomal enzyme concentration. There is also a gene dosage effect of the beige gene on this serum lysosomal enzyme phenotype. The results suggest that the beige and pale ear genes affect the same pathway(s) of lysosome biosynthesis and/or processing. The action of the beige gene may precede that of the pale ear gene in lysosome physiology.

With the aim of knowing the probable magnitude of non-random association between inversion chromosomes and electromorphs, both deterministic and stochastic studies are conducted on the evolutionary change of non-random association, which is defined as the difference in the frequency of a given allele between inversion and non-inversion chromosomes. In these studies inversion chromosomes are assumed to be subject to selection but electromorphs are selectively neutral, and recombination is allowed to occur between inversion and non-inversion chromosomes with a low frequency. The deterministic study has shown that in a variety of selective schemes for inversion chromosomes the non-random association decays at a rate equal to the recombination value in every generation. Thus, if the recombination value is of the order of 10−5 ˜ 10−4, it would take a long time for the non-random association to disappear. Furthermore, the stochastic study has indicated that random genetic drift generates non-random association of inversions and electromorphs in finite populations and the standard error of non-random association often becomes larger than the mean. In addition to these problems the time required for the electromorph frequencies in the inversion and noninversion chromosomes to become equal in a finite population and the probability that the population of inversion chromosomes remains monomorphic for the allele which existed in the initial inversion introduced are studied. Considering all these quantities, it is concluded that data on the non-random association between electromorphs and inversions are not very informative for the study of the maintenance of protein polymorphism. It is also indicated that in the study of association between electromorphs and inversion chromosomes non-random association or Yule's coefficient of association has a better property than the usual linkage disequilibrium measure or correlation coefficient. Implications of this study on some experimental observations are discussed.

A model of centric fusions between the X or Y chromosome and an autosome carrying a selected locus is studied. It is assumed that fusions are not associated with any fitness effects, and that all chromosomes disjoin regularly from their homologues. It is shown that a necessary condition for the fusion to be favoured is that there is a selectively maintained sex difference in allele frequencies at the selected locus. If this condition is satisfied, the initial rate of increase of a rare Y-autosome fusion is about three times that of an X-autosome fusion, with the same parameter values. Computer calculations of the final equilibrium states reached by populations containing such fusions were done.

The 6-phosphogluconate dehydrogenase (6-PGD) E.C. no. 1.1.1.44, zymogram phenotypes of wheat–rye addition lines and triticale were determined. Genes involved in the production of 6-PGD were located on the long arms of C and F chromosomes of Imperial Rye and the long arms of II and IV chromosomes of King II rye. The results indicate that this enzyme is a dimer.