An experiment was conducted with the flour beetle, Tribolium castaneum to investigate genotype-environment interaction. Ninety-two matings of one male and three females were made at random from a large panmictic population. The females were transferred to individual containers after 4 days. Six daughters were randomly selected from each sire–dam pair and individually mated to unrelated males. Three of the six were placed in an incubator (33·3°C., 45% relative humidity) and three in a cabinet at room conditions (22·2–26·7°C., 30–35% relative humidity) and allowed to produce eggs during a 3-day period. Progeny were counted as pupae and larvae. The traits studied were number of pupae and number of pupae plus number of larvae. A transformation to (X +1)½ was required. A conventional least-squares model was employed, and a large environmental effect was observed. In the incubator the mean number of pupae was 13·4 and of pupae + larvae was 20·1 while the corresponding figures for room conditions were 3·3 and 9·2. Genotype-environment interaction accounted for 3·7 to 6·7% of the total variance for (pupae + 1)½ and 2·1 to 8·3% for (pupae + larvae+ 1)½. Heritability of the traits was essentially the same in both environments. The interaction was due to an increasing difference between environments in production associated with increasing breeding values of the sire, and to small changes in rank of breeding values on the two environments. As a result of the interaction, selection in one environment for production on the other would be expected to be only 71 to 72% as effective as direct selection for (pupae + 1)½ and 62 to 86% for (pupae + larvae + 1)½ even though the fraction of the total variance attributed to genotype-environmental interaction was less than 10%.

The selection of animals or plants for high values of a certain character may favour not only genotypes associated with these high values but also genotypes associated with high variability. Any differences between genotypes in variability may therefore be of considerable importance in plant and livestock improvement programmes as well as in evolution. The effects of various selection procedures on variability have been studied in three recent experiments [Falconer & Robertson (1956) Falconer (1957) and Prout (1962)]. In these experiments one line was continued by selecting, in each generation, parents with values of a particular character near the population mean. Manning (1955, 1956) has described the effects of this kind of selection applied to cotton. Robertson (1956) derived and discussed the theory of such selection procedures when certain simplifying approximations can be made We shall obtain some more general results and show that Robertson was incorrect in saying that the selection procedure would lead to gene fixation even if the heterozygotes are less variable than the homozygotes. The importance of the results is discussed in section 8.

1. Inactivation of one X chromosome in somatic cells of female mammals is a form of dosage compensation of sex-linked genes, but the mechanism is entirely different from that operating in Drosophila. The latter is designated as dosage compensation sensu strictu.

2. There is no dosage compensation of barred, sex-linked dilution or slow-feathering in domestic fowls, of almond or faded in pigeons, or of cinnamon in canaries. Among Lepidoptera the same is true of sex-linked melanism in Lymantria monacha and of a locus controlling haemolymph colour in Choritoneura spp. There is no positive evidence that dosage compensation occurs outside Drosophila and mammals.

3. Sex-chromatin in female birds (heterogametic) has been reported by several authors; the genetical evidence is against the possibility that this represents (as in mammals) an inactivated X chromosome. Sex-chromatin in the heterogametic sex also occurs in some (not all) Lepidoptera and Heteroptera; in Heteroptera it usually represents a heteropyknotic Y chromosome.

4. Some complications in Muller's theory of dosage compensation sensu strictu are discussed. Not all ‘compensatory modifiers’ are necessarily sex-linked.

5. The problem of dosage compensation in species with impaternate males is discussed; fused in Habrobracon is not compensated.

A selection of single and double mutant strains was backcrossed repetitively at least four times to each of the wild-types Abbott 4A, Abbott 12a and Lindegren 1A, and then re-isolated. Twelve crisp progeny isolated from the fifth backcross to Abbott 4A were heterogeneous for recombination frequency in the interval between cr and me-6 while thirteen crisp progeny from the fifth backcross to Lindegren 1A were homogeneous for recombination frequency in this interval. Thirteen crisp strains from the fifth backcross to Abbott 12a were homogeneous for recombination frequency between cr and nic-1 loci. However, mutant strains which had been backcrossed independently to Lindegren 1A were heterogeneous for recombination frequency in the intervals cr-aur and aur-nic-1.

