This is a complex field very dificult to explain in a single post reply. The deep wisdom in breeding its someting that are not in the mind of the novices and even in the experient breeder. The bloodline in this case means Genealogy (pedigree). Its a method very well knowing to anyone that using cientifique approach in breeding program. A definition of Combination Capacity is important to know... The hybrid vigor stability environment is in function of the heterozygosis that appeared in mating between the progenitors that has been selected. The aims to developed Hybrids is identify the bloodlines that’s have the high vigor, because the both combine very good. Two types of combination can be distinguished. The General Capacity of Combination (GCC) that’s referring to the ability that one progenitor offspring has the same behavior, when mating with serial different progenitors. The Specific Capacity of Combination (SCC) refers to a behavior from the specific combination, which can vary from the expect base of GCC. This behavior will be very important during the select best puppies, the stud dogs and the best line over all breeding program between the hybrids lines. And the Hybrid lines: . Hybrids lines. A variety hybrid is a progeny mating by two types of progenitors genetically different. On the real world to develop a new line hybrid is more complex that this definition suggest. There phenomenon called Hybrid Vigor is the best benefits to make a Hybrid. But one of the biggest misconceptions of hybrid vigor is that it applies just in F1 mating. Hybrid Vigor applies better on the individuals that have homozygosis genes and with direct offspring of the crossing of the unrelated strains. In other words if you continue to breed the offspring in F2 you lose the vigor. Hybrids can also cause problems if widely divergent genes types due to differences in attributes (genes segregation). The breeder must be a special ability in select the better puppies with “the knowledge eyes” like an artist has. 1-Question: In case of APBT that we have the pedigree back to 1890 in some lines and all those dogs (ADBA) had came from Fighting tested, the pedigree reveal all the important knowlodge about the better combinations betwen the strains involved. Sometimes the gene manifest after 2 or 3 generations. Most of people in this case coud think that the sire or dam did not pass the genes and will do not use the offspring...and lost the oportunity to wait more 1 or 2 generations to produce a stud Dog. All the narrow cross in those pedigrees are allied to wide cross and back cross trying to add and delect good and bad genes. In the case of Health problems the wide cross and narrow cross after its a common strategy. You can see this studying pedigree of APBT.