X-linked modification of the heterozygous phenotypes of X-linked genes has been detected in the X chromosomes of several inbred strains of mice. The effect is similar to that of the alternative ‘states’ or alleles, of the X chromosome controlling element, Xce, identified in T(1; X)Ct X chromosomes. Tests on two such differing X chromosomes have indicated that the phenotypic modification results either from non-random inactivation of the two X chromosomes or from selection operating on the two cell populations differentiated by X-inactivation. The data provide evidence of non-random X chromosome activity in the somatic cells of the female mouse.

The coats of Ta–Ta+ and c–c+ chimaeric mice have been compared with those of Ta heterozygotes and animals heterozygous for an X-autosome translocation which causes c-variegation. With one exception all those features typical of the heterozygotes were found in their chimaeric counterparts, thus showing that the creation of two cell populations by X-inactivation can ultimately lead to the observed heterozygous phenotypes. The one exceptional feature not found in c-variegated chimaeric mice was the progessive increase in the proportion of pigmented hairs in c areas with age which occurs in X-autosome translocation heterozygotes.

The coat of a Modp / + – Ta chimaeric animal was also investigated. Ta hairs showing only the colour effect of Modp were numerous, suggesting that Modp and Ta affect different cell populations. However, Ta hairs showing the structural effect of Modp were very rare. The two observations suggest Modp can operate independently upon two different cell types – melanocytes and hair follicle cells.

The frequency and nature of the changes in ‘state’ of the mouse X-chromosome controlling element (inactivation centre) have been investigated on an inbred background. The results indicate with near-certainty that meiotic crossing over is the responsible mechanism and that the frequency of recombination between the T(1; X)Ct breakpoint and the locus of the controlling element is approximately 3%. Maize-type ‘changes in state’ may occur under other experimental conditions. The data do not distinguish on which side of the autosomal insertion the element lies but when combined with observations of other investigators suggest that the location must be on the Mo-Ta side and very close to Ta.

Data are presented which support the conclusion that in the flecked translocation, T (1; X) Ct, there is a spread of inactivation into both sides of the autosomal region inserted into the X. This would indicate that both parts of the divided X are subject to the X-inactivation process. The data also demonstrate that the inactivation of autosomal genes lying near each end of the insertion are modified by the X-chromosome controlling element system, Xce. Since the element modifies the heterozygous expression of X-linked genes on one side of the insertion, it would therefore be expected that it similarly modifies the heterozygous phenotypes of those on the other side. The data thus support the concept that the controlling element is the master gene, receptor site or inactivation centre which regulates the X inactivation process.

Females heterozygous for the T(1; X)CtX-autosome translocation tend to have lower levels of c-variegation when their rearranged X is inherited from the father rather than from the mother. The difference is not due to a maternal effect. It is postulated that a paternal or parental-source effect, such as that found to modify position effect variegation in Drosophila, is operating but the possibility that a bias in the inactivation of the maternal and paternal X chromosomes is responsible cannot be ruled out.

The non-random X chromosome expression that has been observed with coat markers in female mice heterozygous for the Xce alleles, Xcea and Xceb, has now been investigated with the electrophoretic enzyme marker, Pgk-1. Because the Xce status of the Pgk-1a marked chromosome was not known, PGK expression was assessed in Pgk-1a/Pgk-lb heterozygotes which carried either Xcea or Xceb on their Pgk-1b chromosome. The PGK-1A allozyme was found to predominate in both genotypes but when Xceb was present on the Pgk-lb chromosome the expression of the two allo-zymes was less unequal. This effect was seen in both liver and kidney of adults and to at least the same degree in embryos aged 13·5 and 7·5 days. The results have been interpreted to mean that the non-random X expression derives from a primary non-randomness of the X inactivation process and that a new and more extreme Xce allele, designated Xcec, was present on the Pgr-1a-marked X chromosome.

A new mutant (Wct) has been identified at the W locus of the mouse. The homozygote is poorly viable. Whereas the heterozygote (Wct / +) is only mildly anaemic like Wυ / +, the double heterozygote Wct + / + Ph is considerably more anaemic than Wυ + / + Ph and it and Wsh + / + Ph have significantly raised leucocyte counts. Wct + / + Ph is also unduly radiosensitive to whole body X-irradiation, 50% dying from haematopoietic failure at a dose of 4·59 ± 0·14 Gy, whereas the median for Wct / + was 6·49 ± 0·28 Gy. Serial blood counts of mice after low- or sub-lethal doses of X-rays revealed significantly more profound depression of counts of both red cells and leucocytes in Wct +, and more notably in Wct + / + Ph, than in + / + or Wsh / + (haematologically normal) iso-dosed mice. We conclude that control of haematopoiesis by chromosome 5 is not confined to the W locus but is shared by the linked gene Ph (and perhaps Rw) and that expression of the change is not limited to the erythron but involves the pluripotent haematopoietic stem cell.

