The development of a set of two-locus descent measures is reviewed. The three digenic measures, inbreeding coefficient and parental and recombinant descent coefficients, are considered in detail. The derivations of these three in pedigrees, fixed mating systems, and random mating in monoecious or dioecious populations are given. General expressions for digenic frequencies and disequilibria functions at any time are found by applying the three digenic descent measures to two types of initial populations. The final or equilibrium status of the population is also given. As the inbreeding coefficient is the same as the recombinant descent coefficient in the case of complete linkage, avoidance or promotion of early inbreeding has similar effects on the two coefficients. Estimable components of linkage disequilibrium and other measures of association within and among populations are elaborated.

Eighteen male sterile mutants of N. crassa which block various stages of the sexual development cycle have been functionally categorized on the basis of complementation tests. Nine of these (those which block early stages of the sexual cycle) have also been analysed through recombination studies. On the basis of these data, a minimum of four genes appear to be involved in early stages of sexual development (i.e. probably prior to karyogamy) and several genes are indicated which control later stages of the cycle. Complementation phases with respect to sexual development in heterothallic Ascomycetes are discussed as well as a comparison of results obtained with expectations and with those obtained in related Ascomycetes.

A new method for obtaining mitochondrial mutants of Paramecium aurelia is described. Sensitive organisms were first placed in medium containing a low concentration (0·02 mg/ml) of erythromycin (ERY), insufficient to prevent fission. The paramecia were then transferred through a graded series of increasing ERY concentrations, with a period of growth at each concentration. A high yield of ERY-resistant mutants was obtained.

Continuous growth in 0·02 mg/ml ERY also resulted in the accumulation of a spectrum of mutants resistant to various concentrations of ERY. Many of these mutants are also resistant to mikamycin. On returning the resistant paramecia to media lacking ERY, some clones remained resistant whilst others reverted to sensitivity.

A population genetics model of the role of asymmetric pairing and unequal exchange in the stabilization of transposable element copy number in natural populations is proposed and analysed. Monte Carlo simulations indicate that the approximations incorporated into the analysis are robust in the relevant parameter ranges. Given several simple assumptions concerning transposition and excision, equal and unequal exchange, and chromosome structure, predictions of the relative numbers of transposable elements in various regions of the Drosophila melanogaster genome are compared to the observed distribution of roo/B104 elements across chromosomal regions with differing rates of exchange, and between X chromosomes and autosomes. There is no indication of an accumulation of elements in the distal regions of chromosomes, which is expected if unequal exchange is reduced concomitantly with normal crossing over in the distal regions. There is, however, an indication of an excess of elements relative to physical length in the proximal regions of the chromosomes, which also have restricted crossing over. This observation is qualitatively consistent with the model's predictions. The observed distribution of elements between the mid-sections of the X chromosomes and autosomes is consistent with the predictions of one of two models of unequal exchange.

Chlorpromazine and several other phenothiazines at sub-bacteriocidal concentrations were found to cure an Escherichia coli F′ lac+ strain of its plasmid efficiently. Curing was most efficient at high pH and in complex medium when 70% or more of the bacteria were plasmid-free after 24 h growth of cultures in the presence of the drug.

Two reciprocal mouse translocations T(2; 8)26H and T(1; 13)70H, heterozygous in a Swiss random-bred background, show differences in the spectrum of multivalent configurations and in the segregational behaviour of these multivalent configurations. T26H/+ males mostly contained rings of four (R IV, 53·15%) and T70H/+ males chains of four, missing a chiasma in the shortest interstitial segment (C IV 11, 61·55%). The adjacent II frequency, estimated from metaphase II observations, was 8·47% in T26H/+ and 25·22% in T70H/+. Univalents of the shorter translocation chromosome of T70H are able to divide equationally at first anaphase. The hypothesis is advanced that time differences in chiasma terminalization during metaphase I-anaphase are important for explaining the difference in segregation observed between the two translocations. Translocation-caused non-disjunction is probably low in T26H/+ and 4–5% in T70H/+. Univalents involving T70H/+ are usually capable of co-orientation with the other chromosomes of the translocation complex. The summed percentages of adjacent II disjunction and non-disjunction caused by the translocations were estimated from the relative fertility scores of T/+ males and females versus +/+ males and females as 9·8% and 29·0% for T26H/+ and T70H/+ males, respectively, and 9·4% and 27·8% for T26H/+ females and T70H/+ females. For both translocations, the agreement between the various estimates is good. Chiasma frequencies are much higher in telomeric segments than in proximal segments containing centric heterochromatin.

