Mating systems in which the least related individuals are mated have been designated by Wright as having maximum avoidance of inbreeding. For such systems the initial rate of decrease in heterozygosity is minimum. However, some other systems have a lower rate of decrease in later generations.

Circular mating, in which each individual is mated with the one to his right and to his left, leads to an asymptotic rate of decrease in heterozygosity of 1– λ ˜ π2/(2N + 4)2 compared with 1/4N for maximum avoidance systems. Circular pair mating, in which for example each male progeny is moved one cage to the right, leads to 1– λ ~ π2/(N + 12)2. Other similar systems are discussed.

For minimum gene frequency drift, a mating system should have a constant number of progeny per parent and the population should be broken up as rapidly as possible into the maximum number of lines. The gene frequency variance at generation T within a line is

where N is the number in the line and Ht is the proportion of heterozygotes in generation t. Although the three mating systems, circular, circular pair, and maximum avoidance (and many others) have the same amount of random drift ultimately, at any generation circular mating has the smallest drift variance, VT, and circular pair next smallest.

1. The low but regular positive correlation between body-size and the duration of the larval period in populations of D. melanogaster has been studied by selecting for large size or shorter development time on aseptic defined diets deficient in RNA and comparing the results with parallel selection on unrestricted yeast diets or on media in which RNA is not a limiting factor.

2. There is a striking contrast according to the nature of the diet during selection. On unrestricted diets and where RNA is adequate there is little or no evidence of correlation between the two characters, but on low RNA media there is a striking correlation whether selection is for large body size or shorter development time.

3. This contrast is accounted for in terms of particular changes in larval growth which can be divided into a first stage of growth to a critical size in the early 3rd instar and a second stage thereafter. The duration of the first stage can be greatly prolonged by inadequate diet but the duration of the later stage appears to be virtually unaffected by such variation although the amount of growth and hence final body-size, may be drastically reduced. The different diets which lead to presence or absence of correlation have enabled selection either to extend the growing period, so that the critical stage is reached later at a larger larval size, or to accelerate the growth rate in the later stage.

4. Variation in the final stage of growth predominates on unrestricted diets and is responsible for the greater part of the variation in body size in unselected populations. Stabilization of body-size about an intermediate optimum refers especially to growth in this later stage.

5. Lines selected for fast development on low RNA media are especially sensitive to minor nutritional variation. Probably only under rather special conditions is it possible to shorten the duration of the larval period and this is compatible with the importance of development time in fitness generally.

6. There is evidence that the restriction of early growth, in the 2nd instar, reduces the size of the 3rd instar mouth-parts. Such reduction is correlated with changes in adult size probably because smaller mouth-parts restrict food intake.

7. The pattern of larval growth suggests a flexible system which can be adjusted to different ecological conditions since the same body-size can be attained by adjusting the amount of growth effected before or after the critical stage. Differ ences in this respect will involve characteristic differences in reaction to environ mental variation and particular nutritional conditions are likely to influence the way in which adaptive changes are realized.

By nitrous acid or UV treatment ascospore colour mutants of two kinds, blue and colourless, were obtained in Aspergillus nidulans (wild-type has red ascospores). Four blue mutants were located in linkage group II within 0·5 unit of one another (locus symbol: b11). Of the colourless mutants, four were located in linkage group I within one unit of one another (locus symbol: c16), and one in linkage group IV (locus symbol: c14). In diploids the mutants were recessive. Colourless was epistatic to blue.

In crosses these characters behaved as ‘non-autonomous’ both in the ascospores and in the asci; all the ascospores of the asci in one perithecium as well as the perithecium wall were of the same colour. In crosses between strains with blue perithecia and strains with colourless perithecia, red, blue and colourless perithecia were found; each type included both crossed and selfed perithecia. Red selfed perithecia were of either parental genotype but blue or colourless selfed perithecia always had the corresponding genotype.

The phenotype of the perithecium (perithecial wall and ascospores) is considered to be determined by the homo- or heterokaryotic constitution of the protoperi-thecium which gave origin to it.

1. Data from transduction experiments with Salmonella typhimurium can be interpreted in terms of frequencies of recombination by accepting the arguments of Ozeki that all transduced fragments that include a particular locus are equal in length.

2. From a two-point transduction the ratio of the two classes of transductants is not a measure of the ratio of the frequencies of recombination in the two relevant regions but only provides maximum and minimum values for this ratio.

3. From three-point transductions the frequencies of recombination in the regions between the markers, and between the terminal markers and the ends of the fragment, can be estimated.

4. The difficulties of interpreting data from experiments in which a single factor is transduced are briefly discussed.

The inactive-X theory of dosage compensation postulates that in all somatic cells of adult female mammals one or other of the two X chromosomes is genetically inactive. This means that in a female heterozygous for two non-allelic genes acting through the same cells, and carried one on each X chromosome, one or other gene should act in all cells. Conversely, if the two genes are carried on the same X, then both genes should act in some cells and neither gene in the remainder. This point has been tested by breeding experiments with mice, using pairs of genes affecting coat colour and coat texture. In female mice carrying the colour mutant dappled, Modp, on one X and a translocation including the wild-type alleles of pink-eye, p, and albino, c, on the other, either Modp or the translocation acted in all cells. With the genes tabby, Ta and striated, Str, affecting coat texture, in Str + / + Ta females tabby acted only in the non-Str patches, while in StrTa/ + + it acted only in the Str ones. Thus these experiments confirm that only one of the two X chromosomes is active in the somatic cells of female mammals.

