BRINDLE INHERITANCE IN BULLMASTIFFS
Any dog with a masking gene will exhibit a mask; any dog with a brindle gene will be brindle. The possiblities for bullmastiffs then are these: EmEm, Emebr, and ebrebr. Any bullmastiff having at least one brindle gene will be a brindle. Brindle-coated bullmastiffs may be either Emebr or ebrebr, a brindle carrying a masking gene or a pure brindle. Since the dark brindle color may cover the masked portions, there may be no discernible difference between the pure brindle and one carrying a making gene. The masking gene, of course, becomes visible in the fawn and red-coated varieties. Likewise, any red or fawn bullmastiff cannot carry the brindle gene. Mating two fawns or reds will never produce a brindle coated dog (unless it were a genetic mutation).
Below are described the breeding possibilities and the probably outcomes in litter coat colors. These results are only percentages and are often not proven in one or even several litters; but eventually, after multiple litters, the following percentage would prove true.
Mating two pure brindles having no making gene (ebrebr) would produce only pups that are pure brindle (ebrebr).
Mating a pure brindle (ebrebr) to a brindle with a masking gene (Emebr) would produce all brindles, 50 percent pure brindles and 50 percent with masking genes .
Mating a pure brindle (ebrebr) to a red or a fawn (EmEm) would result in all brindle pups, but all would carry a making gene (Emebr).
Mating two brindles both with a masking gene (Emebr) would result in 25 percent pure brindle (ebrebr), 50 percent brindle with a masking gene (Emebr), and 25 percent red or fawn (EmEm).
Mating a brindle with a masking gene (Emebr) to a red or a fawn (EmEm) would result in 50 percent brindle with a masking gene (Emebr) and 50 percent red or fawn (EmEm), but none of the brindles would be pure brindle.
Mating two fawns/reds would result in a litter of all fawn/red puppies and no brindles.
Of course, it is genetically possible to have a brindle or fawn bullmastiff with no masking gene (Eebr or EE), but since the breed standard prefers the mask, most breeders will not include such a dog in their breeding program, effectively eliminating these combinations.
If you are not sure whether a brindle bullmastiff is a pure brindle or a brindle carrying a masking gene (as in the mating of two brindle bullmastiffs), simply mate that dog to a red or fawn. If all the pups in multiple litters are brindle, then most likely the dog is a pure brindle (ebrebr). If any of the pups are red or fawn, then the brindle carries the masking gene (Emebr). Of course, any brindle that is the result of a mating of a brindle and a fawn/red (which can be discerned from a pedigree that includes coat color) is carrying the masking gene.
Little, Clarence C. The Inheritance of Coat Color in Dogs. Ithaca, NY: Comstock Publishing Associates (Cornell University Press), 1957.