I was told it was the newest genetic evidence that proves the old theory of the Anglo-Saxons killing all of the native Britons..
There is no such thing.
You can't tell if somebody was "Germanic" or "Celtic" or "Romano-British" by looking at their DNA, much less by looking at their descendants' DNA a millennium and a half down the line.
The genetic marker testing that has been done to try to validate theories of migration into the British Isles has been carried out with fundamentally flawed assumptions, and would fail even the most basic tests of scientific rigor. There are no controls that have been sought from individuals not living in either Britain or the Netherlands/Germany/Denmark, so the whole thing amounts effectively to comparing the mitochondrial DNA of a person in Britain and a person in Lower Saxony and saying, "why, yes, these do look awfully similar". There is no merit in such studies.
There is even less merit in studies about skull measurements, which are better suited to nineteenth-century understandings of race than to twenty-first-century understandings of ethnicity.
There have been studies with more valid lines of inquiry that have been conducted in recent years. For instance, one study looked at the mineral content in the teeth of the skeletons found in period grave-sites, in order to determine locations at which the person may have drunk his water. They have yielded some interesting results - one being that, apparently, some of these "Anglo-Saxons" spent considerable time in Skye - but they are of dubious utility in attempting to determine ethnicity, much less political orientation.
Linguistic evidence is not nearly as solid as any of the others, partially because there are no real linguistic 'laws' to govern findings. The fact that the modern inhabitants of England speak a language that is part of the Germanic language family does not mean that they are all descendants of Germanic language-speaking people that moved to England, slaughtered all the native inhabitants, and took their place. Nor does it indicate any sort of 'interbreeding', such that every modern Englishperson must have had at least one ancestor who came from modern Germany. Many, if not most, of the inhabitants of the British lowlands must have elected to learn the English language because it was more advantageous for them to do so. There are some theories bouncing around as to why that happened, but none is particularly solid. Try these
two posts from Guy Halsall's blog, especially the comments, some of which were written by Actual Historical Linguistics Experts like Alex Woolf.