As previously said, My knowledge is from watching quality TV programmes, and having a interest, in getting my DNA tested one day.
From DNA testing, it is possible to identify if you have Celtic, Anglo/Saxon and Norwegian Viking Origins on your Maternal or Paternal side, but that is only two lines, there are so many ancestors inbetween, that the test can't pick up, so it is only part of a persons make up, and not the full story of who you are.
I think there was a school Teacher from Somerset who was related on the Maternal side to a 2,000 year old skeleton near where he lived, so maybe the apples do not fall to far from the tree.
I think I recall Danish (The jutes) being indistinguishable from Angle and Saxon DNA, since they where from Northern Germany and Norman DNA couldn't readily be identified I think.
Kliffy