I think you should assume that the original name was Jetzer because it is obviously Germanic and "Y" is not used in German, except in foreign words, but the "J" would be pronounced as a "Y" (easier for English speakers). There are just three entries for Jetzer in the German telephone book so it is certainly not common, but if you google "Jewish surnames" you will find it is listed as a Jewish name - but it is well to remember that it only takes one male, who could have married out of the faith, to bring a name into a family. My maiden name was Spanish but my gt.gt. grandfather married in an Anglican church in Hull in 1797 and was an English "yeoman". Your guess is as good as mine!