Controlling elements in the mouse: IV. Evidence of non-random<i>X</i>-inactivation

Abstract

SUMMARY The non-random X chromosome expression that has been observed with coat markers in female mice heterozygous for the Xce alleles, Xce a and Xce b , has now been investigated with the electrophoretic enzyme marker, Pgk-1 . Because the Xce status of the Pgk-1 a marked chromosome was not known, PGK expression was assessed in Pgk-1 a / Pgk-l b heterozygotes which carried either Xce a or Xce b on their Pgk-1 b chromosome. The PGK-1A allozyme was found to predominate in both genotypes but when Xce b was present on the Pgk-l b 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 Xce c , was present on the Pgr-1 a -marked X chromosome.