Sterility resulting from treatment of male rats with methyl ethane sulphonate and ethyl methane sulphonate was found to be due to the induction of dominant lethal mutations in spermatozoa and spermatids. Spermatids in the acrosome phase of development were found to be the most sensitive cells and at low doses of both compounds the majority of deaths occurred after implantation. The earlier cell stages, i.e. spermatogenia and spermatocytes, were the most sensitive to isopropyl methane sulphonate and Busulphan (Myleran). The action on spermatogonia was confirmed by spermatid counts and the time of occurrence of oligospermia or aspermia was calculated. Pre-implantation deaths occurred mainly in the early cleavage stages after both isopropyl methane sulphonate and Busulphan; at least part of these losses can be attributed to the induction of dominant lethal mutations in spermatocytes and early spermatids. Isopropyl methane sulphonate appears to be the more effective of the two drugs in this respect.

1. A recessive autosomal gene, dx, preventing the anaphylactoid reaction has been discovered in a stock of Wistar albino rats from the Agricultural Research Council, Compton.

2. Segregation occurred in the F2 generation derived from crossing pure-bred reactor rats and pure-bred non-reactor rats.

3. Non-reactivity was outbred into non-albino stocks and was shown to segregate independently of the albino gene c and not to be closely linked to the colour genes, P and A.

4. The mutation from Dx to dx may have arisen a few years ago in one of the Wistar colonies before it was split into sub-lines.

5. The dx gene is fixed in several brother-sister mated sub-lines and appears to be in equilibrium with Dx in some less inbred closed-populations.

The effectiveness of the assortative mating of selected individuals in increasing selection response was tested, using abdominal chaeta score in Drosophila melanogaster. Three paired comparisons were made. In two sets of lines with 10 matings per line, individual score was used for selection and as the basis for the assortative mating. In the third set with 20 matings per line an index of individual and family score, designed to maximize rate of response, was used.

The intensity of selection was one in ten in all lines. Flies were raised in vials and individual pedigrees were kept.

In all comparisons, assortative mating gave a greater selection response, this being partly due to a greater realized heritability and partly to a greater selection differential. The effect of the assortative mating was largest in the index selected lines. With random mating, the effectiveness of the index selection itself when compared to individual selection was in accordance with theory.

In two comparisons, assortative mating increased the rate of inbreeding. The highest rate of inbreeding was observed with index selection and assortative mating, even though there were here twice as many matings as in the individually selected lines.

In the individual selection lines, the effective population size was 7·4 pairs of parents, compared to the actual value of 10 and in the index lines 7·0 compared to 20. In the former, only one-half of the matings in the initial generations made any permanent contributions to the line and in the index lines only one-third. Within generations and lines, there was a significant positive correlation between the mean score of a family and its inbreeding coefficient.

It is suggested that assortative mating is a method of increasing selection response in some situations. Its particular characteristic is that it becomes more powerful when the heritability is high whereas all of the other environmental aids to individual selection are more effective when the heritability is low.

In these experiments comparisons were made between the magnitude of the interaction of ‘pure’ strains and strain crosses of poultry with two types of environments—location effects and a restricted-feed versus a full-feed rearing programme. The ‘pure’ strains were closed flocks of White Leghorns that had been selected for increased egg production, while the strain crosses were the reciprocal crosses of all combinations of these pure strains. Data from four separate experiments in four consecutive years used for this study involved 8320 laying birds. Six traits of the adult laying birds were used for these analyses.

It was expected that the ‘pure’ strains would differ in performance amongst themselves to a greater extent than the strain crosses, and for the two traits, body-weight at housing and sexual maturity, this was found to be the case in three out of four years. These two traits were affected to the greatest extent by the rearing treatment. Also, the genotype-environment interaction variance was found to be significant and of important magnitude relative to the genetic variance for these two traits. Where the environmental effect was found to be smaller, the interaction variance made up a smaller proportion of the genetic variance.

A study of the gene B6m, previously described as a modifier, revealed that it is a recessive resistance gene of moderate effect when homozygous. Its value in enhancing resistance to bacterial blight when in combination with other genes, in particular B2, is emphasized. The symbol for the gene is simplified to B6.

The induction of dominant lethals after X-irradiation of dictyate, later meiotic stages, and the pronuclear stage after fertilization have been compared using the technique of induced ovulation in mice. The injection of gonadotrophins ensures that the time at which the synchronously dividing oöcytes reach any particular meiotic stage is accurately known. Embryonic lethality up to 13½ days was studied after 0 r., 100 r. and 200 r. acute X-irradiation. Metaphase I, anaphase I and metaphase II were the most sensitive stages, with LD50 's of about 120 r., 130 r. and 170 r. respectively. The dictyate and pronuclear stages were much less sensitive, with LD50's in the region of 500 r., and sensitivity rose steeply during prophase I. Numbers of corpora lutea decreased with irradiation, the decrease being greatest with irradiation at metaphase I and anaphase I. The ovulation of large numbers of eggs, which increased the preimplantation loss of embryos above normal, and the low luteal counts probably masked lethality to some extent. Results generally agree well with those reported in a number of plant and animal species.

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. 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.

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.

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. 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.

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.

The possibility that primary CRM− isolates at the td locus of Neurospora crassa are multisite lesions has been examined. By comparing the behavior of primary CRM− mutants to known multisite CRM− mutants in the genetic tests of recombination and reversion, it can be concluded that primary CRM− mutants are not multisite. The genetic implications of a CRM− and CRM+ mutant have been discussed.