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. (havanesefanciers.com)
So, a dog can be a smooth coat (the dominant gene is expressed, or seen), and have a hidden long coat gene, which it can pass on to the offspring. (chihuahuaclubofamerica.org)
UKC Champion Sky Von Barren Berg CD BN RN HIC CGC TC TDI SV A Stamp Hips and Elbows OFA Shoulders OFA Thyroid OFA Cardiac OFA Full Dentition OFA DM Normal / Normal OFA DNA Bank MDR1 Normal Von Willebrand's Negative Homozygous short coat gene (Sky can not whelp a long coat puppy) (vonwhitehallgermanshepherds.com)