Originally Posted by JetdecksComet
Yes, tobiano is linked to extension (the E/e here) and roan. We don't worry about roan in this scenario, because none of the horses involved are roan (but interesting to note... because of this linkage, you can't have a homozygous tobiano that is roan or a homozygous roan that is tobiano. So, if you see a roan tobiano, you automatically know it's heterozygous for both roan and tobiano.)
Because I know my mare's tobiano came from her brown sire, I know that her tobiano gene is linked to her black gene and not her red gene. It's very, very rare for tobiano to break linkage and "crossover". It could happen, but is very unlikely.
If you want to read more about it: Tobiano Spotting
(Near the bottom see "Understanding Tobiano Genetics.")
Just to further clarify for those playing at home. The chance of it "swpping" linkage is approximately 7%.
Tobiano is "linked" to extension, which means that it is passed down only with that copy of the extension gene. So in this case, we can be fairly certain that the mare has her tobiano linked to a dominant extension gene, so whenever she passes that E to her foals, she will always pass on a To gene. The same thing happens with roan, which is a different mutation on the same locus as tobiano, so, as JDC said, you can't have a horse that is homozygous for tobiano and has roan as well.
Interestingly, dominant white and the only testable sabino are on the same locus as tobiano and roan too - I haven't seen any research that indicates they are extension linked, but I wouldn't be surprised if they are too.