Line breeding has become the cornerstone of my selective breeding program. At some point when I feel I have reached my goal with my line I will close my breeding program and rely exclusively on my line breeding to produce my Bullmastiffs. In genetic terms, inbreeding is the mating of two Bullmastiffs that are related to each other. The opposite of inbreeding is out crossing, the two parents are completely unrelated. Since all pure breeds of dogs trace back to a relatively limited number of foundation ancestors, all pure breeding is, by this definition, inbreeding though the term is not generally used to refer to breeding where a common ancestor does not occur within a five-generation pedigree. When an egg or sperm is created they carry half of the genetic material of the parent. But that half isn't like splitting a pie. It doesn't split down some imaginary middle, of the entire strand, even if some explanations show it that way. Line Breeding increases the probability that the two copies of any given gene will be identical and derived from the same ancestor. The higher the inbreeding coefficient the more likely this is to happen. The technical term is ‘homozygous’ for that gene. The ‘heterozygous’ animal has some differences in the two copies of the gene. Each gene is split but it can combine with the others in many ways.Remember that each Bullmastiff has two copies of any given gene, one derived from the father and one from the mother. If the father and mother are related, there is a chance that the two genes in the Bullmastiff puppy are both identical copies contributed by the common ancestor. Line breeding does not cause genetic problems. Bullmastiff line breeding does increase the chances of creating matched pairs. And if the particular pair that matches up is recessive then you will see in the puppies something you did not see in either parent. It could be a good thing or a bad thing. This is why it is very important to keep records and health clearances. OFA screening in my opinion is useless with out crossed dogs because you would have to have every dog tested on both sides of the bullmastiff puppy in question for many generations back in the pedigree, and I mean EVERY dog. I would also like to bring up some bad words and clarify what they mean. To start with BACK YARD BREEDER. This term is used by breeders to degrade other breeders that they don’t agree with their breeding practices. All my Bullmastiffs live play and breed in my back yard… The other term is PUPPY MILL. This is also used for breeders to degrade other breeders. A true puppy mill is a commercial dog breeding facility that is operated with an emphasis upon profits above animal welfare and is often in substandard conditions regarding the well-being of dogs in their care.