Grandfather's grandfather's grandmother

It occurred to me, that there should be dozens of these in the family tree, but of course adding them up there's just eight. I have three, partly as there is a cousin marriage which consumes the fourth; and then as on the paternal side we can't quite go back that far. Two of the three are Welsh, and two are called Margaret: Rebecca Phillips b. 1780, Margaret Evans b. 1792 (a twin), Margaret Trewhella b. 1784.

Paternal grandmothers' paternal grandmothers are interesting if you look at x-dna. This gets passed on - intact? via son to paternal granddaughter. I need to do some more reading around this.

The one thing I won't comment on too much is y-dna as this doesn't seem very interesting to me at all... Apologies!