Fifty-one metaphase figures from male Mesocricetus newtoni bone-marrow cells were analysed with respect to the distribution of chromosomes. The peripheral location of the sex chromosomes (XY) and that of the chromosome pairs 3 and 9, as well as the non-peripheral location of the chromosome pairs, 16, 17, 18 was revealed. The significance of the peripheral location of the X chromosome and a possible explanation of the characteristic distribution of the chromosomes are discussed.

Purine-excreting mutants of yeast have been obtained by selecting for (a) adenine-insensitive pigment accumulation in a strain carrying ad-2 and (b) by isolating prototrophic mutants which crossfeed to an adenine-requiring indicator strain. The first method yielded both auxotrophic and prototrophic mutants. The auxotrophs were assigned to ad-12 which specifies adenylosuccinate synthetase. The prototrophs were affected by a gene, su-pur, which prevented purine excretion, and the majority of them showed anomalous behaviour of mating-type a associated with the mutations causing excretion. Mutants obtained by the second method were allocated to six unlinked genes. Those assigned to pur-1 to pur-5 are recessive and affected by su-pur. pur-6 is a complex locus with both dominant and recessive alleles; it is closely linked or allelic to su-pur and ad-4. Functional tests involving mutants of pur-3 and pur-6 in combination with pur-1 and pur-4 did not show normal intergenic complementation. The suppressor shows gene-specific dominance/recessiveness. Some metabolic lesions which could give rise to purine excretion are discussed.

Effects of the incompatibility factors in Schizophyllum commune Fries on the process of hyphal fusion are described. A role for the A incompatibility factor in hyphal fusion is indicated. Matings between strains with different mating types have higher fusion frequencies than matings between strains with the same mating types. Evidence is presented that the differences in fusion frequencies are not due to genetical factors other than mating types. When two strains of different mating types are grown in the same culture plate, but separated by a cellophane membrane, the strains are altered in some unexplained manner in such a way that even matings between strains of the same mating type have a higher fusion frequency than occurs in matings between compatible strains not so treated. Matings leading to the formation of common-B and dikaryotic mycelia have comparable fusion frequencies while those leading to the formation of common-A mycelia have a far lower frequency of fusions. It has been demonstrated that high fusion frequency is associated with heterozygosity at the A locus. It is suggested that a repression–derepression mechanism involving a cell wall degrading enzyme or enzymes may be involved in the regulation of hyphal fusion.

A synthetic lethal system involving homozygous combination of the non-allelic eye mutants eye-gone and eyeless is described. The major lethal crisis is towards the end of the pupal instar although a number of double homozygotes die at pupation. The pupal lethal crisis can be overcome in a number of individuals by the experimental removal of the operculum from differentiated pupae. However, only a few of the resulting enclosed double homozygotes proved fully viable, the remainder dying almost immediately after emergence. Dissection of the surviving adults revealed that the brain is located in the anterior thorax.

Plasmodia of P. polycephalum homozygous and heterozygous for a mutation (act) which confers resistance to actidione on the haploid amoebal stage have been tested for resistance to actidione in liquid semidefined medium. Growth of homozygous resistant (act/act) plasmodia determined by protein or pigment assay was inhibited less by actidione than growth of homozygous-sensitive or heterozygous plasmodia. The mutant (act) is therefore recessive but, contrary to a previous report, confers resistance on homozygous plasmodia. The same mutation affects the morphology of homozygous (act/act) plasmodia on agar medium, causing slow growth, fragmentation and excessive sliminess.

Females heterozygous for the T(1; X)CtX-autosome translocation tend to have lower levels of c-variegation when their rearranged X is inherited from the father rather than from the mother. The difference is not due to a maternal effect. It is postulated that a paternal or parental-source effect, such as that found to modify position effect variegation in Drosophila, is operating but the possibility that a bias in the inactivation of the maternal and paternal X chromosomes is responsible cannot be ruled out.

Some features of gene conversion in fungi and their bearing on the hybrid DNA models are discussed. Available experimental data from tetrad analysis seem to give a more complex picture of polarity in intra-genic recombination and of the relations between conversion and post-meiotic segregation, and between conversion and crossing-over, than predicted by the models.

A new hypothesis of the mechanism of gene conversion with special attention given to the aspect of asymmetry in this phenomenon is proposed as an alternative to the mechanism suggested by the DNA hybrid models.

The relations between the quantity of red eye pigment and related pteridine compounds of Drosophila melanogaster have been studied in a variety of genotypes, which include strains selected for high or low pigment content, various derivatives of these lines and also lines in which one or other of the major autosome pairs were represented by homozygous chromosome pairs, derived by random sampling from the base population and also inbred lines. The quantity of red pigment was defined by the optical density when whole heads were extracted in a suitable solvent, while the pteridines were separated by chromatography and their amounts estimated by means of their characteristic fluorescence.

The evidence from selection, inbreeding and chromosome sampling from the base population demonstrated the presence of substantial genetic variation for pigment content and amounts of related pteridines.

The genetic and biochemical properties of the selected lines differed according to the direction of selection. High lines remained heterozygous after many generations of selection and displayed dominance and epistasis in favour of higher pigment content in crosses to the unselected stock. Selection for low pigment content led to fixation of recessive effects, attributable to particular chromosomes. The dominance-recessive relationship in red pigment differences was also applicable to the associated pteridines.

