Controlling elements in the mouse: IV. Evidence of non-randomX-inactivation

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Abstract
SUMMARY: The non-randomXchromosome expression that has been observed with coat markers in female mice heterozygous for theXcealleles,XceaandXceb, has now been investigated with the electrophoretic enzyme marker,Pgk-1. Because theXcestatus of thePgk-1amarked chromosome was not known, PGK expression was assessed inPgk-1a/Pgk-lbheterozygotes which carried eitherXceaorXcebon theirPgk-1bchromosome. The PGK-1A allozyme was found to predominate in both genotypes but whenXcebwas present on thePgk-lbchromosome 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-randomXexpression derives from a primary non-randomness of theXinactivation process and that a new and more extremeXceallele, designatedXcec, was present on thePgr-1a-markedXchromosome.