Breed Genetics

There are three key facts about the American Bully

  1. They are Pitbulls: Analysing each pedigree shows that the American Bully is a Pitbull on paper. We believe it is essentially a Pitbull sub-breed, rather than a separate breed in its own right. They are entirely rooted in the lineage of the American Pitbull Terrier, with every individual dog’s ancestry traceable back to registered American Pitbull Terrier and American Staffordshire Bull Terrier. At some point, Mastiff, Bulldog and other breeds may have been mixed in – but papering (the forging of pedigrees) prevents us from knowing for certain. In any case, other breeds are only a small proportion of their current genetic makeup – they are overwhelmingly genetically a Pitbull Type dog.
  2. They are Inbred: The breed’s genetic diversity is relatively limited. All current American Bullies are descendants of a handful of breeders active in the early 1990s. This has resulted in a lineage that stems from a limited number of original dogs. Most American Bully pedigrees show evidence of some inbreeding, with certain influential progenitors even displaying an extreme inbreeding coefficient of 40% or higher.
  3. They are Genetically Aggressive: A significant number of these breeding dogs are kin to those infamous for producing human-aggressive progeny. Despite this alarming lineage, such breeding dogs are extensively utilized in the UK. A case in point is a breeding female known to have produced multiple human-aggressive dogs, one of which fatally attacked its owner. Notably, this particular bitch had fighting dogs in her pedigree going back 6-7 generations. Her progeny is notably popular in the UK.

Our Evidence: Using online databases, we’ve tracked the pedigrees of some of the most famous Bully XLs. Below you find a images showing the linage of some of the most well-known breeding dogs. The dogs in red ink are registered as American Pitbull Terrier in at least one of the largest databases. The green (IB) refers to a dog present more than once in the pedigree indicating inbreeding. We cover these below.

UKC’s Most Wanted Kimbo, also known as Killer Kimbo, has a unique and complex lineage characterized by intensive inbreeding. Bred by Gustavo Castro, Kimbo is a product of generations of tight inbreeding that resulted in his Coefficient of Inbreeding (COI) reaching an exceptionally high 31.3%. This intensive inbreeding is evident in the fact that he has only one great-grandfather, repeated four times in his pedigree. The continuous use of inbred dogs in his lineage to produce large, muscular dogs resulted in this high COI, a figure which typically brings with it a multitude of health concerns. Despite these potential problems and his reputation for passing on aggressive traits, Kimbo’s impressive size and build kept him in demand as a stud, contributing to many pedigrees in the bully breed world.

More about Kimbo here

Despite his breeding success, Kimbo, notoriously known as Killer Kimbo in XL Pitbull circles, has a dark legacy of producing human-aggressive dogs. Many anguished dog owners reached out to Kimbo’s breeder, Gustavo Castro, sharing their devastating experiences of their dogs attacking people, often their own owners. Despite these alarming reports, Castro disregarded the warnings and continued to use Kimbo as a stud until at least 2017, three full years after Mia DeRouen was tragically killed by Kimbo’s son, Nico.

Adding to the concern, several breeders continue to ignore Kimbo’s unsettling legacy to this day, opting to incorporate his progeny into their breeding programs. The allure of Kimbo’s bloodline, known for its enormous size and solid build, seems to overshadow the underlying problems.

Killer Kimbo’s genes run in the bloodlines of many renowned breeding dogs, including The Unstoppable Juggernaut, ULP’s Legion, ULPs Odd Job, Frank Sinatra, Rside’s King Slayer, Bossy’s Jumanji, Bossy’s Rome, MBS’s Heaven, EBI’s Joker, and Bossy’s Bullion, to name a few.

The pernicious effects of inbreeding typically emerge at around 5% COI, and a significant surge in deleterious recessive mutations—essentially genetic diseases—occurs at 10% COI. These mutations can lead to various health issues, including cancer, neurological disorders, heart disease, and diabetes.

Most reputable kennel clubs advocate for maintaining as low a COI as possible, preferably not exceeding 20%. For context, a mating between siblings or parent-child would result in a 25% COI. Consequently, a COI exceeding this figure implies accumulative inbreeding, meaning the parents, grandparents, or great-grandparents were also subjected to inbreeding.

The Unstoppable Juggernaut (BPKC#: 10292, Club UKC) is directly descended from the UKC’s Most Wanted Kimbo, notorious for his tendency to pass down aggressive traits to his progeny. This genetic link is further emphasized through Juggernaut’s paternal and maternal lineage, with his paternal grandparents and maternal great-grandfather all being full siblings of Kimbo, contributing 50% of genetics from the father’s side and 12.5% from the mother’s. In essence, Juggernaut possesses 62.5% of Kimbo’s genes. It should also be noted that Juggernaut’s lineage is purely American Pitbull Terrier, as certified by the registration of both his parents as American Pitbull Terriers.

Read more about the Unstoppable Juggernaut here

Despite being registered as an American Bully in UKC, he is a purebred American Pitbull Terrier, since both his sire and dam are registered as American Pitbull Terriers. Notably, there is a degree of inbreeding present in Juggernaut’s lineage as exhibited by a Coefficient of Inbreeding (COI) of 7.8%.

In comparison to the average pedigree dog world, a COI of 7.8% might not seem excessively high, but it is important to highlight that it is over the 5% mark, which is considered a threshold beyond which the costs of inbreeding can start to outweigh the benefits. In terms of reference, a COI of 6.25% represents a mating between first cousins, and 12.5% represents a mating between half siblings.

