Complex sentence: It is believed by some Griersons that they are descended from the Clan MacGregor, however, the claim does not command any support from within Scotland and DNA studies have found no trace of MacGregor YDNA in any Grierson descendants tested.
Split into simpler sentences: It is believed by some Griersons that they are descended from the Clan MacGregor. However, there is no trace of MacGregor YDNA in any Grierson descendants tested.