1. A number of mutants affecting changes in the pigment of larval and adult An. gambiae are reported.

2. Two other pigment characters are also described. One of these, diamond, is probably inherited as a single factor.

3. Two of the mutants are sex-linked and the experimental results show An. gambiae to have a Drosophila form of sex-linkage.

4. The sex-linked white-eye mutant is shown to be epistatic to the gene for collar and the characters diamond and red stripe.

5. Attention is drawn to the difference in sex-linkage between An. gambiae, and C. pipiens and Ae. aegypti which may indicate different methods of sex-determination.

Inheritance of DDT resistance has been studied in crosses between the highly resistant ‘T’ strain of A. aegypti (constituted by inbreeding from the TRINIDAD DDT-resistant stock) and the ‘64’ susceptible strain.

Larval DDT resistance derives from a major gene RDDT1 on linkage group II, the order being RDDT1–s–y. Linkage group III may also contribute to larval resistance. Linkage group I makes no contribution.

Adult DDT resistance derives from a major gene RDDT2, 18·2 ± 2·1 units from the market blt on linkage group III. Linkage group II has no influence on adult resistance.

Selection with DDT to retain only RDDT1/+ segregants in larvae of backcrosses RDDT1/+×+/+ did not increase resistance in resulting adults, confirming the difference in genetic mechanism at the two stages.

The F1 progenies from reciprocal crosses between ‘T’ and ‘64’ differed slightly but significantly in larval resistance, modifying the influence of the major gene RDDT1 in the heterozygote.

The early developmental stages of the RDDT1/+ phenotype (up to the fourth larval stage) were more viable than the +/+ phenotype in backcross segregation. The difference in mortality probably exceeded 30%.

Contrary to the conclusion of earlier workers, penetrance of a very rare genetic defect, congenital tremor in White Leghorn chicks, was found to be complete. By appropriate correction of the original data for small family size it is shown that the defect is determined not by one but two pairs of autosomal recessive genes. In the terminology of human genetics, ascertainment of affected individuals corresponded to conditions of ‘truncate’ selection in the case of maternal sibships and more nearly to ‘single’ selection in the case of paternal sibships.

Body and liver weight and the number of nuclei in the liver were significantly higher in F1 hybrid than in inbred embryos developing in the same uterus after artificial heterospermic insemination. The size of 13-day-old F1 hybrids corresponds to the size of 13·4-day in bred embryos. No significant differences in cell size were found between F1 hybrid and inbred embryos of the same age. It is concluded that in 13–15-day embryos heterosis takes place through increase in cell number and not cell size.

The effects of acriflavine on the transfer of various episomes and of the bacterial chromosome were studied using acriflavine-sensitive and -resistant strains of E. coli K-12 as donors and recipients. It was found that the inhibition of transfer of these elements by acriflavine was only slight when donor bacteria were resistant to this acridine dye, regardless of whether the recipient bacteria were acriflavine-sensitive or -resistant. This result seems to support the view that acriflavine inhibits the replication of these elements in donor bacteria which is considered essential for their transfer. With colicinogenic factors, in contrast to F and R, the results were affected by the sensitivity or resistance of the recipient and were in support of the killing by acriflavine of the cells which received the colicinogenic factors.

Interspecific crosses between, different genotypes and species of Nicotiana have revealed S gene polymorphism on a large scale. In the usually yellow-flowered self-compatible species N. glauca a strain occurs that has a self-incompatibility gene which is tightly linked with the bright-red corolla colour gene. In self-incompatible species, there are two kinds of self-incompatibility alleles, SI and SFI, distinguished on the basis of the acceptability of N. langsdorffii pollen. The SI form has a large number of alleles whereas the SFI form has only two, SF10 and SF11. The SI alleles are again divisible into two groups on the basis of the acceptability of N. noctiflora pollen and, on the same criterion, the two SFI alleles, SF10 and SF11, also are distinguishable. N. noctiflora and N. bonariensis plants could be divided into two and three groups respectively on the basis of interspecific compatibility relationships. When plants of each species were crossed among themselves, they formed two and three intra-incompatible and inter-compatible groups respectively, identical with those found on the basis of interspecific compatibility relationships. The significance of this observation is at present not fully understood.

1. Five additional phenylalanine (PHE)-requiring mutants have been isolated but they do not add to the number of loci already known which have been designated phenA (phen2 and its alleles) and phenB (phen6).

