Breeding Guidelines using the DNA Test for Lens Luxation
By George G. Packard
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Assumptions underlying these Guidelines:
DNA Test for Lens Luxation
We have a very accurate DNA test for the presence of a mutated
gene ( allele) which is strongly associated with primary lens luxation
(PLL). (Please read the description from the Animal Molecular
Genetics Lab, University of Missouri at the end of this document) .
Any Sealy can be tested
Any Sealyham being considered for breeding should be tested,
and all the puppies resulting from any mating involving
a carrier or affected dog should be tested. The DNA
test is available through the Orthopedic Foundation
for Animals and costs $65 (2012).
We expect that breeders will understand that testing is part
of the expense of ownership and breeding, and that the price of
a Sealyham puppy may need to be adjusted accordingly.
Determination of Risk
Using the DNA test, we can determine whether an individual dog
at high risk of becoming affected (carrying two
copies of the gene);
at low risk of becoming affected (carrying one copy
of the gene);
at no risk of becoming affected (carrying no copies
of the defective gene).
Loss of good traits in a small gene pool
All Sealyham owners and breeders should understand that because
the total population is so small, the breed is at great risk for
losing diversity in its gene pool. We know that a significant number
of Sealyhams are carriers, including many of our best dogs. That
means if we only use clear dogs (dogs not carrying the PLL gene),
many good traits could disappear within just a few generations,
and the breed could become more susceptible to the emergence of
other genetic diseases besides PLL if the gene pool (the population
of breeding dogs) becomes smaller.
Testing our dogs for the presence of the gene involved in lens
luxation allows us to continue to use dogs in our breeding programs
who have great traits but may be carriers of the lens luxation gene.
Testing also is crucial to provide us with the data which can show
whether the prevalence of lens luxation is decreasing over time
in our breed.
We should not remove good dogs with good inheritable traits from
our breeding programs just because they carry the gene for PLL.
Instead, using the DNA test, we must carefully manage our breeding
program to minimize the risk of producing affected dogs, while maximizing
the diversity of the gene pool and encouraging the flow of good
traits through our breed.
Goals of these Breeding Guidelines
The purpose of this breeding protocol is twofold:
To gradually reduce the percentage of Sealyham Terriers
who are Carriers or Affecteds;
To gradually increase (or at a minimum preserve)
the diversity of the gene pool and breeding stock.
Priorities for Breeding
(NOTE: For a graphical picture of how single recessive gene
inheritance works, see the home page
of the SealyHealthGuard.org (Genetics
There are six different situations a breeder can encounter when
she decides to mate a dog and a bitch and is concerned with selecting
against a single recessive gene trait:
Clear to Clear: This breeding will produce only
puppies who are clear (not carriers).
Clear to Carrier: Any puppy from this breeding will
have a roughly equal chance of being either clear
or a carrier.
Carrier to Carrier: Any puppy from this breeding
will have about a 25 percent chance of being either clear or affected,
and a roughly 50 percent of being a carrier.
Clear to Affected: Any puppy from this breeding will be a carrier, but is unlikely to be affected.
Carrier to Affected: Any puppy from this breeding
will be equally likely to be either a carrier or
Affected to Affected: All puppies from this breeding
are likely to be affected.
Given that three of the above situations are very unlikely to produce
an affected, we can rank those three cases in order of priority
depending on the ratio of Clears to Carriers likely to be produced:
Top Priority: mating a Clear
to another Clear;
Second Priority: mating a Clear
to a Carrier,
Third Priority: mating a Clear to
The risk for producing an Affected in any of these three situations
is very low.
Note: All of these situations require that one
of the dogs be a Clear.
