It turns out that Pete's Y chromosome belongs to the R1a haplogroup, which is actually slightly surprising. We've been trying to trace Pete's paternal line genealogically, and have succeeded in placing his paternal ancestors in Wales in the early-mid 18th century. Since almost everyone with British ancestry is in haplogroup R1b, I wasn't expecting anything else.
R1a distribution in Europe:
Today the R1a1 haplogroup can be found in high frequencies among groups living in the Ukraine, Russia, the Czech Republic and Poland. Haplogroup R1a1 is also found at rates of 50% in Ashkenazi Jewish populations, who ultimately settled in the Rhineland, now Germany, and have a deep and detailed contemporary history.Other places where many men have the R1a Y chromosome: Northern India, Afghanistan, and Tajikistan.
According to the wisdom of the internet and at least one named source, this Y chromosome result is "the only unequivocal indicator of Viking ancestry in Britain." More sophisticated tests and additional genealogical work might tell us whether this is believable in Pete's case, but he's not waiting on further investigations. He's been dancing around declaring, "I come from Vikings" and suggesting potential Viking-y names for our hypothetical future children.
There'll be no living with him after this.