Seeing puppies of a different color doesn't necessarily mean their mom was cheating on their dad with the mailman's dog. Instead, it's all a case of genetics. Even if you've bred two black dogs, there's a possibility some of the pups could end up chocolate, if both parents carry the chocolate gene.
What is Chocolate?
Chocolate doesn't describe all types of brown coats. It's common in dogs such as Labrador retrievers, border collies and Chihuahuas. It's almost more of a liver color, but it can appear faded to look more like chocolate milk.
With a brown dog, his hair is usually a dark shade of tan rather than a liver color. His eyes and nose can be black, standing out from his coat. When a dog winds up chocolate, it means he got only the chocolate gene from his parents and he can't make anything black -- his nose and his eyes reflect the same color as his coat instead of black.
How It Works
Black and chocolate colors are found on the same gene in dogs, noted with a "b" when mapping the genetics. The uppercase B is the dominant black gene, and the lowercase b is the recessive chocolate gene. It takes a chocolate gene from both parents to give the recessive chocolate color a chance, which means both need to be Bb -- one black gene, one chocolate gene. Each parent gives one gene to each puppy. It's the luck of the draw to see whether the puppies end up with BB (black with no chocolate gene to pass on), Bb (still black but with the possibility of having chocolate pups) or bb (chocolate through and through).
Possibility of Chocolate
There's a statistical way to determine the possibility of having chocolate puppies. If both parents are black but carry the recessive chocolate gene -- Bb -- about a quarter of their puppies should be chocolate, statistically speaking. Half of them should be Bb, with another quarter of them BB. That means out of four puppies, one will be chocolate and three will be black, but two of those three black puppies could potentially have chocolate puppies someday. Genetics don't always follow the rules of statistics, so each little will be different, but if a chocolate puppy appears out of a litter of black ones, you know the parents both must carry the chocolate gene.