Note : In the Third Priority case, one of the
dogs can be an Affected (remember that with the DNA test we can
identify a dog with two copies of the defective gene when it is
a puppy, years before it might actually show any symptoms.) Breeding
an Affected may seem counter-intuitive, but bear in mind that if
you have an Affected with many very good traits, breeding that dog
to a Clear will not produce any Affecteds, though it will produce
Testing and Record Keeping
This breeding protocol will fail without the testing of all dogs
being considered for breeding, and without accurate record-keeping
of the results of those tests. Ideally each breeder should have
all the puppies in a litter tested, but if that is not possible,
then at a minimum no dogs should be bred who have not been tested.
Results of all tests should be entered in the on-line SealyHealthGuard.org
data base so that the status of the DNA test of any dog is available
to any breeders, buyers or owners. However, if for some reason a
breeder does not want to participate in SealyHealthGuard, that breeder
must provide the results of any DNA tests of a dog to the person
who buys or acquires that dog.
These guidelines allow us to use any Sealy with good traits in
our breeding programs, whether that dog is a Clear, a Carrier, or
even an Affected, as long as one of the pair of mated dogs is a
Clear. We can test both dogs in the pair we are considering, and
make the breeding decision based on the three top priorities listed
above. Even if breeders do not test their litters, we can still
follow this protocol by testing breeding pairs before they are bred
to ensure that the risk of producing affected dogs is very low.
Appendix to ASTC Breeding Guidlines:
(For more info: www.caninegeneticdiseases.net )
Description of the DNA test from the
University of Missouri College of Veterinary
Research at the University of Missouri led to identification of
a DNA mutation that predicts which dogs are at risk for developing
lens luxation as they age. Working independently and using other
breeds, the researchers at the Animal Health Trust found the same
mutation a few months later. This independent confirmation of the
finding makes both labs confident that the correct mutation has
been identified, and that the test is valid for many breeds. A simple
DNA test will reveal if a dog is NORMAL (has 2 normal copies of
the gene), a CARRIER (has one normal copy and one mutated copy of
the gene), or AFFECTED (has 2 mutated copies of the gene). Wise
use of this test will allow breeders to avoid producing individuals
destined to develop lens luxation, while still retaining many other
desirable traits in their dogs.
Testing and Inheritance of PLL
From pedigree studies done previously, there has been general agreement
that PLL is inherited as a simple recessive trait. This means that
a dog needs 2 mutated, or " bad " copies of the gene to show the
disease. With the PLL mutation identified, and the research groups
able to compare notes on the dogs used in the study, it has become
apparent that there are some exceptions. While the vast majority
of dogs with PLL have tested AFFECTED, as small percentage of the
dogs that test CARRIER are also at risk of developing PLL. Owners
and breeders should be aware of this and understand the implications
of the test results so that they can make well-informed decisions
for the future of individual dogs, and the breed as a whole.
Dogs that test AFFECTED have 2 mutated copies of the gene. The
vast majority of these dogs will luxate at 4-8yrs of age, the typical
age of onset for PLL. There were a few dogs in the study group that
tested as AFFECTED but did not luxate until after 8 yrs of age,
and some dogs testing AFFECTED have died from other causes without
luxating. A search of published veterinary literature revealed that
about 10% of the dogs reported to be clinically affected with PLL
had onset of symptoms after 8 yrs of age. Because of this, the test
results will say " AFFECTED/HIGH RISK " .
As stated earlier, dogs testing CARRIER are at a slight risk of
developing PLL. Carriers have one normal and one mutated copy of
the gene. They could pass either the normal copy or the mutated
copy on to their offspring. Because there were a very few cases
of dogs in the research groups testing CARRIER who did appear to
have PLL, the test results will say " CARRIER/LOW RISK " .
A dog testing NORMAL has 2 normal copies of the gene, is not at
risk for developing PLL, and can only pass a normal copy of the
gene to any offspring.
Breeders and individual owners are now able to test any dog using
the testing kit that can be ordered online through the OFA
website (www.OFFA.org) . DNA is collected using a cheek swab,
and the barcoded sample will be tested by the Animal Molecular Genetics
Lab at the University of Missouri, with results reported directly
to the owner by OFA.