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.

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.

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. 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.

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.

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.

In mice heterozygous for translocation T(2;?)163H, and also for linkage group II markers, the cw locus shows close linkage with the point of rearrangement (about 1−2% recombination). Since T 163 was apparently formed by fusion of two chromosomes near their centromeres, this means that the centromere must lie at the cw end of linkage group II. In homozygotes for T(2; 9)138Ca the genes T and d show significant linkage, indicating that their loci are on opposite sides of the trans-location break. Since, from previous data, the break is known to be proximal to d, it must then be distal to T and, again from previous data, the order of loci hi linkage group IX must be centromere-T-tf-T 138 break.

Two triplications induced in a normal halpoid chromosome complement by TEM (Tr-I) and by mustard gas (Tr-II) are described. Two models which may lead by successive steps to the formation of a triplication are suggested. The induction of two latent breaks is essential in both of them. The origin of repeats in general is discussed. It is concluded that chromosome breaks rather than non-homologous crossing-over constitute the major source of repeats.

Two new wild strains of T. pyriformis, syngen 1, have been examined for new matingtype, serotype and isoenzyme alleles. No new alleles at the mating type (mt) locus were found, but the two alleles previously found were extracted. No new alleles were found at the other five loci examined, but two alleles were obtained for each locus. These observations indicate that a limited polymorphism exists for these loci in natural populations of T. pyriformis, syngen 1.

The extent of certain rII deletions in phage T4 has been mapped by genetic means beyond the terminus of the B cistron. They extend to varying degrees, nearly to the acs locus. The phenotypes of these mutants indicate that there are two or three cistrons between rII B and acs, and that the functions coded for by these cistrons are not essential for growth in any known E. coli K12 strain or E. coli lysogen.