In dogs, genes A, K, and E specify proteins that control the production of light (pheomelanin) and dark (eumelanin) pigments in cells called melanocytes. Gene E specifies a receptor protein that spans the cell membrane and acts as a "switch". The proteins specified by the A and K genes control the switch. When the A protein is bound to the receptor, the switch is "off" and the melanocytes make pheomelanin; when the K protein is bound to the receptor, the switch is "on" and the melanocytes make eumelanin. The proteins specified by various alleles of the A and K genes differ in how tightly they can bind the receptor switch. This demonstrates that:
1. Interactions between alleles of different genes determine coat color in dogs.
2. Dog coat color serves as a good example of incomplete dominance.
3. Genes that code for dog coat color are codominant.
4. The dominance relations of a gene's alleles affect the alleles' transmission.