Six isolate lines of long standing, from the Centre's Brown Leghorn flock, were intermated in all possible directions, providing thirty cross and six pure line samples. Half the birds from each line and reciprocal cross were kept in floor pens and the rest caged in an adjacent battery house.

Sexual maturity, survivors' production to 500 days of age, and November through June egg numbers, all showed two main factors contributing to variations in cross performance: (1) a significant association with parental production levels as measured by mid-parent averages, and (2) an additional hybrid gain. For November to June production in pens, this hybrid gain appeared to approach a constant for the cross groups. Additional sources of variation were present in age at sexual maturity.

The 500-day records for the penned hybrids exceeded the line average by 50 eggs. Caged samples did less well and showed an advantage of only 34 eggs. In this location the hybrid gain for individual groups decreased with increases in mid-parent level.

Egg weight varied negatively with egg numbers only in the upper half of the wide egg-size range. The best layers fell at the lower end of the covariant range for egg size.

The importance of improving closed strains may not have been lessened by the expansion of the hybrid side of the poultry industry, for the pattern of relationships of line and cross production in this flock is consistent enough to justify the inference that the best hybrids come from the best-producing parents.

1. Two-way selection was applied to the growth of mice between 3 and 6 weeks of age when reared on a high plane of nutrition and, in another pair of lines, when reared on a low plane of nutrition. In each generation the growth of all four lines was measured on both high and low planes of nutrition.

2. Growth on the two planes of nutrition was treated as two different characters and the direct and correlated responses of each were estimated. The genetic correlation between the two characters was estimated from the responses of each of the four lines, and from the divergence between upward-and downward-selected lines. The different estimates should be the same if the theory of selection for correlated characters adequately accounts for the responses. Up to generation 7 the agreement was reasonably good, but in the later generations it was not. Four estimates of the genetic correlation up to generation 7 were: 0·75, 0·19, 0·66, 0·57.

3. There was asymmetry between the upward and downward responses, and the realized heritabilities changed over the course of the experiment; so also did the phenotypic variation. In all these respects the lines behaved differently.

4. The conclusions drawn from the final responses are summarized at the beginning of the Discussion.

5. The mice produced by selection for increased growth on low plane, but later reared on high plane, were compared with those produced by selection on high plane. Their growth was the same, but they were heavier, had less fat and more protein, and were better mothers.

1. A random bred population of mice was subjected to inbreeding and the changes of litter size, measured as the number of live young in first litters, were followed.

2. The mean litter size declined at a rate of 0·56 young per 10% increase of the inbreeding coefficient.

3. Selection for large litters within the lines during the inbreeding did not effectively reduce the rate of decline.

4. Out of twenty lines at the beginning of the inbreeding seventeen were lost by the time the inbreeding coefficient reached 76%. Two more were lost later and one survived indefinitely. The three lines that survived longest started at a level below the mean and did not decline in litter size. The one that survived indefinitely reached 99% inbreeding without dropping below the non-inbred control.

5. The three lines surviving at 81% inbreeding were crossed and the litters produced by the crossbred progeny were larger than the non-inbred control by about two young per litter. This gain from heterosis is attributable to selection among the lines on their performance as inbreds. A second and third cycle of inbreeding and crossing yielded no further progress, and the level of the first cross was never regained. This is attributable to the ineffectiveness of the selection applied and to the previous restrictions of the population size.

6. The behaviour of the lines in the inbreeding and crossing point to simple dominance rather than over-dominance at the loci causing variation of litter size.

7. This experiment suggests that, as a means of improvement of farm animals, cyclical inbreeding and crossing does not look very hopeful.

Dissections were made of 16-day-pregnant female mice with the object of discovering the developmental stage at which litter size is reduced by inbreeding. Counts were made of the numbers of corpora lutea, implantation sites, and live embryos, and comparisons were made between females with inbreeding coefficients of 50–60% and non-inbred females. Except in one group the embryos were all non-inbred, so that the comparisons showed the effect of inbreeding in the mother of the litter. No influence of inbreeding in the male parent was found.

The only difference found between inbred and non-inbred females was in the number of eggs or embryos lost before implantation. The greater pre-implantation loss in inbred females was enough to account for the smaller number of young born alive in their litters.

There was no difference between the inbred and non-inbred females in the ovulation rate, measured by the number of corpora lutea, or in the post-implantation mortality of the embryos.

There was a positive correlation between ovulation rate and weight at 6 weeks. For reasons explained in the Discussion, the inbred females did not differ in weight from the non-inbred females. If, under other conditions, the weight declined on inbreeding, the ovulation rate would be expected to decline also.

