R O A N
While the actual mutation for roan has not yet been mapped, geneticists have discovered protein markers that can be associated with roan patterning and have developed a test for those markers. It has been determined that these markers are on the KIT gene. There appear to be many types of roan, though the marker test is not as accurate as a gene test would be. While the tests are fairly accurate, there is room for error.
Dark-headed roans, often called "true" roans, have a 50/50 dispersal of white and colored hairs throughout the body (making them look almost velvety in my opinion), leaving the head, mane, tail, and legs solid. There are also frosted roans, which are genetically roan, but their roaned hairs are more concentrated over the top of their body, sometimes including the mane and tail. Rarely the roan patterning will be skewed and form a brindle-like pattern, but it is important to note that this is not heritable. Roan was once thought to be homozygous lethal, but the marker test has revealed homozygous roan horses so there should not be any danger in breeding roan horses to each other.
Dark-headed roans, often called "true" roans, have a 50/50 dispersal of white and colored hairs throughout the body (making them look almost velvety in my opinion), leaving the head, mane, tail, and legs solid. There are also frosted roans, which are genetically roan, but their roaned hairs are more concentrated over the top of their body, sometimes including the mane and tail. Rarely the roan patterning will be skewed and form a brindle-like pattern, but it is important to note that this is not heritable. Roan was once thought to be homozygous lethal, but the marker test has revealed homozygous roan horses so there should not be any danger in breeding roan horses to each other.
E_ A_ RnRn or E_ A_ nRn or E_ aa RnRn or E_ aa nRn or ee RnRn or ee nRn