2. Two pathways for tyrosine (TYR) synthesis in A. nidulans have been proposed: the well-known one by the transamination of p-hydroxyphenylpyruvic acid and an alternative one, as in animals, by the hydroxylation of PHE.

3. Ten allelic partial TYR-requiring mutants (tyrA), presumably blocked in the transamination pathway, have been isolated after N-methyl-N′-nitro-N-nitrosoguanidine (NTG) treatment of bil;phenA3 conidia.

4. Four partial TYR requirers (at another locus—tyrB) have been isolated after NTG treatment of tyr A7, bi1 conidia. They are presumably blocked in an alternative pathway for TYR synthesis, i.e. in the PHE-hydroxylation pathway.

5. tyrA mutants have been found to be p-fluorophenylalanine (FPA)-resistant and allelic to mutants at the fpA locus. tyrB mutants have been found to be very leaky and FPA-sensitive. tyrA;tyrB double mutants have been found to be exacting TYR requirers.

6. Mutants at loci fpA (tyrA) and fpE (anthranilic acid-requiring) have been interpreted to be p-fluorophenylalanine-resistant due to an oversynthesis of PHE.

Following the transfer to the common pigeon (Columba livia) of a cellular antigen (Cg) from C. guinea, and to Streptopelia risoria of respective members of the group-8 system from chinensis and senegalensis, new combinations of these two antigenic systems were produced from matings of livia and risoria. The hybrid substances associated with the cells of each of the five new combinations of the cellular antigens were related to each other and to the usual F1-livia/risoria cells but, with the possible exception of two, were definitely differentiated from each other. Cross-reactivities of the reagents for the hybrid substances were also observed, primarily with heterozygotes of the group-8 system of Streptopelia. Evidence is thus provided that the interaction takes place primarily if not entirely between the genes of the C-system of Columba and the group-8 system of Streptopelia.

Results of crosses of a large number of Neurosporas of different origin, including several distinct strains of N. sitophila, were utilized to re-examine the question of whether certain wild-type Neurosporas other than N. crassa show biases in the two types of second-division segregation. Segregations for alleles of the mating type and peak loci on a wide variety of genetic backgrounds gave little evidence for excess either of symmetrical or asymmetrical ‘post-reduction’ asci.

1. Chromosome 6B of T. aestivum, to which a small segment of an Ae. umbellulata chromosome had been translocated, was examined both by interference microscopy and in stained preparations.

2. In the stained preparation a constriction was observed in the long arm of chromosome 6B and also a terminal knob that was also observed on chromosome A of Ae. umbellulata.

3. These features were also observed by interference microscopy and in addition the portion of the chromosome 6B of Transfer distal to the constriction had different phase-retarding properties.

4. It is concluded that the part of chromosome 6B distal to the constriction is the part of Ae. umbellulata chromosome A which carries the gene for the leaf-rust resistance of that species and of the wheat variety into which it was translocated.

Six experimental populations of D. pseudoobscura were maintained at three different temperatures for more than 8 years. All the populations have originally descended from the same founders. The populations were examined for evidences of genetic divergence. The pre-adult viabilities of the flies from these populations, and of their F1 and F2 hybrids, were studied at 16°C., 25°C., and 27°C. The results indicate that the populations have diverged genetically from each other. Some divergence is observed not only between populations kept at different temperatures, but also between the replicate populations kept at the same temperature.

Non-directional asymmetry of paired organs is attributed to developmental ‘noise’. The level of asymmetry is inversely correlated to the degree of developmental stability. Children affected with familial cleft lip ± cleft palate have an increased asymmetry of their dermatoglyphics and molar teeth. The action of polygenes with a quasi-continuous distribution is consistent with this observation.

A new mutation, Em, is described for T. pyriformis, syngen 1. This dominant mutation eliminates the normal immature period in heterozygotes but is lethal in the homozygous state.

This paper is dedicated to Dr. Ralph Cleland in honor of his 75th birthday.

The sex factor of colE1a appears related to Ib, the sex factor of colIb, by each of three criteria: mutual inhibition of epidemic spread, antigenic structure of the sex pilus and susceptibility to I phage. The failure of each factor to spread in cultures carrying the other implies that donor ability is subject to a cytoplasmic repressor. Unlike two colI factors, colE1a and colIa (or Ib) can co-exist to give a doubly colicinogenic strain.