1. The plasma proteins of six inbred strains of mice have been studied, using starch-gel electrophoresis.

2. The existence of two alternative plasma transferrin (β-globulin) phenotypes has been demonstrated. Five of the strains have one of these and one strain has the other. Each of the two transferrin patterns comprises three (or possibly only two) electrophoretic bands. The two patterns differ in all of these bands.

3. The two transferrin types recognized are determined by a pair of allelic, autosomal genes (designated TrfA and TrfB). The TrfA phenotype (CBA strain) is determined by the genotype TrfA/TrfA, and the TrfB phenotype (A, C57BL, JU, KL, RIII strains) by the genotype TrfB/TrfB. The phenotype TrfAB, of the heterozygote (genotype TrfA/TrfB), is distinguishable and shows four (or possibly only three) bands. In this way it closely resembles a mixture of equal parts of TrfA and TrfB plasma.

4. No linkage was detected between the Trf locus and sex, the agouti locus or the haemoglobin locus.

5. The possible molecular basis of the action of the transferrin alleles in the mouse, and the widespread distribution in mammals of polymorphism involving the transferrins, are discussed.

1. The four possible kinds of heterokaryon of Coprinus lagopus with no, one or both mating-type factors in common (dikaryon, common A, common B and common AB) were produced. Analysis of hyphal tips of common A and common AB heterokaryons has shown that both nuclei may be present in the same hypha.

2. All four heterokaryons are prototrophic when synthesized from two auxotrophic components with different requirements.

3. When synthesized in this way compatible heterokaryons were stable in all tests, but the other heterokaryons showed different degrees of stability. Common B heterokaryons were the most stable and rarely gave rise to monokaryotic mycelia. Dissociation of the common A and the common AB heterokaryon into either component took place much more easily.

4. Comparisons of the growth-rates of wild-type heterokaryons on complete medium show that common A heterokaryons are less vigorous, and dikaryons more vigorous than their monokaryon components. On minimal medium both compatible and common A heterokaryons are less vigorous than their wild-type monokaryon components. The possible reasons for this are discussed.

5. Fruit-bodies have been obtained from both common A and common B heterokaryons. Both types showed normal segregation at the heterozygous locus (B or A), but showed in addition the segregation of new reactions at the ‘homozygous’ locus.

Two mosaic birds are described, from a sex-linked F1 cross (Rhode Island Red ♂ × Light Sussex ♀). They are sexually normal males and full brothers. Each has the following abnormalities on one side of the body: absence of the maternal dominants S (silver, sex-linked), W (white skin) and Co (Columbian restriction of black), and reduction in size. In one case all the melanocytes appear to be derived from the smaller side, so that the plumage pattern is identical on the two sides; in the other there is a minor deviation (modified expression of Co) from normal F1 plumage pattern on the larger side. Mated to Brown Leghorn hens, both mosaics bred as normal F1 males, with no evidence of germinal mosaicism. Over thirty full sibs were all found to be normal.

A purely paternal origin of the smaller side appears to be excluded in both cases by the lack of striping in the down, and by minor deviations from normal Brown Leghorn phenotype in adult plumage pattern. Origin by some kind of maldistribution of chromosomes at first cleavage division would require the X-chromosome and possibly as many as three autosomes to be affected simultaneously.

Two further unrelated crossbred mosaic males are described. Each has yellow skin (w) on one side of the body. In one case this is accompanied by a slight difference in plumage pattern, reduction in size and abnormal proportions in the limb bones; in the other merely by a slight reduction in size. Loss or non-disjunction of a single chromosome at first cleavage division is a possible explanation of these.

1. The interrelations between environment and the phenotypic expression of genetic differences have not received the attention they merit. Laboratory studies in quantitative inheritance, either by choice of character or experimental conditions, have not shed much light on this problem. Selection for the same character in different environments is likely to involve qualitative differences in physiology and development. Comparative study of such changes will throw light on the genetics of development generally, which in turn is relevant to how far the selection response can be pushed in a given direction. Since statistical variation between individuals must ultimately be interpreted in biological terms, the unnatural barriers between quantitative and physiological genetics must be broken down to clear the way for a greater variety of experimental analysis and a more widely based approach to the interpretation of individual differences in populations. The ecology of the animal provides the point of departure and guide to the kind of environmental variation which should be studied first. Since the suggested approach cuts across the conventional limits of quantitative, physiological and population genetics and exploits the concepts and methods of these alternative approaches to a common end, it is convenient to have a descriptive label. The term ‘ecological genetics’ has been adopted.

