1. Dominant hemimelia (Dh) and postaxial hemimelia (px) both belong to the luxoid group of mouse mutants, tending to cause luxation of limb-bones associated with hemimelia and polydactyly or oligodactyly.

2. Dh is about 4·8 units from leaden in linkage group XIII, while px is about 2·9 units from the microphthalmia locus in linkage group XI.

3. The nature and range of skeletal anomalies found in Dh heterozygotes closely mimic those found in luxate heterozygotes and homozygotes. As in luxate, only the preaxial side of the hind-limb is affected, with (i) polydactyly or oligodactyly (sometimes also syndactyly), (ii) tibial hemimelia, (iii) reduction and fragmentation of the femur, (iv) reduction of the pubis. The more proximal the anomaly the less frequently does it occur; about 4% of Dh/ + mice show no limb anomaly. The defects in Dh homozygotes are similar but usually more extreme, with severe oligodactyly, loss of tibia, fragmentation of femur and reduction of pubis.

4. All mice carrying Dh lack the spleen. Stomach size is reduced slightly in Dh/ + and greatly in Dh/Dh mice. In Dh/ + mice, the left kidney is either flattened antero-ventrally or (less frequently) hydropic; the right seems normal. Dh/Dh mice nearly always have severe hydronephrosis, as well as posterior visceral defects similar to those of the ‘uro-recto-caudal syndrome’. They usually die before weaning, but a few survive to maturity and a female has even bred.

5. Preliminary developmental studies show that Dh/Dh skeletal defects can be traced back to the precartilaginous stage.

6. When homozygous, px affects the postaxial side of the fore-limb and sometimes of the hind-limb also; a large ‘foramen infraspinatum’ is always present in the scapula. There may also be (i) oligodactyly and occasional syndactyly, (ii) ulnar hemimelia, (iii) distal reduction and distortion of the femur. Fore-limbs tend to be more severely affected on the right, but hind-limbs on the left. Extra sesamoid bones occur on the extensor side of digits in all four feet and are associated with extra tendons.

7. Both sexes are sterile and show anomalies of the paramesonephric (Müllerian) ducts. In the female, the vagina is wholly or partly double and the uterine tubes uncoiled, with an abnormal relationship to the ovary. In the male the paramesonephric duct persists in the adult, while the vas deferens runs into the distal end of the vesicular gland instead of into the urethra. The epidermis of manus and pes shows abnormal dark papillae on the extensor side.

8. The mean number of presacral vertebrae is reduced by 0·8 in Dh/ + and 1·6 in Dh/Dh mice. It is also reduced in high-grade postaxial hemimelics.

9. The relationships of these two genes to other luxoid mutants are discussed.

The short-term response to artificial selection in a wild-type population for increased scutellar bristle number, has been interpreted in terms of two alternative threshold models. Both models have been shown to give a satisfactory transformation of the scale of measurement, in that the underlying variable shows an effectively linear response in terms of the accumulated selection differential, and the pheno-typic variance on the underlying scale remains virtually unchanged over a period of generations sufficient to increase the incidence of extra-bristles to almost 100%.

A genetic analysis based on the bivariate model, which to a large extent takes account of the location of the supernumerary bristles, has shown the phenotypic variance on the underlying scale to have the following composition: the additive genetic variance is 27·2 ± 0·9% of the total, and the developmental error or ‘chance’ variance accounts for a further 63·8 ± 2·6%, so that the remaining 9·0 ± 2·7% of the variance must be due to environmental differences among individuals and to non-additive genetic effects.

The relative magnitudes of these parameters are remarkably similar to those observed for two quasi-continuous variables in the same population, viz. abdominal and sternopleural hair number. In view of the ease with which scutellar bristles may be scored, the character can therefore serve as an extremely useful model character for quantitative genetic experiments.

1. Two total and five leaky sorbitol mutants isolated in Aspergillus nidulans by defective growth on the sugar are all recessive. The mutants are closely linked, they appear to represent three linked genes spanned by a deletion.

2. Mutants which complement in heterozygous diploids do not complement in balanced heterokaryons. Failure to complement is a property of the mutants and not the result of a nutritional interaction or an unfavourable nuclear ratio in the heterokaryons.

3. Sorbitol is oxidized by an inducible enzyme system in the wild-type. There are at least two enzymes concerned in the oxidative assimilation of sorbitol, an initial oxidative enzyme, which is defective in the leaky mutants, and a later enzyme defective in the total mutants. There may also be a second non-oxidative pathway for sorbitol metabolism.

