I recently had this insight.
If a grandfather can be (reasonably) sure that his son is his, and his son's son is his son's, then a prior mutation on his Y-chromosome that encourages favoring that grandson will be selected for since it is (almost) definitely in his grandson's Y-chromosome.
This does not work for the X-chromosome. There is no reason for a grandfather to discriminate between the children of his daighters.
Might this be a factor in patriachal lineages such as kings?