The correlated changes in reproductive fitness with selection for single traits for six generations in five Leghorn lines and five generations in three Fayoumi lines were analysed. The Leghorn lines, originating from crosses between four commercial strains, were: line A, selected for high egg production; lines B and C, selected for high and low nine-month body weight; and lines D and E, selected for high and low nine month egg weight. The Fayoumi lines, J, K and L, corresponding to the A, B and D Leghorn lines, respectively, were selected from a random mating population maintained over several years without selection. The breeding populations for the A and J lines each consisted of 16 sires mated to 9–16 dams each, while all the other lines were produced from eight sires, each mated to 9–16 dams. The A and J lines showed no consistent improvement in egg production or reproductive fitness and were used as control lines in this study. Body weight in the B and K lines increased by 55·8 and 45·2%, respectively, while egg weight in the D and L lines increased by 17·7 and 15·1%, respectively. The C line declined in body weight by 21·5%, while the E line declined in egg weight by 16·3%.

A reproductive fitness index was measured as the product of rate of egg production, fertility, hatchability, and percent survival of offspring from housing to nine months of age. The mean values of the fitness index as percentages of the control lines were Leghorn lines B 66, C 81, D 74, E 85 and Fayoumi lines K 54 and L 74.

Reproductive fitness and all fitness components except offspring survival declined more in the high lines than in the low lines. The body weight lines were lower in fitness than the egg weight lines, mainly because of lower rate of egg production The patterns of correlated changes in the fitness index were more regular in the Fayoumi than in the Leghorn lines. The linear regressions of fitness index on direc selection response in the K and L Fayoumi lines were statistically significant. Offspring survival contributed only 14% to the variation in the fitness index in the Leghorn lines and only 8% in the Fayoumi lines. The relative contributions to the variation in fitness index were 35% for rate of egg production, 24% for fertility and 23% for hatchability in the Leghorn lines. The corresponding figures were 35, 30 and 27% for the Fayoumi lines, respectively.

The average linear regression coefficients of fitness index on body weight, calculated on an individual hen basis (selected breeders only) within lines and generations, were - 4·77 for the B line, 11·53 for the C line, and – 1·01 for the K line. The corresponding coefficients of fitness index on egg weight in the egg weight lines were – 0·543 for the D line, 0·947 for the E line, and – 0·712 for the L line. With the exception of the K line, the regressions on body weight were larger in the body weight lines than in the egg weight lines, and, similarly, the regressions on egg weight were larger in the egg weight lines than in the body weight lines. The differences between the regression coefficients of the high and low lines were statistically significant.

Sublines carrying isolated polygenes determining body size, developed by selection and repeated backcrossing to the LG and SM strains of mice, were inbred by full-sib matings with concomitant selection to study the inheritance of body size, using birth, weaning (28-day) and 60-day body weights as indices.

Mean body weights in those sublines backcrossed to SM and selected for large body size were greater than in those selected for small body size, proving that the difference resulting from seven earlier generations of backcrossing and selection, although small, was genetic and could be fixed by inbreeding. The mean body weights of the sublines selected for small body size drifted upward despite downward selection, a phenomenon thought to be due to the pressure of natural selection outweighing that of artificial selection.

In the sublines developed from backcrosses to the LG strain, mice from lines selected for small body size attained a mean body weight greater than that of mice from lines selected for large body size and also than of the parental LG strain. These results were contrary to expectation and a genetic interpretation was offered.

1. The average and the effective numbers of alleles maintained in a finite population due to mutational production of neutral isoalleles were studied by mathematical analysis and computer simulation.

2. The exact formula was derived for the effective number (ne) of alleles maintained in a population of effective size Ne, assuming that there are K possible allelic states and mutation occurs with equal frequency in all directions. If the number of allelic states is so large that every mutation is to a new, not pre-existing, allele, we have ne = 4Neu+1 − 2Neu2, where u is the mutation rate. Thus, the approximation formula, ne = 4Neu+1, given by Kimura & Crow (1964) is valid as long as 2Neu2 ≪ 1.

3. The formula for the average number of alleles (na) maintained in a population of actual size N and effective size Ne was derived by using the method of diffusion approximation. If every mutation is to a new, not pre-existing, allele, we obtain

where M = 4Neu. The average number of alleles as a function of M and N is listed in Table 1.