Delving further into his pedigree, it is evident that Juggernaut’s father was a result of brother to sister mating, and their parents were also full siblings, culminating in Juggernaut’s father having a COI of 31.3%.

However, a more pressing issue is the fact that Juggernaut’s sire is UKC’s The Most Wanted Kimbo, notoriously known as Killer Kimbo. Kimbo’s breeder, Gustavo Castro, was renowned for producing sizeable dogs but was also known for extreme inbreeding practices. Despite warnings about some of Kimbo’s offspring’s instability, Castro continued to use Kimbo as a stud dog. Kimbo’s reputation for passing on the Human Aggression (HA) trait to his offspring was well known, earning him the name ‘Killer Kimbo’ in dog breeding circles. It is crucial to note that Juggernaut has inherited Kimbo’s genes on both his mother’s and father’s side.

ULP’s Legion shares a similar background with his father, Juggernaut, stemming from UKC’s Most Wanted Kimbo Lines. However, the genetic profile is more intertwined, with his parents being full siblings. The impact of inbreeding is evident over several generations, given that Legion’s grandparents too are full siblings, thus amplifying Kimbo’s genetics on both sides of his pedigree. Like his predecessors, Legion’s ancestors are also 100% registered American Pitbull Terriers.

Read more about ULP’s Legion here

ULP’s Legion is a prominent name in the UK bully world, widely sought-after as a sire and frequently highlighted in XL Bully advertisements. However, a closer look at his pedigree reveals extensive inbreeding, with his mother and father being full siblings. This has led to a Coefficient of Inbreeding (COI) of 25%, signifying that he has half the number of distinct ancestors a typical animal should possess, reflected in an Ancestor Loss (AVK) percentage of 46.6%.

Further complicating his lineage, both of Legion’s grandfathers are the same dog: UKCs Most Wanted Kimbo, otherwise known as Killer Kimbo. Kimbo was bred by Gustavo Castro, who gained recognition for producing large dogs roughly a decade ago, frequently resorting to severe levels of inbreeding to expedite the development of size and muscularity. Despite the evident instability in some of Kimbo’s offspring, which even led to a fatal incident where Kimbo’s son Nico killed a four-year-old child, Castro continued to employ Kimbo as a stud dog until as late as 2017.

At this point, it was widely acknowledged in the XL Pitbull / Bully circles that Kimbo was notorious for passing on the Human Aggression (HA) trait to his offspring, thereby earning the ominous nickname “Killer Kimbo”. Notably, ULP’s Legion carries Kimbo’s genetics on both his maternal and paternal sides.

Bossy’s Victoria Secret stands out as one of the most acclaimed XL Bully breeding bitches, considered a paragon of her breed. She can trace her lineage back to 100% registered American Pitbull ancestry.

Bossy’s Willy Wonka also shares an exclusive lineage from American Pitbull Terrier ancestors, exhibiting inbreeding on both his paternal and maternal side. A distinct figure in his family tree is Primieyard’s Momma Mia, who is a renowned bitch in her own right and also happens to be Willy Wonka’s paternal grandmother. Mamma Mia too is 100% American Pitbull Terrier, showcasing inbreeding on both her paternal and maternal lines.

Read more about Willy Wonka here

Willy Wonka is celebrated on both sides of the Atlantic, in the UK and the US. Revered as a legendary figure, he is an incredibly desirable stud dog. Even his frozen semen is available in the UK, a privilege that comes with a hefty price tag of $10,000 for interested breeders. Willy Wonka himself is quite substantial, weighing in at 9 stone 11 pounds, and he is a carrier of both the chocolate and the tri gene.

Examining his lineage further, it’s clear that inbreeding is present on both the paternal and maternal branches of his family tree. His paternal grandfather and great-grandfather are full siblings, and the bloodline of Hatfield Monster Mutt also features in his maternal line. Lor’s Bully Icon appears twice in his maternal line. Moreover, his paternal great-grandmother, Scorpion Kennel’s Paris Hilton, is the product of a brother-sister mating.

Rside King Slayer, Rside Odd Job, and All Star Bullies Bossman Chop are significant names in bully breeding. King Slayer’s father, Odd Job, and All Star Bullies Bossman Chop are frequently mentioned in pedigrees. Bossman Chop’s mother is Victoria’s Secret, adding more dimension to that side of the pedigree. King Slayer, like many others, is inbred on his paternal side with a repeated genetic contribution from Kimbo. The same trait is observable in King Slayer’s sire Odd Job, with Kimbo being his largest genetic contributor.

MBB Nutella and her son MBB Fudge stand as prominent figures in the UK’s bully breeding scene, celebrated for their distinctive and exotic colorations. Their lineage features a noteworthy degree of inbreeding, particularly observable on Nutella’s maternal side with MBB Camo, who has fighting dogs in her pedigree dating back five generations. The inbreeding and fighting dog influence in their heritage have been cause for concern, given the fatal mauling incident involving Nutella’s full brother. Despite this tragic event, the lineage continues to be propagated, as evidenced by MBB Tonka, another sibling, being advertised as a stud at MBB Kennels. Meanwhile, Fudge’s pedigree is further embellished by his sire, the renowned Bossy Willy Wonka, enhancing his popularity as a sire in the UK.

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