What essential trait, inherited from their common ancestor, do all Schnauzer sizes retain today?
Answer
Their temperament and work drive
Regardless of size or modern role, the defining characteristic inherited from the German farm dog ancestor is their intelligent, spirited, and alert temperament and inherent work drive.

Related Questions
In what country did the Schnauzer type of dog originate?What does the name "Schnauzer" primarily refer to?Which size is considered the original and direct ancestor of the other two Schnauzer variations?What was the primary utilitarian reason for the stiff, wiry coat and distinct facial furnishings?What historical role did the Standard Schnauzer frequently perform alongside transportation?What change in work assignment drove the deliberate miniaturization of the Standard Schnauzer?Which breeds are suggested to have been introduced into the Miniature Schnauzer lineage to achieve smaller stature?What major role was the Giant Schnauzer bred to take on, differentiating it from the Standard Schnauzer's role?In which century did Schnauzers first appear in written history and artistic depictions?What essential trait, inherited from their common ancestor, do all Schnauzer sizes retain today?