A mask is a specific expression of the dark coat gene, one which expresses itself only over a certain area of the body, namely the head. A mask may range in intensity and extent. It could be a simple darkened muzzle or cover the entire foreface. It could be a raccoon-type mask around the eyes or include dark ears. A heavier mask may cover the entire head and even extend into the chest.
Obviously a mask will not show on black, but, where it exists, is very noticeable on mixed dark/light and lighter colours. Many Sable and Brindle Havanese have a face mask. Some dogs that carry the mask gene but are otherwise clear colours (red, gold, champagne) also may sport a dark mask.
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