The frequencies of non-disjunction at first and second meiotic division, in both male and female of Drosophila melanogaster, are estimated. Numbers of exceptional individuals scored in several broods of increasing parental age are also presented. These results fail to show any evidence of an age-effect. Work claiming to demonstrate the existence of an age effect in Drosophila is reinvestigated. The importance of these results and their relevance to human studies are discussed.

1. Data is presented in support of the hypothesis that in Salmonella typhimurium the fragments participating in the transduction of a particular linkage group arise by breakages at constant positions in the genome of the donor bacteria.

2. Data from three factor transduction experiments are used to construct a linkage map of the leu-araB region which defines not only the relative order of the markers but also the relative distances between them.

1. The paper described an attempt to see whether differences in co-adaptation between populations of Drosophila subobscura are related to the distance between them. The mean and the variance of body-size, development time and survival were recorded on parent populations and the F1 and F2 of various crosses to test for heterosis in the F1 and decline in performance or greater variance in the F2, which might indicate the break-up of co-adapted gene arrays. Comparisons were carried out at different temperatures and on a variety of larval diets, especially sub-optimal ones in which the larvae were grown on synthetic media. A large number of wild flies were caught at sites separated by about 10 miles along a transect of southern Scotland; these comprised one series of comparisons. For more distant crosses flies were caught at sites in southern England, Denmark, Switzerland and Israel.

2. There were well-defined differences in body-size, and, to a lesser degree, development time between populations from more widely separated localities and these showed evidence of a cline, northern populations having larger body-size. The difference in size between the Scottish and Isreal populations is about 20%.

3. There was no evidence of differences in co-adaptation between populations even in crosses between populations from sites as far apart as Scotland and Israel. The F1's were always close to the mid-parent values and there was no evidence of breakdown in the F2 nor of increased variability.

4. There was hardly any evidence of gene-environment interaction either with respect to different diets or to different temperatures.

5. Records of body-size on flies caught in the wild showed that they are extremely variable, indicating great variation in larval nutrition. Under natural condition stability of growth in body-size is conspicuously lacking in this species.

6. An additional test of co-adaptation was based on the between-family variance of abdominal bristle number of intra- and inter-population matings in the two most widely separated populations. There was no evidence of greater variance in the inter-population series.

7. To test for possible differences in breeding structure, the response to inbreeding was determined for two widely separated populations of D. subobscura and a long-established cage population of D. melanogaster, on an unrestricted larval diet and also on several different kinds of sub-optimal diets. There was little or no sign of consistent differences between the species in their response to inbreeding.

8. This test revealed differences between the two species in their minimum requirements for particular nutrients. subobscura is less able than melanogaster to withstand lower levels of protein and survival is particularly reduced. On the other hand, melanogaster has a considerably higher requirement for choline. Where there are apparent differences between the species in the average effect of inbreeding, the inbreeding effect is greater on the relatively more sub-optimal diet.

9. Comparison of the performance of the immediate descendants of wild flies with those derived from the same site, but kept in the laboratory for some twenty generations, failed to show any differences on several different diets and so there was no evidence that adaptation to laboratory conditions was important.

10. The lack of evidence for co-adaptation apparently conflicts with what has been claimed for other species. Such differences are discussed.

Six years of data collected from two shifts of Cornell random bred control matings each year are analysed. Approximately 900 progeny were tested per year from 50 sires mated to 250 dams.

Means for the important economic traits are presented by years and regressions for nine such traits, together with their standard deviations, are shown. The only significant regressions are those for both egg production and specific gravity score in shift II. An abnormally low egg production the first year for this shift is considered responsible for this result and the reality of the regression is discounted.

Genetic variances are presented by trait and year. Year to year variability is evident, but no trends are indicated.

Estimates of heritability and genetic correlations between traits are presented. These estimates are considered to be quite reliable and will be useful as estimates of genetic parameters when this population is used in selection experiments. Estimates of heritability from the sire components of variance were as follows: 8-week body-weight, 0·29; 32-week body-weight, 0·48; 55-week body-weight, 0·44; age at maturity, 0·09; egg production to 500 days, 0·07; 32-week egg-weight, 0·51; 55-week egg-weight, 0·53; USDA albumen score, 0·29; blood spot score, 0·13; and specific gravity score, 0·34.

Important maternal effects were found for 8, 32 and 55-week body-weight, sexual maturity and egg production. Selection programmes should take into consideration the fact that maternal effects accounted for 13% of the total variation in egg production. Genetic correlations utilizing combined sire and dam components revealed no serious antagonisms between egg production and other economic traits. However, this conclusion must be considered in the light of what influence maternal effects may have, since correlations computed from sire and dam components separately may have opposite signs and be important.