4. In order to check the validity of the diffusion approximations, Monte Carlo experiments were carried out using the computer IBM 7090. The experiments showed that the approximations are satisfactory for practical purposes.

5. It is estimated that among the mutations produced by DNA base substitutions, synonymous mutations, that is, those which cause no alterations of amino acids, amount roughly to 0·2–0·3 in vertebrates. Incompletely synonymous mutations, that is, those which lead to substitution of chemically similar amino acids at a different position of the polypeptide chain from the active site and therefore produce almost no phenotypic effects, must be very common. Together with synonymous mutations, they might constitute at least some 40% of all mutations. These considerations suggest that neutral and nearly neutral mutations must be more common than previously considered.

1. The antibody response was studied in twenty-five monozygous (MZ) and ten dizygous (DZ) pairs o f cattle twins, which were inoculated with red cells from one cattle donor carrying the M, V and U′ antigens. These red cell antigens are controlled by genes at three different loci. Most o f the recipients lacked two or all of these antigens. All had passed puberty at the onset of the experiment.

2. The antibody response against the three antigens differed markedly. Anti-U′ appeared on the average 11 days after the 1st injection and the maximum titre values were high. The V antibodies were produced at a slow rate, while anti-M was intermediate.

3. There was some influence of age on the rate and intensity of antibody production. Older animals gave a faster response and, with regard to anti-M, also higher titres.

4. Twins belonging to the same MZ pair usually produced v e r y similar antibody curves. However, there was at least one noticeable exception, which is given special consideration in the Discussion. The differences between MZ pairs accounted for a considerable portion of the variance in the different measures of antibody response and in most cases this portion remained significant also after the elimination of the influence o f age and sex. The DZ pairs were more variable.

5. The simultaneous response o f two or three antibodies in the same animal was studied. There was a significant positive association between antibody types which suggests the existence o f individual (probably genetic) differences in the general antibody-producing ability.

The effect of fertility differences between homogamous and heterogamous matings are considered. The equilibria are evaluated and their stability deterimined. Completely positive and completely negative assortative mating with two and with three forms, are investigated as special cases of the above fertility structure. In these cases convergence of the genotype frequencies has beer demonstrated.

Mutations in at least five different genetic loci cause some degree of resistance to Chloramphenicol, Tetracycline or both antibiotics in Escherichia coli K12. Two have been identified as chromosomal genes: cmlA near λ prophage and cmlB near pyrD. A third, giving mucoid antibiotic-resistant mutants, is probably capR (Ion) near lac. Another group of mucoid mutants gives marked increases in resistance to Tetracycline and in sensitivity to Puromycin; these mutants do not occur at capR. Although all the mutations have rather small effects on resistance level, the patterns of resistance to the three antibiotics appear to be characteristically different for mutations at different loci.

White-spored mutants of Ascobolus immersus were used to study postmeiotic segregation within a gene. It was found that in two-point crosses between mutants showing postmeiotic segregation, polarization in recombinant asci resulting from this type of segregation resembled that in normal (even-type) conversion. However, the ratio of recombinant asci with even segregation (6:2 asci) to those with odd segregation (7:1 asci) was much higher than might have been expected on the basis of the frequencies of various recombinant asci found in crosses between the same mutants and the wild-type strain. It seems that none of the current models of recombination gives a satisfactory explanation of the results obtained.

Electrophoretic studies on starch gel demonstrated the existence in the European eel of four haemoglobins, not two as believed from agar gel studies. The two minor fractions have been shown to be neither polymers nor methaemoglobin derivatives of the two principal fractions.

The esterase zymograms of eel sera exhibit up to eight fractions. The individual differences observed suggest genetic polymorphism and the same applies to the esterase pattern of various tissues.

This approach is of value for the collection of phenotypic frequency data with a view to solving the Atlantic eel problem.

A simple technique for the selection of auxotrophs of Penicilhium chrysogenum after mutagenic treatment has been devised using nystatin. It relies on the differential sensitivity of germinated and dormant conidia to nystatin and on the gradual loss of antibiotic activity which occurs on complex media at laboratory incubation temperatures. After mutagenic treatment, conidia were plated on MM agar to allow germination of prototrophs and then overlaid with CM agar containing enough nystatin to kill germinated conidia but not ungerminated auxotrophic conidia. When nystatin activity had decreased sufficiently the latter germinated and grew and thus were enriched relative to prototrophs.