The transient distribution of allele frequencies in a finite population is derived under the assumption that there are k possible alleic states at a locus and mutation occurs in all directions. At steady state this distribution becomes identical with the distribution obtained by Wright, Kimura and Crow when k = ∞. The rate of approach to the steady state distribution is generally very slow, the asymptotic rate being 2v + 1/(2N), where v and N are the mutation rate and effective population size, respectively. Using this distribution it is shown that when population size is suddenly increased, the expected number of alleles increases more rapidly than the expected heterozygosity. Implications of the present study on testing hypotheses for the maintenance of genetic variability in populations are discussed.

Recessive transformants (col) obtained from v (vermilion) embryos treated with v+ DNA are shown to map at 1−0·02, a position not distinguishable from that of su(s) (suppressor-of-sable) and in agreement with observations indicating phenotypic allelism of col with su(s) mutants. Recombination in the y–col–gt segment of the X chromosome, over a total map length of O·3 units, was studied among the progeny of colv1/y1gtE6cvv1f and y1colgtE6cvv1f/v1 females. The data from both crosses exhibit the following features: (1) recovery of reciprocal recombinants between y1 and col; (2) recovery of reciprocal recombinants between col and gtE6 and (3) striking negative interference in the y–col–gt segment. These results allow three alternative interpretations: (1) that recombination in the y–col–gt segment results from conventional crossing-over, with high coincidence of crossovers in the two subsegments; (2) that it results from symmetrical gene conversion at the col site (col to col+, and col+ to col), which may be accompanied by single cross-overs in either of the adjacent regions; (3) that col behaves like a transposable element, formally symbolized su(s)+·col, and that recombination in su(s)+·col/su(s)+ (i.e. col/col+) heterozygotes results from transposition of the col element from homolog to homolog, accompanied sometimes by crossing-over, either in the y–col subsegment or in the col-gt subsegment.

The inheritance of resistance to the rodenticide, warfarin, in the Norway rat, Rattus norvegicus, derived from a wild rat population in Scotland was studied in the backcross, intercross and testcross. The resistance was found to be due to a major gene with about the same map position in Linkage Group I as the warfarin-resistance gene, Rw2, which occurs in the wild rat population in Wales. In heterozygotes, the Scottish resistance gene, unlike the Welsh gene, is incompletely penetrant in expression, though the penetrance was found to increase markedly in response to selection. Differences between the Scottish and Welsh types of resistance suggest that the two resistance genes are allelic.

The relationship between parental age and recombination frequency in the offspring of mice has been studied using backcrosses to heterozygous males or females. The mice were allowed to breed until they became infertile. Data for several chromosomes were analysed, mostly from a stock carrying five markers covering approximately three-quarters of the length of chromosome 2.

Analyses of two-point recombination ratios or of multi-point interference ratios revealed no consistent age-related heterogeneity in offspring of male or female heterozygotes. A significant age-related heterogeneity was detected in the offspring of female heterozygotes but not of males, when the number of offspring with no detectable recombination was compared to those with one or more. This difference between the sexes could be related to earlier cytological observations on chiasma frequency in oocytes and spermatocytes. The significance of these findings for the analysis of follicular growth in mammals and in the origin of human trisomic conditions is discussed.

Recently collected strains from Malay peninsula, Taïwan and Japan proved to be similar to previously studied Japanese strains kept for long time under laboratory conditions. It is therefore possible to speak of a Far East race, characterized by slow growth, very high fresh weight and small ovariole number. High heterogeneity between laboratory strains founded from wild caught flies seems also typical. Among the three traits studied, a positive genetic correlation was observed only between duration of development and adult weight. No correlations were found between biometrical traits and the latitude of strain origin. The problem of the origin of the Far East race is discussed.

The role of choline-O-sulphate (COS) as a sulphur storage compound in Aspergillus nidulans was examined by comparing a normal strain and one unable to utilize COS in a sulphur-starvation experiment designed to measure the mobilization of sulphur stores. Efforts to isolate the necessary mutants deficient in choline sulphatase activity produced two nutritionally distinct classes of mutants unable to utilize COS. They were found to be allelic on the basis of genetic complementation and fine structures mapping and represent either leaky or tight mutants with respect to choline sulphatase activity. One of these mutants with no detectable choline sulphatase activity was selected for a growth experiment which demonstrated that COS is a major, though not the only source of the endogenous sulphur supply which can be mobilized during growth in sulphur-limiting conditions.

Evidence is presented that the distribution of the combinations of the sexes in twin lambings has subnormal dispersion: there are more opposite-sexed pairs than same-sexed pairs. When adjustment is made for the occasional presence of monozygotic twin pairs, the departure from expectation (on the assumption that the sexes of DZ twins are distributed binomially) is highly significant. Thus sheep litters seem to show a feature characteristic of litters of pigs and (probably) mice and rabbits. This is consistent with the suggestion that p, the probability of a male zygote, varies systematically from one zygote to another within a litter.

Maternal inheritance of extranuclear mitochondrial genes has been demonstrated in Aspergillus nidulans using the ‘blue’ ascospore colour mutants in combination with heterokaryon incompatible strains. It appears that heterokaryosis is not a prerequisite of sexual outcrossing, and that recombination of extranuclear mitochondrial markers does not occur in the sexual stage of the cell cycle.

In devising maps of the positions of gene loci along chromosomes, by measurements of crossing-over, it is usual to assume that the chiasmata which give rise to the crossing-over are randomly located, unless there is evidence to the contrary. If chiasmata are in fact preferentially located at certain points or regions this not only affects mapping, but also is of interest for population genetics since it may favour the development during evolution of blocks of genes relatively undisturbed by crossing-over. Preferential localization of chiasmata is well known in a number of disparate organisms (Darlington 1965; John & Lewis, 1965), but has so far been little studied in mammals. Evidence is presented suggesting tha chiasmata are in fact non-randomly located in certain chromosomes of the mouse.

The resistance plasmid RP4 can be used to mediate conjugation in Acinetobacter calcoaceticus. Transfer of chromosomal genes occurs only on solid surfaces and not in liquid matings. Two RP4-carrying donor strains which donate the chromosome in different directions from different origins have been isolated. As a result it is possible to demonstrate that the linkage map of A. calcoaceticus is circular.

The fate of RP4 in recombinants was examined. Although frequent loss of the complete plasmid was detected, segregation of plasmid antibiotic resistances was not observed.

A possible mechanism by which RP4 could interact with the A. calcoaceticus chromosome is discussed.

Lysogenization by phage P22 carrying amber mutations in genes 12 and 18

By Amiram Ronen and Myron Levine

Department of Genetics, The Hebrew University of Jerusalem, Israel, and the Department of Human Genetics,

The University of Michigan, Ann Arbor, Michigan 48109, U.S.A.

Vol. 27, No. 2, p. 337: add to Acknowledgements –

‘In addition, this research was supported by Grant No. 738 from the United States–Israel Binational Science Foundation (BSF), Jerusalem, Israel.’