Two mutant alleles of the meiotic locus, mei-9, have been examined for their effect on magnification of a rod Xbb chromosome and transmission of a ring Xbb chromosome under magnifying conditions. Our results indicate that the effects of these two mutations are allele-specific: mei-9a strongly inhibits both rod chromosome magnification and ring chromosome loss under magnifying conditions, while mei-9b has a smaller inhibitory effect on rod chromosome magnification and on the transmission of ring chromosomes under magnifying conditions. These observations can be explained by a difference in leakiness between the two alleles. Our results demonstrate that mutants defective in excision repair and repair replication inhibit ribosomal gene magnification. This suggests that a component of the excision repair pathway is involved in the process of magnification.

1. A number of mutants affecting changes in the pigment of larval and adult An. gambiae are reported.

2. Two other pigment characters are also described. One of these, diamond, is probably inherited as a single factor.

3. Two of the mutants are sex-linked and the experimental results show An. gambiae to have a Drosophila form of sex-linkage.

4. The sex-linked white-eye mutant is shown to be epistatic to the gene for collar and the characters diamond and red stripe.

5. Attention is drawn to the difference in sex-linkage between An. gambiae, and C. pipiens and Ae. aegypti which may indicate different methods of sex-determination.

A strain of Ustilago maydis has been isolated, after treatment with nitrosoguanidine, which lacks an extracellular endonuclease capable of causing two strand breaks in DNA. Genetic analysis suggests that the strain carries two mutations and is probably also deficient in a second nuclease.

Interspecific crosses between, different genotypes and species of Nicotiana have revealed S gene polymorphism on a large scale. In the usually yellow-flowered self-compatible species N. glauca a strain occurs that has a self-incompatibility gene which is tightly linked with the bright-red corolla colour gene. In self-incompatible species, there are two kinds of self-incompatibility alleles, SI and SFI, distinguished on the basis of the acceptability of N. langsdorffii pollen. The SI form has a large number of alleles whereas the SFI form has only two, SF10 and SF11. The SI alleles are again divisible into two groups on the basis of the acceptability of N. noctiflora pollen and, on the same criterion, the two SFI alleles, SF10 and SF11, also are distinguishable. N. noctiflora and N. bonariensis plants could be divided into two and three groups respectively on the basis of interspecific compatibility relationships. When plants of each species were crossed among themselves, they formed two and three intra-incompatible and inter-compatible groups respectively, identical with those found on the basis of interspecific compatibility relationships. The significance of this observation is at present not fully understood.

A technique has been developed for achieving synchrony of division by selection for immediate pre- and post-fission cells of Paramecium tetraurelia. Cell populations are fed granular tantalum, followed by density-gradient centrifugation. Dividing cells, which do not feed, are separated from feeding interphase cells. The selected cells are division-synchronous for at least two generations when resuspended in fresh culture fluid. This technique provides a rapid means of obtaining large numbers of synchronously dividing paramecia for cell-cycle studies and has general application for selective enrichment of non-feeding sub-populations of cells.

We describe herein, a new unstable mutant of the vestigial locus, isolated from a French natural population. From this mutant vestigialalmost (vgal) wild-type flies (vgal+) and extreme vg phenotypes (vge) arose spontaneously without genomic shock. The occurrence of vgal+ or vge alleles depends mostly on the breeding temperature; vgal+ revertants arose principally at low temperature (21 °C) and vge at 28 °C. These events occur mainly in the male germ line and the phenomenon appears to be premeiotic. Our results with in situ hybridization experiments and Southern blots show that the vgal mutation is due to a 2 kb DNA insertion, which is a deleted hobo element. Genetic and molecular analyses show that two distinct events may underly the wild-type revertants. One is the excision of the resident hobo element, the other a further deletion (about 300 bp in the example characterized herein). The vge mutation is probably due to a deletion of vestigial sequences flanking the hobo insertion.

Diplotene and diakinesis chiasma frequency in oöcytes of the mouse cannot be studied successfully with the present technique. Metaphase chiasmata have been examined in thirty-nine oöcytes. It is deduced that the total diplotene map length in females is about 2300 cM. compared with 1950 cM. in males. There is sexual dimorphism in the frequency of chiasmata, which is paralleled by similar dimorphism in frequencies of crossing-over, measured genetically.

The two sexes differ in the duration of various stages of meiosis. In adult males the pachytene stage, lasting for about 7 days, is directly followed by diplotene and diakinesis, after which the metaphase stage sets in. The sex bivalent in males develops visible chiasmata much earlier than do the autosomes and it precedes them in anaphase separation. Quick terminalization of chiasmata in it leads in a fair proportion of cases to precocious separation and in less than 1% of cases to cytologically detectable non-disjunction of sex chromosomes.

In females the pachytene stage appears in oöcytes of the embryo and is followed by the dictyotene stage, which last still ovulation, i.e. between 35–40 days and several months. Since in the oöcyte chiasmata are formed and move during the dictyotene stage, it follows that stainable materials of the chromosomes are not necessary for the formation and movement of chiasmata and are concomitant with pairing and anaphase separation. It follows also that the time for chiasma formation and movement is in females at least five to six times longer than in males. In old oöcytes in which time is available for maximum terminalization of chiasmata, non-disjunction may appear with detectable frequency. This mechanism may also operate in cases of Mongolism in man, where non-disjunction of an autosome has been recently cytologically established and higher frequency of incidence of the condition for old mothers has been known for some time.

