This is a good video explaining chimerism. It deals with cats and human beings, but the general concept is the same for horses. (Thank you, Joanie, for sharing it!)

I sometimes get asked why I do not often discuss brindles. A large part of it is that, as a fraternal twin myself, I find the type of chimerism responsible for brindle in horses – called tetragametic chimerism – deeply disturbing. There are other forms that do not involve the fusing of two embryos into one organism, as the video explains. The video talks about microchimerism, where genetic material passes from the fetus to the mother. There is also blood chimerism, where genetic material passes between twins through a shared portion of the placenta. A certain portion of fraternal twins are blood chimeras. Because I have had a conflicting blood type test in the past, and because my twin had a different blood type, it is possible that I am part of that group. (It is also possible that I had perfectly ordinary blood and a careless individual performing a blood test.)