A newly described neurologic disorder is found to affect multiple family members in three generations that were available for study. In the first generation, two sisters and one brother were affected. In the second generation, all of the children of the first-generation sisters were affected, but none of the descendants of the first-generation son. In the third generation, all of the children of the affected second-generation women were affected, but none of the descendants of the second generation men. The mode of inheritance exemplified here is