Flaxen is a modifier trait that is only seen on Chestnut, and Chestnut based, horses. It is where the manes and tails are lighted to a blonde, white, ivory, etc, colour. Some flaxen horses have lighted hair at the bottom of their legs. Flaxen is seen in most breeds; some breeds like the Belgian and Haflinger express a bold version of flaxen that when paired up with Pangare will have people confusing them as Palominos!
There is a theory where there is one recessive flaxen gene (shown as ‘f’ when writing out a genome), which is often presented as genetically tested correct on most colour sites and even in colour books, I certainly think there are far more then one gene at work as it hasn’t be genetically proven yet. This is because there are many different expressions of flaxen. Some horses will have completely white manes and tails to horses with only a small few white hairs in the manes.
Check out the Flaxen Chestnut album to see Flaxen Chestnut horses: