Forty-five temperature-sensitive mutants of Aspergillus nidulans which are defective in nuclear division, septation or distribution of nuclei along the mycelium have been isolated, and most have been subjected to complementation analysis and mapped to chromosome. Thirty-five of the mutants were unable to complete nuclear division at the restrictive temperature. Twenty-six of these mutants exhibited a co-ordinate drop in both spindle and chromosome mitotic indices at 42 °C, indicating that they fail to enter mitosis. These mutants have been assigned to the gene symbol nim. Nine mutants exhibited a co-ordinate rise in spindle and chromosome mitotic indices at 42 °C, indicating that they are arrested in mitosis. These mutants were assigned the gene symbol bim. Five mutants failed to form septa and were given the gene symbol sep; and five mutants had an abnormal nuclear distribution and were given the gene symbol nud. All of the mutations were recessive. Most of the mutants were in different complementation groups. Mutants in the same complementation groups were phenotypically similar, but phenotypically similar mutants were not necessarily or usually in the same complementation group. There was no evidence for genetic clustering of phenotypically similar mutants. The mutants were located on all eight chromosomes.

Both maternal and zygotic expression of many essential genes are required for normal development. For some of these genes, absence of maternal function yields striking embryonic defects. The experiments reported here examine two questions about such genes: (1) Are embryonic effects of maternal deficits a common property of maternally-and-zygotically active genes? and (2) Is use of the maternal products of these genes restricted to early embryogenesis? A comparison of times of lethality of mutant sons of normal and mutant-heterozygous mothers has been made for six mutations in the zeste-white region of the Drosophila X chromosome. Four of the mutations are defective in single cistrons and two are deficiencies that between them remove thirteen essential loci. All of these mutations had previously been shown to have both maternal and zygotic effects, and all of them had been tested, using homozygous germ-line clones, for the effects of complete maternal defects. For several of them, homozygous germ-line clones cause embryonic defects. Of the six, only one, Df(l)K95, shows a shift from larval to embryonic lethality when the mothers are heterozygous, and even in that case lethality occurs at the very end of embryogenesis. These results have two implications: (1) maternally-derived transcripts do not always serve a solely embryonic role; and (2) an embryonic effect of a complete maternal deficit does not by itself demonstrate an embryo-restricted function for the maternal transcript.

Genetic recombination through the parasexual cycle, or some very similar system, was demonstrated. Diploid strains were very unstable, yielding about 95% haploid conidia from three-week-old cultures This high frequency of haploid segregants was at least partly attributable to greater sporulation of haploid mycelium, but may also reflect a higher frequency of haploidization than that found in Aspergillus nidulans.

Mitotic crossing-over also occurred frequently, and gave segregants homozygous for some markers but heterozygous for others. It was also detectable by changes in the phase of linked markers occurring during vegetative growth. Some heterozygous segregants were either aneuploids, or were formed by double mitotic crossovers.

Haploid segregants, derived from nuclei which had previously undergone mitotic crossing-over, were often recovered. This coincidence of mitotic crossing-over and haploidization in one nuclear lineage, together with the probable occurrence of double mitotic crossovers, makes mitotic analysis less clear cut than in Aspergillus nidulans.

Maximum likelihood estimates of codon and base frequencies from observed amino acid composition of proteins were obtained based on models capable of revealing dependency between base arrangements in the three positions of a codon. Results showed that many of the proteins analysed revealed dependency between base arrangements in the first and second codon positions (first-order interaction). Also, in a number of proteins the interactions between base arrangements seemed to involve simultaneously more than one first order interaction and/or a second-order interaction (among base arrangements in the three codon positions). It was of interest to observe that the model of random base arrangements did not fit the observed amino acid data in almost all of the proteins that were analysed. More than ten amino acids contributed to this deviation from randomness.

Using models which describe the change, by natural selection, of the actual numbers of genes rather than their relative frequencies, it is demonstrated that the equation familiar to geneticists, i.e. dp/dt = sp(1 − p), is appropriate under a wide range of circumstances. It was pointed out that, for realistic treatment of the evolutionary process through which gene substitutions are repeated, the models must have the property such that the total population number remains constant or nearly constant throughout the process, and is not appreciably influenced by the genes being substituted.

The load or cost for a gene substitution was studied assuming a haploid population and the effects on the load of such factors as epistatic gene interaction in fitness, finite population number and slow change of environment were investigated. The load may become very large under a strong ‘reinforcing’ type epistasis between advantageous genes. In a finite population, the load for one gene substitution may be inflated by about unity if the product of the effective population number (Ne) and the selection coefficient (s) is large but Nesp0 is much smaller than unity, where p0 is the initial gene frequency. On the other hand, slow change of environment may decrease the load somewhat. It was concluded that despite these and other complicating factors, Haldane's original formula, –logep0, for a haploid population (−2 logep0 for the case of a diploid without dominance) is still useful for assessing the approximate amount of selective elimination that accompanies the process of gene substitution in evolution.

