Pattern White and Mismark Aussies are one and the same. Pattern White pups simply have more white than they should. They can be either merle or solid. They may have "too much" white trim, one side of their face (or both) may be white, or they may have color on their head, but very little anywhere else on their body. On rare occasions, they can even be completely white. Most often, they are born into litters where one parent is a merle and the other is a solid, but can occur in other litters as well. Sometimes it is possible to predict that there may be Pattern White pups in the litter (based on the amount of white on the parents), but it can also come as a complete surprise.
(Remember, all pictures with blue borders are links.)
Basic White Genetics
Original genetic research determined that there were 3 genes for white trim. They were "Irish Spotting" (S^i - typical "Lassie" white markings), Piebald Spotting Gene (S^p - non-symmetrical random white pattern), and Extreme White Spotting (or Extreme Piebald) Gene (S^w). More recent research is tending to believe that there are more than three, all of which can be paired in various combinations.
In fetal development, color starts at the "top" of the animal, and works its way down. Dogs with only a tiny bit of white on their chest, nose, and the tips of their toes probably do not carry any white trim genes at all. Their pigmentG just didn't quite extend all the way down. Less white is dominantG to more white, so it is possible for a dog to carry genes for more white than what you can see (so a pair of dogs with very minimal trim can still produce pups with more white than expected).
Is this a problem?
It can be. In order for any animal to be able to hear, the hair cells in the inner ear need to have pigment. If these cells are not, the nerve endings will atrophy and die in the first few weeks of life and deafness (full or partial) will result (see ASCA - White Deafness for more details). Note that you cannot tell the color of hairs in the inner ear by looking at any visible part of the dog's ears (including the hair around the ear canal). Some Pattern White pups will be deaf (or have partial hearing loss), while others won't, depending on where the pigment ends up.
The trim gene that seems to cause the most trouble is Extreme White Spotting (or Extreme Piebald) Gene. In a single dose (remember, all genes come in pairs), it gives a white blaze, collar, and socks. In a double dose, it turns the dog almost pure white. This is seen most often in Boxers (referred to as "flashy"), but many other breeds also carry this pattern. When two dogs of this pattern are bred, 1/4 of the pups will be almost pure white (known as "pattern white" or "color headed white"). Some breeds (such as Boxers, ACDs, etc) are always homozygous for this gene (meaning that all of them carry 2 copies of this gene). Usually, these breeds are also heavily ticked.
The White Aussie Project 2003-2005