The mouse X-chromosome controlling elements, detected by their influence on the position effect variegation caused by the X-autosome translocation T (1; X) Ct, have been found to modify the heterozygous phenotypes of two X-linked genes. It is proposed that X-inactivation can be incomplete, the level of inactivation or the frequency of cells in which inactivation is incomplete being dependent upon the ‘state’ of the controlling element located in the X. The data suggest that this is a consequence of a reversal, or partial reversal, of inactivation of the X as a whole in some cells rather than a vairable spread of inactivation along the length of the X.

Previous studies have shown that the mouse X chromosomal locus, Xce, which causes non-random X chromosome inactivation, is closely linked to the Is(X; 7)Ct X-autosome translocation. This has placed it either near Ta on one side of the breakpoint or near jp on the other. Linkage tests with Movbr and Ta now demonstrate that the locus in fact lies close to Ta. The data also provide genetic evidence which establish that the C3H/HeH X chromosome carries the Xcea allele of this gene and the JU/FaCt and C57BL/GoH X chromosomes carry the Xceb allele, and further suggest that the X-linked modification of the heterozygous phenotypes of X-linked genes observed by various other investigators are all attributable to differences at the Xce locus. Evidence of a maternal influence upon Movbr phenotypes is also presented. This appears to operate independently of the X-inactivation process, probably through an effect of differing levels of copper in the milk in early life upon the mutant coat colour in the young.

X/Y male mice carrying the sex reversal factor, Sxr, on their Y chromosomes typically produce 4 classes of progeny (recombinant X/X Sxr ♂♂ and X/Y non-Sxr ♂♂, and non-recombinant X/X ♀♀ and X/Y Sxr ♂♂) in equal frequencies, these deriving from obligatory crossing over between the chromatids of the X and Y during meiosis. Here we show that X/Y males that, exceptionally, carry Sxr on their X chromosome, rather than their Y, produce fewer recombinants than expected. Cytological studies confirmed that X-Y univalence is frequent (58%) at diakinesis as in X/Y Sxr males, but among those cells with X-Y bivalents only 38% showed normal X-Y pseudo-autosomal pairing. The majority of such cells (62%) instead showed an illegitimate pairing between the short arms of the Y and the Sxr region located at the distal end of the X, and this can be understood in terms of the known homology between the testis-determining region of the Y short arm and that of the Sxr region. This pairing was sufficiently tenacious to suggest that crossing over took place between the 2 regions, and misalignment and unequal exchange were suggested by indications of bivalent asymmetry. Metaphase II cells deriving from meiosis I divisions in which the normal X-Y exchange had not occurred were also found. The cytological data are therefore consistent with the breeding results and suggest that normal pseudo-autosomal pairing and crossing over is not a prerequisite for functional germ cell formation. The data support the concept that Y short arm-Sxr pairing and crossing over may be the mechanism responsible for the occurrence of the Sxr variants reported in the literature.

In Cattanach's X-autosome translocation a piece of autosome of linkage group I has been inserted into the X-chromosome and a piece of X may have been reciprocally translocated to the autosome (Cattanach, 1961; Ohno & Cattanach, 1962). The present communication reports investigations to locate the autosomal insertion in the X-chromosome linkage map and provide evidence pertinent to the question of the possible reciprocal nature of the rearrangement; a brief summary of the results has already been reported (Cattanach & Isaacson, 1965).

Triethylenemelamine induces chromosomal aberrations in Drosophila and in mammals. In the present paper the effects of intraperitoneal injections of the drug on the fertility of male mice are related to its mutagenic activity. Males were given prolonged or short treatment with the drug and were mated to females at frequent intervals.

Male fertility was greatly reduced after either treatment, a result similar to previous findings in the rat. Sterility for the first few weeks was due to dominant lethals causing embryonic mortality, and to oligospermia from 30 days after the injection. Embryonic mortality was reduced after lower doses, being maximal between 11 and 14 days after the injection, and little or no oligospermia resulted from this treatment.

The drug probably acted directly on spermatogenic cells; spermatids being most sensitive to the induction of lethals and spermatogonia to destruction; these estimates were based on the known rate of spermatogenesis in the mouse. The sensitivity of the stages of spermatogenesis and the translocation frequency induced by the drug have some similarities to the effects of X-rays on the testis.