Data from three experiments bearing on the relative stability of the four mating systems required to test the restrictions against inbreeding and unequal family size were examined in relation to the results given by Robinson & Bray (1965). Tribolium castaneum was the experimental animal used in these experiments.

An analysis of variance indicated that both restrictions were probably effective (P approximately 0·10) in reducing the phenotypic variability of control population means. It seems likely that the apparent gain in stability obtained when the inbreeding restriction was used in addition to the restriction against unequal numbers is due to non-random genotypic proportions which would affect estimates of genetic variability based on assumptions of random mating.

All 153 crosses between 18 tomato varieties were grown in F1, F2, F3 and F4. The F2, F3 and F4 were derived by selfing one plant of the previous generation. The F2 plant chosen to give the F3 was selected (1 in 20) for early yield; and the F3 plant chosen to give the F4 was similarly selected. Flowering date was an unsatisfactory character. In crosses segregating for the Mendelian gene ‘uniform’, a significant excess of heterozygotes was selected. The parents transmitted variability of yield (as well as average yield) to their offspring. The division of yield into its components fruit number and fruit size is useful because (1) much of the heterosis in yield can be viewed as a combination effect between these components, (2) the components responded differently to selection, (3) different components showed phenotypic dominance in different directions. The average yield and fruit number responded as expected to selection; fruit size did not. The F1 generation means for all three characters exceeded the parental means; but the crosses were grouped more compactly about their generation mean than were the parents, so that heterosis rarely occurred in crosses involving the best parents. The yields of each cross were analysed into parental main effects (general combining abilities) and interactions (specific combining abilities). No useful prediction of interactions could be made in any generation, either from the same generation in different years or from different generations in the same year. The main effects (general c.a.) were analysed into a part due to regression on parental yield, and a deviation from that regression. No useful prediction of the deviations from parental regression could be made in generations which had responded to selection. The actual advance under selection of different crosses, although not uniform, was unpredictable. During advance under selection, the parental means gave predictions of the (relative) performance of each generation's crosses which were as good as predictions based on the previous generation. This may, of course, be connected with the fact that the parents were inbred and that the amount of heterozygosity decreases in each successive generation. These results indicate, therefore, that in an inbreeding species propagated by seed, the early hybrid generations tell us nothing more than do the parental yields about the relative performance of the inbred lines that can be selected from those hybrids. (This generalization from Lycopersicon esculentum to inbreeders as a whole may, of course, be false.) The relative performance of F1 hybrids, on the other hand, is better predicted from other F1 crosses involving the parents concerned than from the yields of those parents. There was phenotypic dominance of high yield, large fruit number and low fruit weight. The extent of this dominance was not enough to invalidate the analysis by general combining abilities; and since it varied from year to year and from generation to generation, the emphasis that should be given to the top-parent (in contrast to the bottom-parent) in predicting the yields of crosses after selection, is as yet unpredictable.

The zymogram phenotypes of triosephosphate isomerase (TPI) were determined for a large number of aneuploid derivatives of Triticum aestivum cv. ‘Chinese Spring’ and for six wheat-alien species chromosome addition series. Examination of the available compensating nullisomic-tetrasomic and homoeologous groups 3 and 5 ditelosomic lines of Chinese Spring disclosed that T. aestivum possesses two systems of dimeric TPI isozymes, designated TPI-1 and TPI-2. The genes TPI-A1, TPI-B1 and TPI-D1 were located in Chinese Spring chromosome arms 3Ap, 3Bp and 3Dp, respectively and the genes TPI-A2, TPI-B2 and TPI-D2 in chromosome arms 5Aq, 5Bq and 5Dq, respectively. TPI-1 genes were also located in Hordeum vulgare cv. Betzes chromosome 3H, T. longissimum chromosome G, Elytrigia elongata chromosome 3E, and Secale cereale cvs. Imperial and Dakold chromosome 3R. TPI-2 genes were found in Betzes chromosome 5H, T. umbellulatum chromosome 5U, T. longissimum chromosome F, and Imperial and Dakold chromosome 5R. These gene locations provide evidence of homoeology between the alien chromosomes in which the genes are located and the chromosomes of homoeologous groups 3 and 5 of Chinese Spring, respectively. Evidence was obtained for the presence of a TPI-R2 gene in each of the T. aestivum cv. Kharkov -S. cereale cv. Dakold chromosome addition lines studied suggesting that this gene is present in the wheat genome in each member of this addition series.