In general, recombination frequencies in crosses between strains of the same wild-type ancestry including Abbott 4 and Abbott 12 were significantly higher than those in crosses of Lindegren × Abbott 4 or Lindegren × Abbott 12 ancestry but exceptions were found. Recombination frequencies between the same markers usually, but not always, increased on inbreeding and the changes in frequency were non-uniform in the marked region in some crosses.

The limitations of backcrossing as a means of transferring a mutant marker into a wild-type genome were discussed. It was concluded that inducing marker mutations anew in a given wild strain, preferably Lindegren 1A, might be more successful in obtaining constancy of map distance.

The pleiotropic effect of the rose-comb gene (R) on fertility when combined with artificial selection against the single-comb type (r) results in an interesting example of operational over-dominance. An evaluation of the equilibrium frequency of the rose-comb gene based on this over-dominance concept provides a plausible explanation for the relatively high frequency of single-comb birds appearing in the Wyandotte breed of fowls.

This paper presents the first part of a further analysis of data collected many years ago by Clark and Hersh on allometric growth in Notonecta undulata, the water boatman. The data consist of measurements of a number of dimensions, taken each instar, on 72 individuals reared separately from egg to adult. These individuals were the progeny of nine wild females, so that a rough estimate can be made of the magnitude of genetic effects by comparing the variances within and between families. The paper examines the allometric relationship between femur length of the three legs and body length.

The statistical problems in this type of analysis are discussed. There are significant deviations from linearity when the six points for femur length are plotted against body length on a double log graph, but these are small compared with the overall linear trend. The growth coefficients (k) are calculated for the three femurs against body length for each individual, and are analysed for differences between femurs, between sexes and between and within families. This analysis shows that there are significant differences between the three femurs in growth coefficient, but the growth gradient, or pattern of k values between the three femurs does not vary significantly from one individual to another. On the other hand, mean k for the three femurs shows significant individual variation and also a significant variation between families. From this it appears that about one third of the phenotypic variance in mean relative growth rate of the femurs is due to genetic effects.

Cultivated plants of Nicotiana alata are self-incompatible and are of two kinds: normal (N); and exceptional (M). N plants are reciprocally compatible with N. langsdorffii; M plants are compatible only as males. M plants contain an unusual allele, SFI, which has a dual action in the style: it rejects both self-pollen, and Sf pollen from N. langsdorffii. The overall results agree with the assumption that the SFI gene produces two kinds of specificity in the style: primary specificity, which is responsible for the rejection of Sf pollen; and secondary specificity, which is responsible for the rejection of self-pollen as in SI alleles generally. The genetic sub-units concerned must be closely linked; there was no evidence for their dissociation in the 599 plants studied.

In both compatible and incompatible pollinations, SFI pollen grows more slowly than SI and, in addition, appears to depress the normal rate of growth of SI pollen. In consequence, crosses SfSf × SISFI ♂ yielded significantly fewer S.I. plants than the 50% expected. The two kinds of pollen grew at comparable rates, however, when F1 (M × M) plants involving parents from different original sources were backcrossed to SfSf ♀. Progenies then showed the expected 1:1 ratio of S.I. to S.C. plants. These results are assumed to be due to differential behaviour of the SFI allele according to its genetic background. The change in background would be from a degree of homozygosity, in plants from the same source, to a degree of heterozygosity, in crosses between plants from different sources.

The high incidence of the SFI gene in N. alata is considered to be due to the advantage it confers on a self-incompatible population when it is overlapping with a related self-compatible population (having the Sf gene). Plants carrying an SFI allele, by rejecting the Sf pollen, will restrict the spread of inbreeding and so be favoured by selection.

The origin of the SFI and Sf alleles are discussed in relation to the author's hypothesis of S-gene structure.