4. In diploids complementary pairs of mutants oxidized sorbitol at 75% the rate of the wild-type but non-complementary mutants did not oxidize the sugar. In balanced heterokaryons none of the pairs of mutants oxidized the substrate. It is concluded that failure of inter-genic complementation in the heterokaryons is the result of a failure of either enzyme formation or enzyme function. Models to account for differences in enzyme formation in heterokaryons and diploids are suggested.

Lysis of stock 7 killers with ½% sodium deoxycholate, yields preparations with one half as much killing activity as homogenates, while lysing virtually all of the bright particles known to have killing activity in homogenates. The killing activity which persists is associated with particles which sediment at the same rate as the refractile bodies, the only visible remnant of the bright particles. It is concluded that killing activity is probably associated primarily, if not exclusively, with the refractile bodies of the bright kappa particles. Partially purified killer particles when stored at 27° C. for more than one day, progressively lose their ability to produce spinning of sensitives but retain their ability to produce paralysis. Loss of activity in crude preparations of refractile bodies kept 2 hours at 31° C. can be retarded by homogenates of either killers or sensitives.

Five dimensions—wing length and width, thorax length, head width and tibia length—have been measured on samples of twelve populations of Drosophila subobscura taken from different parts of the species range, extending from Scotland to Israel. The populations had been started from thirty or more pairs of flies and maintained in the laboratory for eight to eleven generations. They were reared for measurement under standard conditions, so that any differences between them must be of genetic origin. The localities from which the populations came ranged over 25° of latitude and 15° F. in mean annual temperature.

The population means of the five dimensions all showed strong positive correlations with latitude and slightly lower negative correlations with mean annual temperature. There is, in consequence, a very uniform cline in the five dimensions, each increasing as we move northwards. This pattern differs from that found for essentially the same dimensions of D. robusta by Stalker & Carson (1947), where all the correlations with latitude are lower and those for head and thorax size are of opposite sign in the two species.

A partial correlation analysis and reference to selection experiments suggests that there are two distinct groups of genes involved in these clines: (1) a group of genes causing increase in relative wing and leg size, and responsible for the positive correlation of these dimensions with latitude in both species, and (2) a group of general size genes causing correlated changes in all dimensions, which have been selected in opposite directions in the two species, with the result that head and thorax size increase in subobscura but decrease in robusta as latitude increases. The ratio of wing length to thorax length has a high positive correlation with latitude in both species.

Further analysis of the data available on both species brings out the following points:

1. In subobscura there is considerable variation between localities remaining for all dimensions when the correlations with latitude are eliminated. This component of variance is much larger in proportion to the within-locality variance in subobscura than in robusta.

2. Comparison of the percentage regression coefficients of dimensions on latitude or temperature shows that individual dimensions change more rapidly in proportion to both geographical indices in subobscura, but the ratio of wing to thorax length ?changes more rapidly in robusta.

3. The ‘environmental’ regression of dimension on temperature of rearing in the laboratory, measured only for robusta, is very much higher than the corresponding genetic regression between populations on mean annual temperature for all the robusta dimensions and different in sign for head and thorax size. But this environ mental regression is uniformly only about twice as high as the corresponding genetic regression for each character in subobscura. Unexpectedly, the environmental and genetic regressions on temperature of the wing/thorax length ratio are equal in robusta.

4. D. subobscura is about 20% smaller in head and thorax size, and 26% less in wing width, than robusta, when both species are reared at 18° C.

5. The basic within-culture variances of the five dimensions are 50–100% greater in subobscura than in robusta for all dimensions. Variability in D. melanogaster is about the same as in D. robusta.

Some of the possible implications of these results are discussed.

1. Ovalbumin was shown, by starch gel electrophoresis, to exist in two genetically different forms, A and B. It was suggested that these are determined by two alleles at one locus, named Ov.

2. Ovalbumin of each genetic type is electrophoretically heterogeneous. Dephos-phorylation of each type with human prostatic phosphatase and calf intestinal phosphatase removed some of the heterogeneity, but some remained.

3. The genetic difference was shown not to reside in the fragment released from ovalbumin by the proteolytic enzyme subtilisin.

4. The genetic difference was evident in the ovalbumin present in the fluid contained in right oviduct cysts.

1. An auxotroph of Salmonella typhimurium, pro-401, was isolated in a strain that was unstable at the su-leuA locus. The auxotrophy of pro-401 is probably due to the attachment of a controlling episome to the proline region of the genome where it suppresses gene expression.

2. The controlling episome frequently transposes over short distances so that all clones consist of cells, mixed for the site at which the controlling episome is attached; homologous transductions yield prototrophs.

3. The controlling episome can transpose to a different complementation group; homologous transductions yield abortive transductants; syntrophism occurs between cells that are ‘mutant’ in different complementation groups to give reversions consisting entirely of auxotrophic cells which are called auxotrophic reversions.

4. The controlling episome transposes over very short distances and never to beyond the limits of this proline region of the genome; no wild-type reversions were found.

5. The controlling episome can be located at relatively distant proline sites in different clones; prototrophs from transductions between clones that are separated by many subculturings can be 100 times more frequent than from homologous transductions.

6. The controlling episome has its frequency of transposition to different complementation groups increased by UV; irradiation increases the frequency of auxotrophic reversions.

7. The controlling episome continues to transpose in stored cells.

8. The pattern of reversions of pro-401 is different in these studies from its pattern two years previously. This is discussed.

A mechanism for gene conversion is proposed which overcomes many of the difficulties that any copy choice model encounters. It is suggested that along with general genetic pairing of homologous genomes at meiosis, effective pairing over short regions of the genetic material occurs at the molecular level by the separation of the strands of the DNA double helices, followed by the annealing of strands from two homologous chromatids. If the annealed region happens to span a heterozygous site, mispairing of bases will occur. Such a situation may be analogous to that in DNA which is damaged by mutagens; the same or similar repair mechanisms may operate, and these, by adjusting the base sequences in order to restore normal base pairing, would bring about gene conversion in the absence of any genetic replication. The model indicates how precise breakage and rejoining of chromatids could occur in the vicinity of the conversion, so that conversion would frequently be accompanied by the recombination of outside markers. The model also proposes that the distance between two mutant sites on a fine structure map depends not so much on the frequency of a recombinational event occurring between them, but rather on the degree of inhibition of the processes of genetic pairing by the mutants themselves.

The model will explain almost all the data in a formal way, and it has the advantage over copy choice mechanisms for gene conversion in (1) being compatible with semi-conservative replication of DNA, (2) not invoking DNA synthesis during or after genetic pairing, (3) providing a molecular mechanism for close specific pairing, (4) making it unnecessary to postulate sister strand exchange or a process akin to this, (5) suggesting why rates of gene conversion in opposite directions are sometimes unequal and (6) providing an explanation of the clustering of mutant sites, a basis for map expansion and for the apparently capricious departure of fine structure maps from additivity. Although the model proposed is a general rather than a specific one, it suggests that the process of conversion and intragenic recombination is more complex than is usually believed, since it depends on several interacting factors. Nevertheless, it is hoped that the introduction of a model with this complexity will help to stimulate specific experiments, and that these will provide definitive information which would never be obtained if simpler models of conversion and intragenic recombination were believed to explain the genetic data sufficiently well.

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.

The results of the analysis of 393 mitotic recombinants within the paba1 region of Aspergillus nidulans can be summarized as follows:

1. Reciprocal, non-reciprocal and probably inexactly reciprocal exchanges occur in intra-cistron mitotic recombination.

2. Negative interference observed in intra-cistron mitotic recombination in the paba1 region of Aspergillus nidulans is more intense than that observed in meiotic recombination in the same cistron.

3. In intragenic mitotic recombination multiple exchanges can involve three or even four chromatids within one effective pairing segment.

The XXY male mouse, of otherwise normal chromosome complement, is characterized by small testes, few spermatogonia, and spermatocytes in which the first stage of meiosis up to the pachytene stage can occur, although with a very low frequency. Pachytene autosomes are morphologically similar to those of the oöcytes. The sex vesicle is smaller and more condensed than in the normal male.

The ovalbumin locus and locus II (Lush, 1961) were found to segregate in a closed flock of White Leghorns. Three of the nine expected genotypic classes were absent and from this it is concluded that the two loci are very tightly linked.

The paper on the analysis of data from transduction experiments with Salmonella typhimurium, which was published in Genetical Research (Dawson, 1963), unfortunately contained an error on p. 419. In calculating the limiting values X/Y, the formulae for d and d′ are correctly stated, but their solutions are transposed. The last and third last lines on p. 419 should therefore be interchanged. In the arithmetic example on pp. 420 and 421, a similar interchange of the calculations of d and d′ is necessary. The maximum value of a/b is then:

and the calculation of the minimum value should be:

These values of 2·20 and 1·23 are to be compared with the values of 2·23 and 1·24 in the published paper.