The third chromosome mutant, hairy1, adds a varying number of microchaetae to the scutellum. The genetic relationship between this character and scutellar bristles was investigated using the conventional techniques of full- and half-sib analysis of variance and covariance, direct and correlated response to selection for microchaetae and scutellars respectively and diallel crosses at two stages in the programme. There was a good correspondence between the predicted and realized divergence resulting from selection for increased and decreased microchaetae. The correlated response in scutellar bristles appears to be accounted for primarily by genetic changes in microchaetae.

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.

Analysis of F1BC1 and BC2 progeny from matings of inbred DBA/2 and C57BL mice confirm the existence of a single major gene for C 57 BL-resistance to Friend virus. The doses of virus used did not obscure expression of this Mendelian unit.

Matings of Drosophila pseudoobscura were observed in Elens-Wattiaux chambers, using individuals of three karyotypes: AR/AR, AR/CH and CH/CH. In each chamber two karyotypes were represented, with frequencies 10:10, 2:18 or 18:2. The males of the heterokaryotype AR/CH tend to have an advantage in mating compared to the homokaryotypes, provided that both kinds of males are equally frequent, i.e. the ratio 10:10. This advantage is further increased when the heterokaryotype is a minority, the ratio 2:18. When the homokaryotype, AR/AR or CH/CH, is a minority (18:2) it is equally or more successful than the heterokaryotype in securing mates. Among females, the mating success is independent of frequency, or the minority females have sometimes only a slight advantage.

Ca and P metabolism in the grey-lethal mutant mouse has been investigated. This animal is hypocalcaemic and hypophosphataemic and the serum alkaline phosphatase is elevated far above the normal. Increased bone ash content has been attributed to an elevated bone Ca. From in vitro experiments involving steady-state distributions of calcium and phosphate it is concluded that active bone metabolism is reduced in mutant bone. This reduced ratio may be attributed to an increased passive ion solubility.

These results tend to the assumption of an hormonal imbalance in this mutant system. An increased production of thyrocalcitonin has been postulated to account for these findings.

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.

Genetic mapping of suppressors of the ilv-188 mutation is described. Suppressors of this mutation have been mapped at four ochre suppressor loci: sup L, sup M, sup N and sup O. The ochre suppressor described by Brenner & Beckwith is shown not to suppress ilv-188 whereas the suppressor described by the same authors suppresses ilv-188 and may represent the same locus as supO suppressors. The mapping by P1 transduction of the sup L, sup M and sup N loci is described.

Freckled (Frd) is a dominant mutant of D. melanogaster, lethal in double dose, which manifests itself by melanization in the adult stage, while nothing can be noticed in the larva and/or in the pupa. Its localization has been determined at nearly 102 of the second chromosome.

Among the offspring from backcrosses involving Frd, some individuals that do not carry the Frd gene exhibit a weak Frd manifestation in the adult stage, or a stronger one, mostly lethal, in the larval or pupal stage. Weak Frd, indicated as [Frd], is transmitted indefinitely without any recognizable linkage with the chromosomes, although its frequency is influenced by the 1st and, probably, by the 3rd chromosome. No matroclinous transmission has been found. It is concluded that [Frd] is probably controlled by an extrachromosomal entity, not located in the cytoplasm. The possible relationships between Frd and [Frd] are discussed.

Observations of mutation and phage-mediated transduction of rhamnose-fermentation properties were made on two groups of wild-type rhamnose-non-fermenting (rha−) strains of Salmonella typhimurium isolated in several different countries. The ‘FIRN’ group included twenty-three strains in the biotypes 15, 16, 17 and 18, and the ‘non-FIRN rha−’ group included fourteen strains in a new biotype, provisionally designated 19 X d. The two groups showed different patterns of mutational behaviour and the pattern was uniform for the different strains in each group. Non-FIRN rha− strains gave rise directly to colonies of rha+ mutant bacteria in platings on rhamnose minimal media. FIRN strains were unable to do this, but produced rha+ bacteria by two successive mutations, the first of which yielded weakly fermenting (rhaw) bacteria capable of fermenting rhamnose in peptone water, but not of utilizing it for growth on minimal medium.

Rha+ recombinant bacteria were produced in transductional crosses between any one FIRN strain and any one non-FIRN rha− strain, but not in any cross between one FIRN strain and another FIRN strain or between one non-FIRN rha− strain and another non-FIRN rha− strain. These findings suggest that although the sites of the rha− mutations are different between the FIRN and non-FIRN rha− groups, they are located at identical sites on the chromosome in all strains in either group. It is concluded that all FIRN strains have probably descended from a single ancestral strain which underwent mutations at two sites in the rhamnose region of the chromosome and that all non-FIRN rha− strains have descended from a single ancestral bacterium which underwent a mutation at a third site in the rhamnose region.

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.

A formal genetic analysis of the heterothallic, colonial green alga Eudorina elegans has been described. Wild-type strains were found to be sensitive to different drugs when grown on minimal agar containing very low concentrations of these drugs. Mutant strains resistant to high concentrations of drugs have been isolated. These are msr-500 (resistant to 500 μg/ml of DL-methionine-DL-sulfoximine), ery-r-100 (resistant to 100 μg/ml of erythromycin) and sr-100 (resistant to 100 μg/ml of streptomycin). The wild-type phenotypes sensitive to these drugs have been designated as mss-500, ery-s-100 and ss-100 respectively. The sr-100 also showed cross-resistance to other antibiotics belonging to the streptomycin group.

