When a horse has white hairs mixed into the coat (of whatever color), it falls into one of two categories.
Grey is caused by a single, dominant gene called "G". It causes the horse to have progressively more white hairs every year until it is virtually all white. The rate of greying can be fast or slow. Some horses go from very dark to nearly white in just a couple of years, while others progress much more slowly and take many years to change color.
Roan is a general term meaning a mixture of white hairs in the coat that is not progressive, that is, it does not increase every year like grey does (although they can vary with the seasons). There are several different genes that can cause roaning, some of which are well understood, some not so much.
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