this is thought to be due to the paternal x chromosome being 'imprinted ' , that is , somehow marked as different , during either male gametogenesis or fertilization , causing primary nonrandom x-inactivation in tissues that differentiate early , such as trophectoderm and primitive endoderm , from which yolk-sac endoderm is derived .
C0043292
x chromosome
C0043297
x-inactivation