Primary Lens Luxation in Miniature Bull Terriers

In September 2009, the genetic marker for Primary Lens Luxation in miniature bull terriers was discovered. The Orthopedic Foundation for Animals made the genetic test available a month later, and mini bull owners from around the world jumped on the opportunity to learn their dogs' PLL status.

The OFA test gives owners one of three results for their Mini Bull: Clear, Carrier or Affected/At Risk. A dog with a result of Clear has two normal copies of the PLL gene and will never luxate. Carriers have one normal copy and one abnormal copy of the gene, and run a very low risk of luxating. Affected/At Risk dogs have two abnormal copies of the gene and will likely experience lens luxation at some point in their life.

Now that this test is available to breeders, the next question becomes how to ethically breed mini bulls - who are already swimming in a shallow gene pool - taking into consideration their PLL status in addition to heart results, temperament, size, looks, and overall health.

Nobody contests that a PLL Clear can be bred to another Clear. The resulting pups from such a litter would all be free of PLL, which is the end goal for all Mini Bulls. And everyone also agrees that a PLL Affected dog should never be bred to another Affected dog - all of the offspring would also be PLL Affected. After that point, however, we have to brush off our old high school Biology book and review the genetics section for guidance as to how to proceed.

Here's how the other breeding combinations work out:

a) Breeding a Carrier to a Clear will result in each pup having a 50% chance of being Clear and a 50% chance of being a Carrier.

b) A Carrier bred to another Carrier will result in each pup having a 50% chance of being a Carrier, a 25% chance of being Clear, and a 25% chance of being Affected.

c) Breeding an Affected dog to a Carrier will result in a 50% chance of being a Carrier and a 50% chance of being Affected.

d) An Affected dog bred to a Clear dog will result in 100% of the offspring being carriers.

Knowledge of genetics is obviously key here for any breeder who is planning a litter of Mini Bull pups. While everyone has their own opinion on the topic, the general consensus among breeders seems to be to avoid breedings where there is the chance that an Affected pup might be produced. Others argue that, since there are real risks in a breed this small for inbreeding if otherwise good dogs are excluded from the gene pool, a breeding that would result in a low chance of producing Affected pups could be done as long as the breeder is prepared to deal with the special needs of Affected pups that might result from such a breeding.

And of course, it is possible and very likely that any given breeding would not follow the exact statistics quoted above. For example, in a Carrier to Carrier breeding that resulted in four pups, all four of those pups could be Affected, all four could be Clear, all four could be Carriers, or any mix therein. It is therefore theoretically possible to breed an entire litter of PLL Clear pups by breeding two Carriers together, although the statistics show that would be just as likely as that breeding producing all PLL Affected pups.

So what should a puppy buyer expect, now that the PLL test is out? Ethical breeders should be prepared at a minimum to disclose the PLL status of both parents to potential puppy homes and to provide them with a copy of the OFA paperwork that certifies that status. Also at a minimum, in the case of breedings that may result in Affected pups, all the pups in the litter should be tested and the actual OFA results for that pup should be provided to the puppy buyer (although many believe that all pups should be PLL tested prior to being placed, regardless of the parents PLL status).