All it takes is a few genes to determine how the pigment spreads in a dogs skin and coat.
Pigment in a dogs hair determines the color of its coat. The white patches are actually hairs that have no pigment as the white patches are basically hair without pigment.
Puppies inherited one set of genes from both of their parents so they will probably look like one of them or the genes might get mixed resulting in very odd-looking dogs.
The difference between the parents is odd.
Black coats with white patches on the chest are known as Irish spots. A dog with this coat type has white patches along the dogs edges if we may use that term.
These are the tip of the tail and the muzzle as well as the area above the paws and the chest.  A trim is also called this pattern.
The white patch on the chest of some dogs can look like a tie or it can be only a small spot just above the breastbone.
Since no gene has yet been identified that determines how pigment spreads in a dogs coat its impossible to say why this occurs.
Its true that the pigment in dogs with this Irish spotting is concentrated in the hairs covering the body where the vital organs are located.
A piebald dog has mostly white hair with only the head back and root of the tail colored.