Hybrid dysgenesis has been described as a syndrome of aberrant traits including sterility, male recombination, and mutation, which occurs in some inter-strain hybrids of Drosophila, but only from one of the two reciprocal crosses. In a series of experiments in which hybrids of various pedigrees were tested for sterility, it was found that a case of hybrid dysgenesis could be most easily interpreted as the interaction of two components. One component was found to be a polygenic Mendelian factor linked to each of the major chromosomes of π2, the paternally contributing strain (‘P strain’). These chromosomes were capable of causing sterility when inherited from either parent, provided the appropriate maternal component was also inherited. The ability to transmit this maternal component was designated ‘cytotype’ to indicate that it is a property of the entire cell. It was possible to classify nearly all hybrid females as either P or M cytotype on the basis of their ability to produce sterile daughters. All daughters of the M-cytotype mothers were susceptible to the sterilizing effects of the π2 chromosome, whereas all, or nearly all daughters of P-cytotype mothers were immune. When more than one of the π2 chromosomes were received by daughters of M-cytotype females, chromosomal interactions could be detected statistically, but the model of independent action remained a useful approximation. Cytotype was shown to be determined by chromosomal factors, but with limited cytoplasmic transmission. This unusual mode of inheritance can be compared with other cases of hybrid dysgenesis where the behaviour resembles that of self-replicating cytoplasmic particles which are dependent on certain chromosomes. The lack of sterility from intra-strain crosses can be explained by the fact that chromosomes capable of causing sterility also induce the P cytotype, and thus prevent sterility in the next generation.

Geographic variation was studied at 15 electrophoretic loci (40 alleles) in Italian populations of Ornithogalum montanum Cyr. ex Ten. (Liliaceae). Homogeneity of allele frequencies was assessed by G tests; gene-frequency patterns were described by spatial autocorrelation statistics; matrices of genetic and environmental distance were compared through a series of Mantel's tests, and the zones of highest overall gene-frequency change per unit distance (steep multi-locus clines, or genetic boundaries) were identified. Nineteen allele frequencies appear heterogeneously distributed, but only 3 of them show significant spatial structure. Only 2 allele frequencies are correlated with 1 environmental parameter. Large genetic differences are observed between spatially close populations. These findings support a model of differentiation in which the genetic relationships between isolates do not depend on their spatial distances, but reflect mainly population subdivision and restricted gene flow.

Clones of V79 Chinese hamster cells resistant to the steroid compound ouabain have been induced by mutagen treatment with MNNG (= N-methyl-N′-nitro-N-nitrosoguanidine). The frequency of resistant colonies was increased from spontaneous value of 2 × 10−6 to a frequency of 1·2 × 10−3 with MNNG treatment.

The highest number of induced mutants appeared when an expression time pf 40 h was allowed between MNNG treatment and addition of ouabain.

Time-dependent behaviour of linkage disequilibrium when there was initial linkage disequilibrium is studied in a finite island model assuming neutrality. Explicit expressions for linkage disequilibrium parameters are obtained. From these expressions, the initial and the ultimate decay rates of linkage disequilibrium parameters are found to be increased and decreased, respectively, by finiteness of the population when recombination rate, migration rate and inverse of subpopulation size are of comparable order. Thus, linkage disequilibrium created in the past may persist longerin smaller subdivided populations. Also, differentiation of the gametic parameter of linkage disequilibrium among subpopulations is found to diminish quickly compared tothe linkage disequilibrium in the whole population. Implications of these results for the interpretation of linkage disequilibria in natural populations are discussed.

A method has been developed which will permit the mapping of the genetic material of Pseudomonas aeruginosa. This has been made possible by the use of potassium nitrate in the mating medium and other modifications which have increased the frequency of recombination. In certain circumstances, the ilvA12 marker may be transferred by more than 1% of the donor cells.

A genotype of wheat deficient for a pair of chromosomes stabilizing chiasma frequency against extremes of temperature was investigated to determine the position and duration of temperature sensitivity with respect to first metaphase of meiosis. Temperature changes over a critical range, followed by sequential sampling and measurement of chiasma frequency, showed a relatively short temperature-sensitive stage, the position and duration of which were dependent on the final temperature used. Comparison with meiotic timings made independently showed that the temperature-sensitive stage occurred in the premeiotic interphase. Euploid wheat was shown to have a stage in chiasma formation sensitive to high-temperature treatments at a similar time.

Comparison with the work of others showed that the sensitive stage lay between the last premeiotic mitosis and the start of DNA synthesis. This modification of chromosome pairing at a much earlier stage than has been previously demonstrated is further evidence that the processes of chromosome pairing and crossing-over are probably more complex than formerly envisaged.