It is possible that the differences in duration of various stages of gametogenesis are connected with the period at which gametic selection is operating: in spermatogenesis after the second meiotic division, in oögenesis prior to first meiotic metaphase.

The level of histidine decarboxylase in mouse kidney normally differs between the sexes with females higher than males. In a strain derived from feral Danish mice (DAN), however, both males and females have the same, high, HDC activity due to the males being insensitive to repression by testosterone. Genetic analysis indicates that this insensitivity is caused by a variant allele of a new gene in the histidine decarboxylase gene complex, Hdc-a; the Hdc-ab allele in C57BL/10 confers high sensitivity to testosterone whereas the Hdc-aw allele in the DAN strain confers low sensitivity. In addition, the DAN strain has a novel haplotype for the other three known elements of [Hdc]: the allele Hdc-sd of the structural gene, the Hdc-cd allele of the gene determining enzyme concentration, and the oestrogen-inducible allele Hdc-eb.

A novel central partial t haplotype was generated by screening for a recombination event between overlapping distal and proximal partial haplotypes. This haplotype contains just two elements Tcrt and Tcd-3t —involved in the t-specific transmission ratio distortion phenotype. Breeding analysis of males that carry this chromosome provides evidence that Tcd-3 is, indeed, a distorter locus and not a second responder. Furthermore, the data indicate that a single well-defined distorter locus is insufficient to overcome completely the self-destructive, low transmission ratio distortion phenotype expressed by the t allele at the t complex responder locus, although a small, but highly significant, effect was observed.

A centromeric heterochromatin (Hc) comparison of two substrains of the mouse inbred strain 101 with another two, revealed differences in at least two and possibly in up to six chromosome pairs. The differences could have arisen by the inclusion of chromosomes from the inbred strain C3H/He.

A quite specific kind of sterile F1 female, called SF females, arises only when females of strains denoted reactive are crossed with males of the other class (inducer). It was previously shown that this sterility results from a nucleocytoplasmic interaction between the maternal reactive cytoplasm and a factor, called I, which may be born by any one of the paternal chromosomes. In SF females, but not in their brothers, a varying proportion of reactive chromosomes are able to acquire irreversibly the I factor, independently of any classical genetic recombination with the inducer chromosome(s). During this process, called chromosomal contamination, the contaminating chromosome(s) do not undergo any apparent change. The present paper deals with the efficiency of both original inducer and contaminated chromosomes to yield a more or less intense SF sterility. The Otanu inducer laboratory strain contains at least two types of X chromosomes (called strong and weak) which differ genetically with respect to their inducer efficiency. Reactive third chromosomes were contaminated by these strong or weak X chromosomes and their inducer efficiencies compared. Results show that they are on average stronger when they have been contaminated by strong X chromosomes than when contaminated by weak ones. Such a correlation favours the hypothesis that chromosomal contamination is due to the insertion of some genetic element(s) into reactive chromosomes.

Genetic variance and fixation probabilities are evaluated for a model of a quantitative trait at a balance between mutation, selection and drift in which many alleles can segregate at each locus. If the distribution of effects of new mutant alleles is such that mutations are unconditionally deleterious, as might be the case in natural populations, genetic variance maintained is proportional to the number of mutations occurring in the genome per generation, but is independent of the number of loci at which they appear. If selectively advantageous alleles can occur these tend to interfere to a greater extent with each others' fixation and increasing mutation rate leads to a decrease in the genetic variance as a fraction of the variance maintained in the absence of selection. Fixation probabilities of new mutant alleles approach that for neutral alleles with increasing mutation rate at a locus irrespective of their effects on fitness. The additive genetic variance contributed by the locus may appear to be ‘decoupled’ from the fixation rate of mutant alleles.

The covariance of an ancestor and the average of its descendants in generation t(CPt) is formulated for a breeding system which is a mixture of selfing and outcrossing. This covariance is partitioned into least squares additive (σA. At) and non-additive (σD. Dt) components summed over individual loci, and the combined inbreeding effects of all loci (H′), such that CPt = σA. At + (s/2)t[σD. Do + H′]. For the mth locus the covariance mσA. At = 2(1 + F)Σipiα(0)iα(t)i, in which pi is the frequency of the ith allele whose additive effect (α(t)) depends on the generation for which it is defined. For distant descendants α(∞) is equal to half of the derivative of the population mean with respect to the frequency of the allele. The covariance CP∞ = σA. A∞ thus relates directly to permanent selection response measured in the equilibrium population, any additional responses observed in earlier generations being due to temporary disturbances in population genotypic structure. It is only for these distant descendants that the least squares additive component has any direct interpretation in terms of selection response. The definitions of α(0) and α(∞), lead to two distinct definitions of the average effect of an allele substitution for a model with two alleles (Fisher, 1941), and to a clarification of their significance for this breeding system.