Using a mouse Y chromosomal repetitive sequence that differentiates between the Mus musculus musculus type Y chromosome and the M. m. domesticus type Y chromosome, we studied the Y chromosome in M. m. molossinus, M. m. castaneus and M. m. subspecies specimens recently trapped in Japan, Taiwan and China as well as Asian mice maintained at the Jackson Laboratory and Litton Bionetics. Here we report that the M. m. musculus type Y chromosome predominates in Asian house mice and that Japanese mice maintained at some laboratories may not represent typical M. m. molossinus.

For a model in which quantitative traits are assumed to be determined solely by additive genes at many loci, formulae are developed for the variance among replicated small populations of size N, maintained without selection, of the additive genetic variance, heritability, genetic correlations and similar parameters. The base population is assumed to be in linkage equilibrium, but it is argued that most of the variation in the within-line additive variance (VAt at generation t) is due to linkage disequilibrium caused by sampling. If is the squared correlation of gene frequencies averaged over all pairs of loci at time t, the coefficient of variation (CV) of VAt equals , with similar formulae for other parameters.

The formulae are evaluated for models of loci distributed uniformly along the chromosome but much of the disequilibrium is due to loci on different chromosomes. For unlinked loci CV(VAt) reaches √4/(3(N)), and for mammalian models, this value is not greatly exceeded. The variance in successive generations has a correlation of at least one-half due to the maintenance of linkage disequilibrium. The magnitude of this variance in parameters and their autocorrelation with time shows that accurate predictions cannot be made about genetic parameters in the base population from single replicate results.

The inheritance of alleles at ten enzyme loci of Stagnicola elodes (Lymnaeidae) was analysed by laboratory crosses. Alleles for alanine aminopeptidase (2 alleles), leucine aminopeptidase (2 alleles), aspartate aminotransferase (2 alleles), β-hydroxybutyrate dehydrogenase-1 (4 alleles), 6-phosphogluconate dehydrogenase (2 alleles), esterase-2 (2 alleles), esterase-3 (2 alleles), esterase-7 (2 alleles), esterase-13 (2 alleles) and esterase-18 (2 alleles) segregate in the F2 generation according to Mendelian expectations. Contingency X2 tests for linkage done on F2 progeny from crosses between strains homozygous for alternate alleles at two loci showed that β-hydroxybutyrate dehydrogenase-1, esterase-13 and esterase-18 are presumably linked.

Numbers of bristles are reduced in the dorsocentral regions of achaete Drosophila melanogaster. In achaete tissue of mosaics the effect is not uniform, and near the clone boundaries bristle numbers are significantly higher than they are elsewhere in the clone. It is argued that the cause of this non-autonomy stems from ‘factors’ that spread into the achaete clone from surrounding non-achaete cells.

Midget, a new dwarfing gene in the house mouse, was found in a line (NS) selected for small body size. Backcrosses to, and heterozygous matings of NS mice gave midget offspring in the expected 1:1 and 1:3 ratios. Backcrosses to two other lines of mice selected for small body size resulted in fewer midget offspring than expected and in a reduced phenotypic expression of the gene, though matings between these midget offspring gave all midget progeny.

A characteristic check in growth occurred between 15 and 21–24 days of age, and midgets rarely exceeded 10 g. at 6 weeks of age. They were 1·2 g. lighter than NS litter-mates at 3 weeks, and 3·6–4·4 g. lighter at 6 weeks. Fostering or transferring 3½-day-old midget embryos to the uteri of large mothers slightly increased the growth-rate, though the characteristic growth check still persisted. Transfer as embryos did not influence the midget phenotype.

Midget males were of near-normal fertility. The females were semi-sterile due to irregularity or absence of oestrous cycles, failure to ovulate, or the death of embryos before implantation. Each of these factors, which were also found in NS mice, could be remedied by the administration of exogenous hormones.

The genetic factors underlying the expression of midget are discussed, and the physiological traits associated with this and with other dwarfing genes are compared.

Data were collected on the distribution of nine families of transposable elements among second and third chromosomes isolated from a natural population of Drosophila melanogaster, by means of in situ hybridization of element probes to polytene chromosomes. It was found that the copy numbers per chromosome in the distal sections of the chromosome arms followed a Poisson distribution. Elements appeared to be distributed randomly along the distal sections of the chromosome arms. There was no evidence for linkage disequilibrium in the distal sections of the chromosomes, but some significant disequilibrium was detected in proximal regions. There were many significant correlations between different element families with respect to the identity of the sites that were occupied in the sample. There were also significant correlations between families with respect to sites at which elements achieved relatively high frequencies. Element frequencies per chromosome band were generally low in the distal sections, but were higher proximally. These results are discussed in the light of models of the population dynamics of transposable elements. It is concluded that they provide strong evidence for the operation of a force or forces opposing transpositional increase in copy number. The data suggest that the rate of transposition perelement per generation is of the order of 10−4, for the elements included in this study.