On genetic analysis, the msr-500 and ery-s-100 were found to be inherited in a Mendelian way. These alleles are not linked to each other or to the mating type locus. The inheritance of mating type was found to be due to a single gene difference.

The inheritance of ss-100/sr-100 was found to be non-chromosomal and was characteristically uniparental, always transmitted through the female parent. The evidence for the non-chromosomal gene (NC genes) controlling sr-100/ss-100 phenotypes in this organism has been derived from the exceptional zygotes in which the male parent apparently transmits streptomycin resistance to the progeny. Although ultraviolet or gamma-radiation resulted in normal survival curves of the exposed cells, no mutant deficient in any nutritional requirement was isolated.

1. Approximately 25000 mice have been produced over twelve generations of selection in nine lines. The experimental design involved a 3 × 3 factorial arrangement of direction of selection with mating system. The primary character measured was 6-week body weight.

2. Consistently high phenotypic correlations between mates have been achieved, positive in the assortative lines, negative in the disassortative lines. Correlations were low and inconsistent in direction in the random-bred lines. These correlations have had very little, if any, effect in redistributing the genetic variance as estimated from the variance component analysis; the expected higher variances in the assortative lines and expected lower variances in the disassortative lines have not appeared, thus leaving heritability unaffected.

3. Selection differentials likewise show no consistent advantage for the assortative lines, so that the progress from selection has been virtually identical in all three mating systems in each direction.

4. Assortment of mates, either positively or negatively, for characters of even moderate heritability appears to have little influence on the outcome of selection. On the other hand, selection has been singularly effective in modifying the mean 6-week weight, with progress markedly greater in the downward direction; indeed it appears that the lower limit, may already have been approximately attained.

5. Environmental effects operating in the various generations have affected all lines in remarkably consistent fashion.

Erythromycin-resistant mutants were separated into two categories showing Mendelian and non-Mendelian inheritance of resistance respectively. The cytoplasmic factor was located in the mitochondrion based on the fact that the loss of the mitochondrial genetic unit associated with the petite mutation (the ρ factor, assumed to be the mitochondrial DNA) resulted in a corresponding permanent loss of resistance. Conversely, the petite mutation in gene resistant mutants had no effect on the inheritance of resistance.

Analysis of vegetative and sexual progeny of isolated zygotes from a number of crosses between resistant and sensitive strains indicated that different mitochondrial types, both sensitive and resistant, co-exist in the yeast cell and are distributed at random among daughter cells under non-selective conditions. Only resistant mitochondria are in evidence if zygotes are put down to proliferate on medium containing the antibiotic.

Resistant strains were tested for retention of resistance following anaerobic culture (known to cause disruption of mitochondria). The cytoplasmic mutants retained their resistance but two gene-resistant mutants tested both lost their resistance. It was suggested in the latter case that resistance is dependent on the existence of intact mitochondrial membrane (possibly inner membrane). In the former mutants the mechanism of resistance may be the alteration of one or other of the components of the protein-synthesizing system of the organelle.

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.

Twenty-four lines were bred from a base population of outbred Q mice by continued full-sib mating. Inbreeding did not cause depression in mean birth weight. However, convincing evidence of inbreeding depression in foetal weight at 17½ days' gestation was found; this depression in the rate of prenatal growth was attributable both to inbreeding in the litter and the mother. It was also inferred from the results of crossing the partially inbred lines that inbreeding in the litter caused depression of placental weight. It is concluded that the genes controlling the rate of prenatal growth exhibit directional dominance but that the resulting inbreeding depression in this trait does not lead to a lower birth weight, possibly because of the compensating effect of a longer period of gestation.

Esterase activity was detected in starch-gel electropherograms of plasma samples from two species of teleost: flounder and plaice. In both species polymorphism was found. In flounder its genetic origin was indicated by population genetical data. Differences in staining intensity of the individual bands were found to be correlated with length of the animals.

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.

The genes responsible for u.v. sensitivity in ten sensitive mutants of E. coli strain B (Bs strains of R. Hill) have been mapped by transduction. The uvr genes of all the mutants able to reactivate u.v.-irradiated phage (HCR+), including Bs2, 4, 5, 6, 7, 9 and 10, were linked, 50–95%, with malB. The gene of Bs12 (HCR) was also linked to malB as is uvr A. The gene of Bs8 (HCR) was linked to gal, that of Bs3 (HCR) to his. Transduction of mal+ from a strain with uvr A, or a mal+ derivative of Bs12, to Bs2, 4, 5, 6, 7, 9 or 10, yielded about 30% u.v.-resistant transductants. A mal+ transduction with Bs2 mal+ as donor and other Bs strains as recipients yielded < 0·1% u.v.-resistant transductants. The malB-linked uvr genes of all mutants (except Bs3 and Bs8) were transducible with metA. The quasi-reciprocal crosses and three-point tests suggested the order of markers as metA malB uvr (HCR+) uvr (HCR). The sensitivity gene of Bs9 was exceptional in that it appeared to lie between metA and malB. The sensitivity gene of Bs11 could not be mapped because transductants to that strain were not possible, nor did it act as a recipient in sexual recombinations.