Gregg,
This snow line is an Incomplete Dominant trait. To save a little typing, I will cut a paste a couple of things from my web site (
www.reptiliangems.com). I also have links to definitions from Answers.com, Wikipedia, and FreeDictionary.com. The links are on my ABOUT OUR SNOWS page. In my humble opinion, most people are going by an incorrect definition of Co-Dominance, which leads to even more confusion when you add the term Incomplete Dominance. Hope the following definitions help.
Incomplete Dominance: When there is a blending of the two parental phenotypes, producing third phenotype different than either parent. One classic example of Incomplete Dominance, is when breeding some flowers, crossing a red flower with a white flower, produces a pink flower. In this case, one allele dominates the other, but only partially, producing a third intermediate phenotype, intermediate between those of the parents.
Co-Dominance: Is when both parental traits are FULLY expressed in the offspring. An example of Co-Dominance often given, is in cattle when one parent has red fur, and the other has white fur, the offspring is born with both red and white fur. As with Incomplete Dominance, a third phenotype is produced, but there is no blending. In other words, the red fur is still red, and the white fur is still white, but they are both present together. There is no blending with Co-Dominant traits. Because neither Allele dominates the other, they are both expressed fully in the first generation. In the example of the red and white cattle, the red hair is still red, and the white hair is still white, but they are both present together (hence the term Co-Dominant). You don't have to breed offspring together to get a Co-Dominant trait.