1. The rate of production by X-rays of new genetic variation in two quantitative characters in Drosophila melanogaster (sternital and sternopleural bristles) has been investigated, using ‘plateaued’ populations which had reached the limit under artificial selection and, for sternital bristles only, populations which had been made genetically invariant by inbreeding. The genetic variation was always measured by the response of the population to selection. The X-rays dose given in any generation was always 1800 r. to adults.

2. Seven plateaued lines had eight cycles of alternate irradiation and selection, each with its non-irradiated control. All the responses were small but in three lines they were significantly greater after irradiation.

3. Selection was applied to three different inbred lines, genetically marked to detect contamination, after varying periods of irradiation. At the same time, the inbred lines and lines derived from them which had been mass mated in bottles were selected. The irradiated populations showed a greater response. The new genetic variance produced by the irradiation was approximately 10−5 units/r. The estimate of the dose required to introduce new variation equal to that in a standard outbred population was 500,000 r.

4. The effective population size was an important factor in the interpretation of some of these results on the long-term effects of radiation. By observing the variation between replicate lines in the frequency of a gene with a visible effect under these culture conditions (i.e. in a single culture bottle) the effective population size was estimated at sixty. Outbred populations kept under these conditions for many generations showed a reduction of genetic variability in agreement with this value.

5. To investigate the possibility that the deleterious genes produced by irradiation would interfere with the response to artificial selection, a standard outbred population was irradiated and selected. In spite of the observed high frequency of recessive lethals produced, the response to selection was very similar to that of the standard population.

1. Inheritance of flower colour in Delphinium ajacis is controlled by a locus with three stable alleles: pb (blue) is dominant to pl (lavender), which is dominant to p (pink). An unstable allele, p*, exists in a number of states, distinguished by the pattern of their mutation to pb and p during the development of the sepals.

2. The rate of occurrence of blue sectors and spots in the developing sepals of p*p* plants is twice that in the sepals of p*pl plants. The rate of mutation to pb apparently falls during the development of the sepals and this is probably due to p* mutating also to the stable p allele.

3. The frequency of pb and p gametes from p*p* plants is twice that from p*pl plants.

4. The pb mutants from p* show no evidence of instability; the controlling element has transposed from the locus.

5. Some p mutants from p* are apparently stable; others can reacquire instability, especially during sexual reproduction. The latter are tentatively interpreted as due to transpositions of the controlling element to other sites within the gene.

6. The instability of p* is only shown in the presence of a dominant activator.

7. Evidence is presented of transposition of the controlling element from p* to a locus controlling the colour of the foliage.

Mi-spotted (misp) is a new mutation in the mouse at the microphthalmia (mi) locus. It has no obvious visible effect in either the heterozygote (misp/+) or homozygote (misp/misp) and was discovered by virtue of its interaction with white (miwh). Mice of genotype miwh/misp are pale yellow with white spots; yellow areas later become ‘sooty’ at the first moult.

Though mice of genotypes a/a; misp/+ and a/a; misp/misp cannot be visibly distinguished from those of a/a; +/+, the amount of tyrosine-2-C14 incorporated in melanin in skins of 5-day-old mice of these genotypes differed. Assays of tyrosine incorporation were extended to include other non-agouti genotypes differing only at the microphthalmia locus. The amount of tyrosine incorporated was greatest in +/+ followed in order by misp/+, misp/+, misp/misp, miwh/+, miwh/misp, and miwh/miwh.

Pigment granules were examined in club hairs of these same genotypes for different regions of the hair shaft. Hairs of misp/+, misp/misp, and mi/+ could not be distinguished from +/+ either in number, kind or arrangement of granules. Hairs of miwh/+ showed reduced cortical and medullary pigment, especially distally, which was even more pronounced in hairs of miwh/misp. Medullary granules of miwh/misp varied greatly in size with a few large yellow-brown granules.

The chromosomes of the two parental forms and those of the hybrid do not show any detectable numerical or large structural differences. The pairing in the hybrid is frequently abnormal in that both free ends (highly charged with stainable substances) of many chromosomes show strong pairing, while midsegments repel each other. This partial failure of paring suggests that in the two parental forms gene mutations rather than chromosomal structural changes are responsible for incipient divergence.