2. This introductory paper is the first of a series dealing with experiments orientated along these lines. Since environmental variation largely consists of variation in the quantity and composition of the diet, the growth of individuals from a cage population of Drosophila melanogaster and also other strains has been studied on a variety of aseptic, synthetic diets. Body size and duration of the larval period are taken as measures of growth. There is a well-marked ability to regulate body size, by extending the duration of development, provided the diet is not too deficient. When the diet is further reduced development time is further lengthened and body size is reduced as well.

3. To test for genetic differences in reaction to the diet, strains have been created by selecting for large or small body size, and their performance, together with that of the cross between them, has been compared with the performance of unselected individuals on alternative diets for the first few generations of mass selection. There is evidence of gene-environment interaction quite early in selection, and after six generations striking differences were detected. It is concluded that genetic differences in reaction to different sub-optimal diets are widespread in the population.

4. The within-culture variance is increased by growing larvae on progressively more deficient diets and is approximately twice as great on a low-protein diet as on the usual live yeast medium. This increase is attributed to the segregation of genetic differences which are unimportant and contribute little to the variance under more favourable conditions.

5. Comparison of body size and development time in repeated tests with two diets lacking fructose or deficient in ribonucleic acid revealed evidence of a plasticity of response to minor nutritional variation which is characterized by a positive association between body size and the duration of the growth period. This relationship is the reverse of that associated with crude variation in the diet which leads to a negative association between development time and body size. This plasticity of response probably represents an aspect of physiological homeostasis. Genetic differences in the magnitude and direction of this response probably contribute to gene-environment interaction generally, and this probably accounts for apparent discrepancies in alternative estimates of the response to selection for large and small body size when these are based on deviations from the unselected. This suggests the need for determining how far body size may be increased either by altering the growth rate or by extending the growth period, and also how far strains differentiated in such respects differ in their reaction to controlled differences in nutrition.

1. The gene for Pintail (symbol Pt) shows no dominance. Heterozygotes are viable and fertile; many homozygotes die early in life, but the survivors appear to breed normally. Pt reinforces the effects of Sd and T on the tail; there is no evidence for interaction between Pt and + /vt.

2. The gene produces a shortening of the tail, but the most marked effect on the adult is in the great reduction of the nucleus pulposus of the intervertebral discs, which may lead to tail kinks in some animals.

3. Pintail embryos can be recognized from the eleventh day stage of development onwards by the possession of a much smaller notochord than normal. This reduced notochord is the result of a reduced rate of cell division which first manifests itself in 10-day embryos. Perhaps this reduced rate is the result of altered relations of the notochord with tail-gut and neural tube in 10-day embryos.

1. Four main types of interaction between paired mycelia of Coprinus lagopus have been defined in terms of the extent of nuclear migration.

2. Nuclear migration was demonstrated in matings between monokaryotic mycelia with common A alleles. No extensive migration of nuclei was found in common B or common AB matings.

3. The speed of nuclear migration in common A matings was slower than in compatible matings. Migration occurred to approximately the same extent in both kinds of matings.

4. In heterokaryon-monokaryon matings compatible and common B heterokaryons acted only as donors. Common AB heterokaryons acted as donors and as acceptors of compatible or common A nuclei. Common A heterokaryons always acted as donors and frequently acted as acceptors of compatible or common A nuclei.

5. A simple explanation is suggested for the frequently observed fact that in an incompatible di-mon mating both nuclei of the dikaryon may migrate and eventually eliminate the nuclei of the established monokaryon.

1. The X-ray mutability of wild-type isoalleles of the white-eye loci in Drosophila melanogaster has been determined.

2. No difference in the mutability of the isoalleles was found. However, an appreciable difference in mutation rate of the two halves of the w loci was found.

3. The bearing of these data on the problems of isoallelism and pseudoallelism are discussed.

1. Strains from a cage population of Drosophila melanogaster were selected for increased body size on the live yeast medium and on two aseptic synthetic media, (1) deficient in protein and (2) with all nutrients reduced to one-third the normal concentration required for growth to normal size. Both these media reduce body size by about 25%.

2. In two strains, mass selected on the live yeast medium, the response continued fairly steadily for at least sixteen generations, when the experiment was discontinued. By this time body size had been increased by some 30%.

3. On the sub-optimal diets the number of selected parents per generation was twice as great as on the live yeast medium, for technical reasons, but the response ceased abruptly after seven or eight generations of selection at a level considerably below that attained by the strains selected on the more favourable diet. Also, when selection was continued on the live yeast diet, no further progress occurred.

4. Flies selected on the different diets and also unselected flies have been grown on the alternative conditions to see how the deviation from unselected is affected. For both strains selected on sub-optimal diets, the deviation from unselected is appreciably greater on the medium used for selection than on the live yeast medium.

