Cytologic evidence for preferential inactivation of the paternally derived X chromosome in XX mouse blastocysts.

A total of 941 mouse blastocysts obtained from two types of crosses in which one of parents carried Cattanach's X/autosome translocation was studied cytogenetically by quinacrine mustard fluorescence. The rearranged X (Xt) and the normal X (Xh) were distinguished by size. Karyotype analysis was successful in 721 embryos, of which 205 were heterozygous for Cattanach's translocation. A single heterochromatic and brightly fluorescent X chromosome was identified in 154 metaphase spreads from 89 blastocysts consisting of 32--96 cells. The paternally derived X chromosome (Xp) was heterochromatic in 87% and 88% of the informative cells from the crosses XnXn x XtY and XtXn x XnY, respectively. This preferential choice of Xp at the blastocyst stage might have an important bearing upon the preponderance of cells with an inactive Xp in the chorion and yolk-sac splanchnopleure.