In the analysis of recombination in Aspergillus nidulans coincidence values of about 1 are found in 3-point tests using markers more than a few map units apart. In comparable tests in which the marked intervals were very short (0·1 map unit or less), coincidence values of over 100 had been found. To account for this difference it was proposed that a necessary condition for recombination, termed ‘effective pairing’, was realized at any particular point on the chromosome in only a small fraction of a population of cells at meiosis. It was supposed that when effective pairing occurred it extended over a very short segment of the chromosome and that the probability of recombination in the effectively paired segment was high, i.e. about 1. Coincidence values greater than unity would be a necessary consequence of such a situation provided the intervals in question had a total length not much greater than that of effective pairing segments.

The experiments described in this paper were undertaken in an attempt to measure the mean length of effectively paired segments, their distribution, and the frequency of exchange within them. The data suggest a mean length of about 0·4 map unit, a mean exchange frequency of about 0·6, and a distribution which is variable, perhaps random.

The occurrence of localized negative interference suggests a way in which a number of difficulties encountered in relating the experimental evidence concerning the time and mechanism of recombination with the cytological evidence concerning the sequence of events at meiosis might be resolved. The data indicate that the frequency of recombination between linked loci is a measure principally of the frequency of effective pairing between them. If effective pairing is synonymous with homologous contact between chromosomes, and evidence is presented which suggests this may be the case, it becomes possible to construct a simple model which is compatible with the view that recombination takes place before chromosomes are paired, in the cytologically observable sense (i.e. before zygotene), at meiosis.

The recombination events occurring within effectively paired regions are generally, although possibly not exclusively, reciprocal. Non-reciprocal recombinants have been encountered in Aspergillus and other organisms, characterized by the occurrence of 3:1 ratios in tetrads. On the basis of evidence currently available it does not seem necessary to invoke a special mechanism of recombination, distinct from crossing over, to account for the formation of non-reciprocal recombinaiits. A single mechanism of recombination of the copy-choice type which, although primarily a reciprocal process, is nevertheless not necessarily exactly so or always so in detail, will account for the observed results.

The effect of hybridity on reproduction in mice has been studied in two environmental temperatures: 21° C. (‘normal’) and −3° C. (unfavourable). Mice of inbred strains A2G and C57BL are maintained as permanent breeding stocks in each of the two temperatures. In each temperature the two strains were crossed, and the reproduction of F1 pairs, to the age of 28 weeks, compared with that of the parent strains.

At 21° C. the expected superior fertility of the F1 mice was found: the number of litters produced was not affected, but there were more young produced per pair. The number of young per pair reared to 3 weeks was about twice the mean of the parent strains at the same temperature.

At −3° C. the difference was greater. The F1 pairs produced more and larger litters than the parent strains, and deaths between birth and weaning at 3 weeks were few. The number of young per pair reared to 3 weeks was nearly five times the mean of the parent strains. Part of the superiority of the F1 mice, at both temperatures, was due to the fact that they began to breed earlier.

Despite the larger litters produced by the F1 pairs, at 21° C. the mean weight of the F2 young at the age of 3 weeks was intermediate between those of the parent strains. At −3° C. it was the same as that of the heavier parent strain, namely, A2G.

As a further test of resistance to cold, F1 mice born in the warm environment were transferred to the cold at the age of 22 days and there placed each alone in a cage, with nesting material. In these conditions they had a higher survival rate than the young of either of the parent strains. They also grew faster than the A2G mice.

The F1 mice were not only more fertile than the inbred mice, but also more uniform in breeding performance. This difference was especially marked in the less favourable environment.

These observations are in conformity with the view that heterosis is a consequence of heterozygosis; and that it depends on an enhanced ability to withstand disturbances of developmental and physiological equilibria.

1. Two rabbit sires were used for insemination of sixty-eight females. Insemination was either homospermic (one sire at a time) or heterospermic (mixed semen from the two sires). Each offspring could be traced to its sire by genetic marks. The sires differed in weight and were known to give offspring differing in mature weight and named Large and Small offspring. The object of study was the birth weight of these offspring in logarithmic transformation.

2. After heterospermic insemination, there was no evidence of any real ‘heterospermic vigour’ in the average birth weight of the litter as a whole.

3. After heterospermic insemination, the difference in birth weight between the two kinds of offspring was accentuated (enhancement effect). This confirms a previous experiment.

4. The enhancement effect is ascribed to competition among embryos. It is not attributable to postulated changes in the gestation period. It appears to arise from an increase in the birth weight of Large offspring, together with a possible decrease in the birth weight of Small offspring. These changes in birth weight are attributable to changes in prenatal growth rate.

5. The bearing of this work on Russian experiments with heterospermie insemination is discussed.

Sixty-six individual tests on an array of skeletal variants showed an essential absence of parent-offspring correlations in the inbred strains C57BL, A and CBA in the mouse. It is concluded that these strains, and inbred strains of mice in general, are genetically homogeneous except for the differentiation of genetically distinct sublines as the result of mutations.

In Great Britain by far the greater part of the genetically effective dose of man-made radiation to the human population is due to high-intensity irradiation of the gonads in medical radiology; about a third of it is received by post-natal males, a third by post-natal females, and a third by foetuses, mainly in the later stages of gestation (Osborn & Smith, 1956).

1. A population of Paramecium aurelia, syngen 9, in Blackford Pond, Edinburgh, has been sampled over a period of 6 years and the types of immobilization antigen present identified.

2. Three antigen-determining alleles at the G locus and six alleles at the X locus were found.

3. No significant changes in relative numbers of the various alleles were apparent over the period studied, nor was there any difference between opposite ends of the pond.

4. An appreciable proportion of G-type heterozygotes was found, but the numbers were significantly less than would be expected by random mating alone. Autogamy is presumed to be responsible for this deficit.

5. Whether there is any natural selection in favour of heterozygotes over homozygotes in unknown. A possible method for obtaining information on this subject is discussed.

(1) In heterozygous condition, the gene for Patch (symbol Ph) produces spotting with sharply defined pigmented and white areas. The extent of the spotting is under the control of the genetic background.

(2) The Ph/Ph homozygote is inviable and dies before birth. In 9-day embryos, clear liquid is found flanking the notochord; in addition, there may be excessive amounts of liquid in the pericardium, the circulation, the tissues, and under the epidermis. The more extremely affected Ph/Ph embryos die at about 10 days. About one-third survive to later stages of pregnancy. Such ‘cleft-face’ embryos have a large bleb of liquid in the middle of the face which interferes mechanically with the formative movements of the nose and palate, and many subepidermal blebs elsewhere.

(3) Ph is closely linked to the gene for dominant spotting with macrocytic anaemia (W, Wv) in linkage group III, the crossover percentage being 0·077. There is a striking interaction in the double heterozygote Ph + / + Wv which is almost white, but nearly equally striking interactions occur with other spotting genes (s, bt and Miwh).

(4) By itself, Ph has no detectable effect on the coat colour, but in the double heterozygote with Wv, it slightly increases the dilution effect of the latter in the regions which remain pigmented. Similarly, Ph by itself has no appreciable effect on the red-blood picture of 13–14-day-old animals, but in the double heterozygote with Wv, it probably slightly increases the mild macrocytic anaemia produced by that gene. It remains unknown whether the Ph/Ph homozygote has an effect on the blood. Unlike W/W and Wv/Wv, Ph/Ph has no appreciable effect on the primordial germ cells.

(5) The relationship between Ph and the W-series is discussed.

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. 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.

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. 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.