5. The response to selection for larger body size on deficient diets can be attributed partly to better adaptation to these conditions. This inference is supported by several lines of evidence. The within-culture variance, which is clearly greater when selected flies are grown on deficient diets, declines with effective selection. Also in the low-protein strain, for which data are available, the duration of the larval period is shortened in the early stages of selection, while egg production considerably exceeds that of unselected flies grown on the same diet.

6. Other effects, of the kind normally selected for on the live yeast medium, also contribute to the variation and selection response on the deficient diets. At first they appear to act more or less independently of the genetic changes which favour increased size via improved adaptation to the diet, but continued selection soon leads to mutual incompatibility between the alternative pathways in growth. Since no further progress occurred when selection was continued on the live yeast medium, the earlier selection had probably lowered the level of adaptation to the live yeast medium. A new genetic situation had been created in which it was impossible to gauge the amount of further progress by reference to the behaviour of strains selected on the live yeast medium from the beginning.

7. Estimates of heritability, based on cumulated selection differentials, are rather similar in the different diets and range between 0·30 and 0·38. On the live yeast medium, the estimate provides a fair guide to future progress, whereas, on the deficient diets, the predictive value is nil since response ceases immediately after the generations which provide the data for the estimates.

8. By comparing the deviation from unselected on the media used for selection and also the other media, alternative estimates of genetic correlation in performance in different conditions can be computed. The estimates were sufficiently divergent to cast doubt on the practical utility of the statistical procedure, which takes no account of the likelihood that individual variation in body size in different environment represents to greater or lesser degree the effects of segregation on different processes of growth and metabolism.

9. Since the course of selection is influenced by nutritional conditions, comparisons of response to selection for the same ‘character’ such as body size, in populations or species adapted to different conditions, must allow for the likelihood that unequal differences between the conditions in which selection is carried out and those in which the animal normally lives may be an important cause of differences in response.

1. Mutant stocks whose phenotype is affected by temperature were kept at 18° and 25° C., and selection was applied in an attempt to reduce the magnitude of the phenotypic difference caused by the temperatures. The stocks used were Bar, dumpy, cubitus-interruptus and aristopedia-Bridges.

2. When the stocks were kept for one or more generations at one temperature, then transferred for the next generation or two to the other, and so alternately, selection applied against the phenotypic effect of the temperature was only slightly effective in reducing the differences between the high- and low-temperature phenotypes. It is suggested that this was due mainly to the fact that such selection would be expected to operate on quantitatively-acting genes as well as on genes controlling developmental buffering.

3. With family selection, in which the offspring of a pair-mating was divided into two lots, one kept at each temperature, and selection was made on the basis of the differences in family means, progress was rapid. In the unselected Bar stock, the facet number at 18° was initially about three times that at 25°; after six generations of family selection the difference had been reduced to about 10%, the phenotypes at both temperatures being about intermediate between those seen at the beginning of the experiment. With aristopedia the reduction in the difference in family means was also striking, but in this case was achieved by increasing the abnormality of the 25° phenotype, and hardly at all by lowering that of the 18° population. It is pointed out that this is probably the result of an inadequate scoring system.

4. Although the selection in Bar stocks was effective in increasing developmental canalization against external environmental changes (temperature), it had little or no effect on the asymmetry of the facet numbers.

1. Published data suggest that mean left-right asymmetry in number of sternopleural bristles of D. melanogaster declines when inbred lines are crossed, while the corresponding variance for sternite bristles remains unchanged. Some genetic tests were undertaken to analyse this difference in behaviour of the two characters.

2. A progeny test on a wild stock showed that a small amount of genetic variance in sternopleural asymmetry was present, equivalent to about 2% of the total phenotypic variance.

3. It was possible to increase and decrease the level of sternopleural asymmetry in two wild stocks by selection. These experiments gave an estimated heritability of some 2–3%, in close agreement with the progeny test. Change in asymmetry did not necessarily lead to a change in mean count.

4. Homozygous lines, consisting of individual third chromosomes from the Renfrew wild stock made homozygous in an inbred line genetic background, were intercrossed, and the average indices for a number of characters of eight inter crosses involving eight lines were compared with their mid-parent averages. Thorax length was 2% greater and its variance 32% less in the crosses; total sternopleural count and its variance did not change significantly, but the asymmetry variance declined by 18%. In contrast, the corresponding asymmetry or independent variance for numbers of sternite bristles was 6% higher in the crosses, although the total sternite count and its variance did not change. These results fit in with previous work.

