|Image is from Wikimedia Commons under a Creative Commons license|
This is Betty, a Blenheim Cavalier King Charles spaniel. Blenheim is perhaps the most popular of the four color variants seen in the breed, and all Blenheim dogs are basically the same genetically. So, what's this dog's genotype?
First, it must be determined what form of red the dog is. There are two forms: sable and recessive red. It is far from unheard of for sable dogs to look like this, with little to no black in the coat. This phenomenon is known as clear sable. In the Cavalier breed, red dogs are actually all recessive red. This is most clear when puppies are quite young as, unlike sable dogs, red Cavalier puppies show no black hairs. In dogs that will grow up to be clear sable, there will still be a visible black overlay that fades with time. Expanding on this, it can be determined that all Cavaliers are actually tan pointed, and that the red Cavaliers are hiding this under their recessive red coat. This is thanks to the recessive red gene not allowing any black pigment in the coat at all. So, Betty would have to be atat ee recessive red (hiding tan point).
The intensity of the red seen in Cavaliers is also quite dark. As such, going with the theory that Chinchilla controls red intensity, Betty must be CC dark red.
Lastly, Blenheim and tricolor Cavaliers are expressing the piebald gene. This is what causes the prominent white markings seen in their coats. Though some dogs can have rather a lot of white, I don't see evidence of extreme white in the breed. Even dogs with a lot of white for a Cavalier fit within what can be expected from the piebald gene. Betty can be included in this, and thus she must be spsp piebald.
So, that's atat CC ee spsp or dark red piebald (carrying tan point).