The metabolic pattern in all lines with reduced pigment content is compatible with the assumption of reduced enzyme activity at particular steps of the pathway leading to the drosopterins (red eye pigments). The two steps accessible to study are subject to genetic variation in the base population, while inbreeding or selection for low pigment content leads to genetically fixed alterations at one or other of these steps. The genetic analysis was consistent with the biochemical evidence.

Increase in pigment content above the normal level, either by selection or chance fixation, is accompanied by correlated increase in all the precursors. Several alternatives are possible but it is suggested that this may be due to an increase in early precursors, before the stages which have been altered in the low pigment lines.

Attention is drawn to the similarity in genetic behaviour between pigment content and body size. Particular emphasis is laid on the value of selection as a means of creating biochemical differences which offer a basis for relating biochemical function and genetic behaviour.

The effect of ultraviolet radiation (u.v.) on λ lysogens of exrA strains of Escherichia coli was studied. exrA strains could be lysogenized with, as well as support the vegetative reproduction of, λ. However, though spontaneous induction of λ occurred in exrA(λ) strain at 10% the frequency of exrA+(λ) strain, exrA(λ) strains were not induced by u.v. Because λ was not induced in exrA(λ) strains, lysogens of these strains were no more sensitive to u.v. than were non-lysogens.

The heat-inducible mutant λhcI857 could be induced in exrA strains at elevated temperatures. Furthermore, u.v. irradiation of exrA (λhcI857) strain did not prevent the heat induction of this λ mutant. The exrA mutation appeared to interfere only with the inactivation of λ repressor by u.v.

Among the exrA strains studied was strain Bs1(exr A uvrB). Whereas the λ lysogen of strain Bsl could not be induced by u.v. and was no more sensitive to u.v. than its non-lysogen, the exrA+uvrB(λ) derivative of strain Bsl could be induced by u.v. and was more sensitive to u.v. than its non-lysogen.

Twenty-three exotic strains of four species of Neurospora (N. crassa, N. sitophila, N. intermedia and N. tetrasperma) were grown in minimal medium and their enzyme phenotypes were compared. The exotic strains did not differ substantially with respect to the specific activities of the biosynthetic enzymes, ornithine transcarbamylase (OTC), carbamyl phosphokinase and aspartate transcarbamylase. Exotic strains of N. crassa and N. sitophila were crossed with laboratory mutants of N. crassa to determine whether there were significant differences among the exotic strains in the gene complexes underlying their enzyme phenotypes. Most of the exotic strains carried similar OTC structural genes and similar genetic modifiers of OTC activity: their OTC structural genes were expressed normally in the genomes of the parent strains, in the alien genomes of standard laboratory mutants and in the mixed genomes of parent × mutant hybrids. One exotic N. crassa carried a distinctive OTC structural gene that elevated OTC activity when transferred into the genome of a standard laboratory mutant. A second exotic N. crassa carried distinctive genetic modifiers of OTC activity: two mutations, c- and hi, interacted in the exotic parent to normalize OTC activity and rate of growth, but hi led to extremely high OTC activities and to a reduction in rate of growth when separated from c- by interstrain recombination.

Evidence is given that me-7 and me-9 are separate but contiguous gene loci.

The pattern of polarized recombination throughout the me-7 me-9 region indicates the location of a recombinational discontinuity between the known me-7 alleles and the known me-9 alleles.

Recombination events may include sites in both genes but such events are not preferentially associated with parental combinations of flanking markers.

Recombination events extending into both genes provide an extra criterion for the ordering of the sites within the me-7 locus. The order so obtained confirms that deduced from the flanking markers of me+ recombinants from me-7 × me-7 crosses.

In me-7 × me-9 crosses but not in me-7 × me-7 crosses, the map order derived on the assumption that single exchanges are more frequent than apparent triple exchanges is the reverse of that derived from prototroph frequencies. It is concluded that the former criterion is more likely to have provided the correct order and the anomalous prototroph frequencies reflect the polarity of gene conversion within me-7.

Eight experimental populations of Drosophila pseudoobscura were made in laboratory population cages. All were polymorphic for ST, AR, CH, and PP gene arrangements in their third chromosomes, and all the chromosomes were of the same geographic origin. The initial frequencies of the gene arrangements were, however, different in the different populations. Natural selection has caused frequency changes in all the populations, from which we have attempted to infer the adaptive values of the different karyotypes. There was no evidence that the adaptive values were different in the populations begun from different initial frequencies of the inversions, although our ability to demonstrate such differences is severely limited. The selection process in these populations has been complex, and the simple model of constant adaptive values does not fit.

Frequency distribution of the length of time until fixation (excluding the cases of eventual loss) of a selectively neutral mutant in a finite population was obtained. With 4Ne generations (Ne, effective population size) as the unit length of time, the distribution has mean μ1 = 1, standard deviation σ = 0·538, skewness γ1 = 1·67 and kurtosis γ2 = 4·51 (see Fig. 1).

Contrary to previous reports, evidence has been presented which indicates that Xan-b61, a Xantha mutant of barley, does not have an auxotrophic requirement for leucine. It has been shown that this mutant, when grown under appropriate conditions in the absence of leucine, will synthesize leucine and will continue to grow for an extended period of time.