In the case of an allele which is going to become fixed in a population, the average number of generations for which the population assumes particular gene frequencies is investigated, using the diffusion approximation. Explicit formulas were obtained and they were checked by computer simulations. As a particular case, it is shown that if a new mutant that is selectively neutral is eventually fixed in a population of size N, it spends two generations on average at each of the intermediate frequencies (1/2N, 2/2N, …, (2N−1)/2N), and the variance at each frequency is four generations.

Through a series of backcrosses the centromere region of linkage group I of Neurospora crassa was transferred to the N. sitophila genome, and through another series of backcrosses the centromere region of linkage group I of N. sitophila was transferred to N. crassa. Strains thus synthesized showed, in further crosses, that the N. sitophila centromere region acts as a dominant enhancer of cross-over frequencies across linkage group I of the two species.

An inverted segment in one of the chromosomes of Sordaria brevicollis has been found to have a marked effect on recombination within a gene in another chromosome. There are two effects on the intragenic recombination pattern, namely, a change in the polarity of recombination and a change in the frequency of parental and recombinant outside markers. The results support the idea that a mechanism exists for cancelling crossovers shortly after they have been formed as a means of controlling their number.

The effects of heterozygous inversions and of the C(3)G mutation on recombination between alleles of the rudimentary locus of Drosophila melanogaster were analysed. It is shown that the presence of either of these genetic alterations significantly changes the frequency and type of intragenic recombinants which are produced.

Young mice of the strains A/Y and B10.D2 were repeatedly treated with diethylsulphate (DES) in different doses (36 to 1100 mg/kg). At the age of 9 weeks they were mated to females of the strains A. CA (‘A’ group) and C57BL/10Eg (‘D’ group) respectively. 2101 progeny of these matings were tested for histocompatibility by skin grafting. The spontaneous H-mutation rates were 6·96 × 10−4 per gamete in the A group and 9·6 × 10−4 per gamete in D group. In progeny of treated males the H-mutation rates were 0 in A group and 5·79 × 10−3 per gamete in D group, showing apparent effect of paternal DES treatment on mutation frequency in the last group. Two mutations of the H-2 locus were found, which together with the other three H-2 mutations published so far yielded a mutation rate of 5·18 × 10−4 per gamete. The mutation rate of the H-2 locus is higher than the expected rate per H-locus, indicating a great genetic complexity of H-2.

tsd1–1 strains of Ustilago maydis are blocked in DNA synthesis under restrictive conditions. On recovery from the block, both intergenic and allelic recombination are enhanced, and aneuploids are generated. UV is synergistic in the induction of recombination. Direct assays of allelic recombination have shown that gene conversion occurs during the block in DNA synthesis.

Eight newly isolated 9 + 0 mutants each mapped at one of the four previously known loci. Short flagellum mutants were at three loci, two of which (pf7 and pf8) were closely linked; the third, pf21, was unlinked to these two and mapped on linkage group II. The long flagellum mutants lf1 and lf2 were on linkage groups II and XII respectively. Mutants pf8A and lf1 were both recessive to wild-type. There was no evidence for non-Mendelian flagellum mutants.

A class of rII mutants revertible by both base analogues and acridines is described. The members of this anomalous class are base-substitution mutants suppressed by a phase-shift mutation in a phage gene mapping outside the rII region.

The frequencies of genetically determined electrophoretic variants of two enzyme systems in the parthenogenetic crustacean D. magna have been followed in two isolated populations. In both populations a marked excess of hetero-zygotes was found in the later samples. It is concluded that the observed changes in gene and genotypic frequencies are due to natural selection as both migration and genetic drift can be excluded.

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.

The highly selective technique of integrative suppression has been used to determine the ability of autonomous transmissible plasmids to integrate into the E. coli chromosome.

All the F-like transmissible plasmids appear to be able to integrate, suppress the initiation defect and form Hfr-type donor strains. However, there is no evidence of integration with the I-like transmissible plasmids examined.