When I first read that genetic research showed that Californian Zinfandel was the same as Italian Primitivo, I was quite pleased. But when I read a few years later that Primitivo’s genetic origin is in Dalmatia on the Adriatic coast, I was even more pleased. Which is perhaps why I have become partial to a Zinfandel sold by Marks & Spencer a few minutes from my home. In fact, I am drinking it as I write. Even I, the impartial one, need a simple decision rule when it comes to so many good wines that now crowd the market.