5. Tests on a similar set of homozygous lines in which the third chromosomes came from the SP wild stock, and on some long inbred lines from the Pacific wild stock, gave discordant results. Of eight SP lines examined, four were homozygous for a gene polychaetoid, and four were homozygous for a genetic effect causing sockets without bristles to occur among the sternopleurals. Both types had much greater sternopleural variance and asymmetry than the Renfrew lines, and both indices declined sharply in intercrosses leaving these genetic effects heterozygous, but neither declined if they were left homozygous in the crosses. Similarly high sternopleural variances were found in the Pacific lines, but only the total variance declined in males and only the asymmetry variance declined in the females, when they were intercrossed. All the four Pacific lines tested appeared to be homozygous for a genetic effect which caused a variable number of dorso-central and scutellar bristles to be replaced by sockets without bristles, and an occasional extra scutellar bristle to appear. This effect was also probably responsible for the high sternopleural variances.

6. Males of the Pacific inbred lines and intercrosses were compared when reared on the normal live medium and on a synthetic diet in reduced concentration, which reduced body-size by 23% (thorax area). The inbred lines were reduced more than the F1's in total sternopleural count and its variance, but the F1's were reduced more in sternopleural asymmetry, by the restricted diet.

7. The problems of interpreting these experiments, in view of our ignorance of the biological functions and attributes of the sternopleural and sternite bristles, are discussed. It is concluded that we have no basis yet for deciding whether sternopleural bristle number is of adaptive significance, but this is considered improbable.

8. The experimental evidence suggests that sternopleural asymmetry cannot be considered a measure of general developmental stability, particularly as the level of asymmetry can be reduced by selection well below that of typical wild stocks.

9. The scaling problems arising when the mean asymmetry of lines with different mean counts are to be compared, are examined, and it is suggested that the ratio of asymmetry to total count does not eliminate scale effects.

10. Developmental and anatomical differences between the sternopleural and sternite bristles suggest a possible reason why they behave differently when inbred lines are intercrossed.

In haploid strains of Aspergillus nidulans, asci arise from croziers. The two nuclei of the young ascus (the penultimate cell of the crozier) fuse, and the zygote nucleus immediately undergoes meiosis. At diakinesis and first metaphase eight bivalents are seen: three large (one, Chromosome 2, with a satellite), two medium sized (Chromosomes 4 and 5), two small (Chromosomes 6 and 7) and one very small (Chromosome 8). The perithecia of haploid strains are packed with asci and have very few sterile hyphae.

Diploid strains (heterozygotes made by Roper's technique) are very different from haploids in that the perithecia contain many sterile hyphae with little cytoplasm and coiled hyphae with dense cytoplasm made up mainly of uninucleate cells, and there are few asci in a perithecium. Croziers are absent: some of the cells of the coiled hyphae become asci. At diakinesis and first metaphase, the same chromosome configurations are seen as in a cross between the haploid strains from which the diploid was synthesized; the chromosomes are bivalents. The young ascus thus has a single (diploid) nucleus which undergoes meiosis, and asci develop apogamously. No evidence of nuclear fusion in the young ascus and of a tetraploid meiosis was obtained. Cultures from ascopores isolated by micro-manipulation from perithecia of diploids were all haploid. Meiosis appears to proceed normally to first metaphase, but typical later stages, and asci with spores, are rarely seen. First anaphase frequently fails to occur, and the chromosomes clump together.

In the strain ad2 y, obtained from a normal strain by X-irradiation, nine bivalents are present, one a very small fragment. In crosses between ad2 y and a normal strain, and in a diploid made from them, the fragment pairs with Chromosome 6. Asci in the cross between ad2 y and a normal haploid are highly irregular, often with less than the usual eight spores.

The American raspberry variety Chief is shown to carry three dominant genes, A5, A6 and A7, each capable of conferring strong resistance to Amphorophora rubi strain 1.

Chief also carries three genes, A2, A3 and A4, for resistance to A. rubi strain 2. A2 is a dominant gene conferring full resistance by itself; A3 and A4 are dominant complementaries, neither gene by itself having any effect on resistance. A5 is linked with either A3 or A4 with a crossover value of 10%.

The gene A1 from Baumforth A, which confers resistance to strains 1 and 3, when combined with A3 gives resistance to strain 2 also. Thus the three strains of A. rubi at present recognized on raspberries in Britain can be controlled by using either the combination A1A2 or A1A3.

Recently, a publication under this title has been distributed by Drs O. G. and M. J. Fahmy. On top of every page, it carries my name and not that of the authors. This has already resulted in its summary having been printed mistakenly as ‘author's summary’ of a paper of mine in Biological Abstracts. Since this curious publication is likely to lead to more confusion, its origin is explained below.

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.