What is the 統 in 傳統？ The 傳 in 傳統 is easy to understand. Tradition is transmitted. As the dictionary(MOE) says: 世代相傳.

But what about the 統？It has many meanings, including verbal ones. 


One (絲的頭緒) refers to the main thread of silk. So is 傳統 the essential, important part of transmission?
Another says: 相繼不絕的體系、關係。如：「血統」. So is it the uninterrupted transmission?
A third one says: 綱要, the essentials, similar to my first guess.
A phrase you'll often hear is: "It's a tradition in our family."

Now extend family to 民族

The word tradition comes from: trans 过 + dare 送给 (Originally in the sense of Jewish instruction, Mosaic Law, it seems.)

传统: 传承血统 pass on bloodline

Whereby, 血统 does not just indicate the dna I think, but all the idiosyncrasies that come with it, the dances, the funny hats, the strange and wonderful delicacies, the social expressions of that bloodline.