A large number of mutants that affect the morphology of the ascus of Neurospora crassa has been isolated by means of a screening technique that utilizes the filtration principle to separate colonial type mutants from wild type. In relationships with each other and with wild-type alleles, the mutant genes exhibit zygote dominance and recessiveness. A zygote complementation test for placing the recessive mutants into functional groupings is described. Results obtained by use of the test place the mutants into seven functional groups. Dominance relations in the developing ascus of Neurospora are discussed.

Thirteen selected distal markers have been mapped by time of entry studies and linkage analysis. They appear to fall on two distinct linkage groups and with the ten previously described there are nineteen markers on one group and four on the other. There is as yet no clear evidence that the two groups are linked, but this possibility has not been excluded.

Of the thirteen markers described, six can be mapped by measuring their time of entry into suitable recipients. It should now be possible to establish the order of any further marker and to provide more accurate information for any marker transferred within 35 min from the time of mixing the cells.

A comparison of the gene-environment interactions of the eyg mutant in two different genetic backgrounds clearly demonstrates that the properties of the genetic background play a major role in the control of the gene-environment interactions of this mutant. Similarly, modifier background is important in the determination of the sensitive stages in eye development to a cholesterol-deficient diet.

The phenotypic identity of the eyeless and eye-gone mutants suggests a close underlying metabolic and developmental relationship. Possible inter-relations of these two mutant genes are discussed in the light of their gene-environment interactions in a standardized genotype.

The sites of mutations affecting host-controlled modification (HCM) have been mapped in E. coli K and E. coli B by conjugation and transduction experiments between mutants. These mutations all map close to the serB locus on the opposite side to the marker thr. Non-parental HCM has been observed among colonies obtained from Pl transduction experiments between HCM mutants. Control experiments have shown that these non-parental recombinants can not be accounted for by reversion of either parent and must result from recombination between mutants. Several genetic models are suggested which could account for these recombinants and an attempt is made to distinguish between various models by testing for complementation between mutants in a zygotic induction complementation test.

The karyotype of Spalax leucodon (2n = 56) from Moldavia (Rumania) shows 27 pairs of autosomes: two metacentrics, three submetacentrics, eight suhtelocentrics and 14 acrocentrics. The X chromosomes are the longest subtelocentrics and the Y chromosome is a submetacentric of medium size. In metaphase I we have found a partial homology between the short arm of the X and the Y chromosome.

The population of S. leucodon from Moldavia shows considerable differences of karyotype as compared with other populations of the same species from the south of Europe. This species presents a high degree of chromosomal polymorphism in different populations.

Colicin factors Ib-P9 and Ela-16 both convert their hosts to chromosomal donors. However, negative fluctuation tests suggest that chromosomal transfer does not result from the formation of stable Hfr variants.

Attached-X stocks appear to be deficient in a small region, probably heterochromatic, which is normally covered by an homologous region on the short arm of the Y chromosome. There is an homologous deficiency in the ec dx stock Y chromosome. When this Ys deficiency occurs in crosses involving attached-X stocks, the matroclinous females are not recovered.

The basic recurrence equations given in our previous paper have to be modified when males and females are differentiated. The modifications we gave are incorrect because we ignored the deviations from Hardy–Weinberg equilibrium when sampling and selection are applied separately to the two sexes. Appropriate corrections are presented in this paper.

The extent and phenotype of acriflavin-resistant rII deletions have been examined. The properties of these deletions confirm that acriflavin resistance may result from a loss of function at the ac locus and that the ac locus coincides with the rII distal portion of the dispensable region which is adjacent to the rIIB cistron.

Phenethyl alcohol is shown to induce cytoplasmic respiratory deficiency (petite mutation) in various strains of Saccharomyces cerevisiae. Apart from petite induction, phenethyl alcohol also affects cell growth both in respiratile and non-respirable media. This sensitivity to growth inhibition appears to be complex, and does not correlate with the degree of induction of respiratory deficiency.

I + × I + crosses were studied, using various Coll plasmids. Exclusion due to presence of Coll in recipients was removed by starvation, with production of I – phenocopies. Superinfection immunity prevented normal replication of transferred genes: thus, Itrp + donated to an I + trp– recipient led to formation of abortive recombinants.