Seven different genes, me-1 to me-7, controlling the steps in the synthesis of methionine in the Basidiomycete Coprinus lagopus have been tested for the production of prototrophs on minimal medium. Cultures carrying me-1 and me-7 produced prototrophs spontaneously at a rate of 2·6 × 10−5. These prototrophs were the result of mutations of suppressor genes and not due to back-mutation of the me-gene.

An intensive study of sixty-four mutants of an independent origin suppressing me-1 has revealed five different suppressor loci.

Tests for complementation between suppressor mutants and for their recessiveness to the wild allele were made in dikaryons.

All suppressor mutants were recessive to the wild allele. The five suppressor loci were all separated from one another by recombination, twenty-eight map units being the smallest distance between any two pairs. Mutants of the same locus did not complement one another, with few exceptions. Mutants of different loci, as tested in the trans-position in a dikaryon, complemented one another with the exception of pairs between sup-3, sup-4, and sup-5. Sup-3 and sup-4 are 28 units apart and are independent of sup-5 and yet they did not complement. This unique example of long-distance non-complementation is discussed in terms of gene action.

Welshons & Russell (1959) have presented data to show that the XO chromosomal constitution in the mouse is female. This conclusion was based on results of genetical tests with sex-linked markers and on chromosome counts. All XO females were matroclinous, that is, they had inherited their X-chromosome from their mother. Females of this type will arise when non-disjunction occurs in the meiotic divisions of the father and results in spermatozoa without a sex-chromosome. Alternatively, the paternal sex-chromosome may be lost from the fertilized ovum if non-disjunction of sister-chromatids occurs during the first cleavage division. This latter explanation has been urged by Ohno, Kaplan & Kinosita (1959), who found no evidence for non-disjunction of the X-and Y-chromosome in an extensive cytolosical examination of the mouse testis.

Two apparently very similar quantitative characters, the numbers of hairs on the sternopleural region and on the abdominal sternites of Drosophila melanogaster, show unexpected differences in their genetic behaviour. In particular, the amount of left-right asymmetry of the sternopleurals (i.e. the mean absolute difference in numbers of hairs on the two sides of the fly) tends to decline when inbred lines are intercrossed, and can be both increased and decreased by straightforward selection; the corresponding index for the sternite hairs—the uncorrelated variance between two sternites, or the mean absolute difference between the numbers of hairs on each—appears, on the other hand, to be susceptible neither to selection nor to change when inbred lines are crossed (Mather, 1953; Reeve & Robertson, 1954; Reeve, 1959).