The many negative estimates of interaction components (SH, genetic by environment) and (I, sire by dam) indicate that these elements of the model contributed little to the variation of the traits studied.

The inheritance of three lymph protein fractions separable by starch-plate electrophoresis has been studied. Each fraction is under the control of a separate gene, the dominant allele determining its presence and the recessive its absence.

The gene for the fastest fraction is sex-linked, whilst those for the other two are autosomal and linked.

1. The four mating-types I, II, III, IV known for P. bursaria, syngen 1 are determined by specific combinations of dominant and recessive alleles at two unlinked loci. Mating-type I is formed by cells with the genotypes AABB, AABb, AaBB and AaBb; type II is controlled by the genotypes aaBB and aaBb; the double recessive, aabb is type III, genotypes AAbb and Aabb bring about type IV.

2. Clones which are exceptions to these rules for mating-type determination occur in low frequency (2–3%) in one line of descent. All can be explained by assuming either a mutation of dominant gene B to its recessive allele b or to a loss of the B locus. The data suggest that in four clones, the macronuclear but not the micronuclear genotype is affected; in two clones both nuclei are aberrant.

3. The cytogenetic events of conjugation were verified by the use of the new genetic markers. No evidence for uniparental nuclear reorganization, autogamy, was found.

Acridine orange does not interfere with the distribution of the F-particles among the daughter cells but inhibits the multiplication of the episome.

In growing cells the generation time of the episome in the presence of acridine orange is longer than the generation time of the cell.

This lowers the number of episomes per cell to less than two copies at cell division.

F-particles rapidly multiply after transfer until a relatively small fixed number (3–4) is reached.

1. Sensitivity of Drosophila melanogaster male germ-cells to chromosome breakage by X-rays has been measured by the structural changes found in the salivary gland chromosomes of larvae from irradiated fathers and untreated mothers. The genetical effectiveness of irradiation on the same males was measured by the frequency of sex-linked lethals.

2. Assessed by the overall percentage of germ-cells carrying structural changes, sensitivity follows the well-known pattern: it is highest in spermatids and decreases over spermatozoa and spennatocytes to spermatogonia.

3. This overall sensitivity has been analysed on three levels:

(a) The proportion in successive broods of ‘positive’ males, i.e. males which respond to irradiation. The results indicate that spermatozoa and spermatids in most or all males are equally sensitive to X-rays, while in regard to spermatogonia the males represent a heterogeneous population consisting of positive and negative males. This is fully confirmed by statistical analysis which was carried out by Dr B. Woolf.

(b) The proportion of ‘sensitive’ germ-cells per positive male, i.e. germ-cells carrying at least one structural change. While the proportion of sensitive cells per positive male is about 1:3 for spermatozoa, spermatocytes and spermatogonia, it is higher for spermatids. This is the main reason for the observed increase in overall sensitivity from spermatozoa to spermatids and for its subsequent drop in spermatocytes. On the contrary, the drastic drop in overall sensitivity from spermatocytes to spermatogonia results entirely from a drop in the proportion of positive males.

(c) The distribution of breaks in sensitive cells. The clustering of breaks in sensitive cells is highest in spermatogonia, somewhat lower in spermatids, and lowest in spermatocytes and spermatozoa. It indicates that spermatogonial cells are a heterogeneous population, consisting of cells that are either highly sensitive to X-rays or not sensitive at all.

4. The fact that the sensitivity of spermatogonia varies both between and within individual males is tentatively attributed to the presence or absence of spermatogonial mitoses.

The detached cilia from sexually reactive cells of Paramecium bursaria will agglutinate with the cilia of intact sexually reactive animals of a complementary mating type. No such reaction will occur if incompetent cells are used or if the cells are of the same mating type. Particulates other than cilia do not adhere to tester cells; the cilia which carry the specific mating-type substances are restricted to the ventro-lateral surface of the animal.

Studies of the heat inactivation of the ability of detached cilia to agglutinate confirm in detail the hypothesis of Metz which holds that cell unions leading to conjugation are brought about by the interaction of two pairs of complementary substances, A and a and B and b; the former pair of substances is more heat labile than the latter. The data suggest that animals of each of the four mating types carry a unique combination of two substances, namely, AB, aB, ab, and Ab.

When zygotes formed by conjugation between Hfr and F- cells of E. coli are transferred from an amino-acid rich medium to one lacking amino-acids the probability of genetic recombination is reduced. This result is interpreted as indicating that recombination can only occur during a limited period after zygote formation and that the metabolic imbalance caused by the change of medium leads to temporary inhibition of recombination. By an hour after zygote formation the yield of recombinants is no longer affected by transfer to amino-acid free medium.

In crosses in which the ratio of Hfr to F- cells was varied it was found that with one group of Hfr strains the yield of recombinants increased linearly with donor concentration until the number of donor and recipient cells were approximately equal. With other Hfr strains marked deviations from linearity were observed at Hfr: F− ratios greater than 1:5 and in crosses with one particular F− strain the yield of recombinants actually decreased progressively as the Hfr concentration was raised beyond this value. The deviations from linearity observed with this group of strains have been shown to be due to damage to F− cells which mate simultaneously with several Hfr cells.