It is suggested that the stainable substance of the chromosomes acts as a carrier mechanism ensuring orderly pairing, segregation and numerical constancy of bivalents in the hybrid where the partner chromosomes have become partly non-homologous.

The distinction between long and short chromosomes is typical for the avian karyotype, its evolutionary significance is discussed.

1. Hybrid (C3H♀ × 101♂) male mice were given two doses of 600 r. acute x-irradiation eight weeks apart and outcrossed at the end of their sterile period. Their fully fertile sons were outcrossed and daughters of these sons were backcrossed to them, in order to study the rates of induction in spermatogonia of dominant and recessive lethal and visible mutations, as well as dominant semi-sterility.

2. F1 litter-size decreased by 15·2% at birth and 15·8% at weaning age, as compared with controls. This decrease was very largely due to dominant lethality acting at about the time of embryonic implantation. There was also a highly significant increase in the incidence of heritable semi-sterility, the estimated rate of induction of reciprocal translocations being 3·3% per gamete.

3. The dominant lethality was shown to be partly a secondary consequence of induced translocation heterozygosis. The estimated overall rate of dominant lethal induction was 10·6% per gamete, with ‘primary’ dominant lethals induced at a rate of 4·0% per gamete.

4. The estimated mutation rate to dominant visible mutations was 4·6 × 10-7/gamete/r., but this was based on only two mutations in the irradiated series.

5. In the backcross generation there was again significantly more embryonic death in the irradiated series, mainly at the small mole stage and this was attributed to induced lethal mutation. The survival in the irradiated series was 96·80% of that in the controls and from this the rate of induction of recessive lethals was calculated to be 2·5 × 10-4/gamete/r. There was no evidence of the induction of lethals acting later than 14 days' gestation.

6. The estimated rate of induction of recessive visible mutations was 1·8 × 10-5/gamete/r., but the results showed heterogeneity, probably due to personal factors.

7. No significant sex-ratio differences were found.

8. The results were compared with those of specific locus and other relevant experiments. The rates of induction of recessive lethal mutations and of recessive visibles were both lower than might have been expected. On these results the mouse was only 4–5 times more sensitive than Drosophila to the mutagenic effects of radiation.

1. F1 (CSH♀ × 101♂) male mice were given 1200 r. 60Co Υ-irradiation over welve weeks and mated twelve weeks after the end of irradiation. The incidences of foetal and neonatal lethality and of semi-sterility in their offspring were compared with those in controls.

2. Embryonic survival to 15 days in the irradiated series was 98·2% that of controls, while the incidence of semi-sterility was 0·9% compared with nil in the controls. Neither of these differences is significant.

3. Comparison of these results with the significant rates of induction of dominant lethals and translocations in a previous experiment, in which a dose of 1200 r. acute x-irradiation was given to males of the same hybrid stock, show the existence of a dose-rate effect. Its magnitude cannot at present be accurately estimated.

4. The reasons for this effect are discussed. It is concluded that the main cause of the lowered translocation yield is that restitution of breaks will be favoured at low dose-rates, with less opportunity for the formation of interchanges. This could partly account for the lowered rate of induction of dominant lethals as well, but a full explanation is not yet possible.

1. A mild diabetes insipidus is associated with the oligosyndactyly caused by the dominant gene Os. No recombinants were observed between the diabetes and the oligosyndactyly so the diabetes is probably a pleiotropic effect of the Os gene.

2. There are one or more modifying genes which in combination with Os enhance the manifestation and cause a severe diabetes insipidus. This affects both sexes and becomes progressively worse in older animals, the average water intake reaching 50–60 ml. per 24 hours, or 1·7 times the body weight.

3. The modifying gene or genes, in the absence of the Os gene, themselves produce a mild diabetes insipidus, which becomes severe in some old females.

4. The severe diabetes insipidus associated with Os and the modifying genes together is probably renal in origin.