And although a black streak through the tail indicates the dog (or bitch) carries tri, its absence does not mean there is no tri gene.
I have had tris from a bitch with not a sign of black fur in her tail, and actually no tris anywhere near close up in her pedigree.
Whispering in their ears during pregnancy is no help either. Trouble was a lovely tri and we were so hoping for her to pass her color on in at least one son. So I kept nagging her - But no. Five red and white girls eventually emerged !