Hereditary Cataract
Hereditary Cataract in
Staffordshire Bull Terriers has been recognised as an inherited condition since
the late 1970`s. Affected dogs develop cataracts in
both eyes at an early age. The condition is not congenital, so the lenses are normal
at birth but cataracts appear at a few weeks to months in age, progressing to
total cataract (and resulting blindness) by 2 to 3 years of age.
The mutation or change to
the structure of the gene, probably occurred
spontaneously in a single dog but once in the population has been inherited
from generation to generation like any other gene. The disorder shows an autosomal recessive mode of inheritance: two copies of the
defective gene (one inherited from each parent) have to be present for a dog to
be affected by the disease. Individuals with one copy of the defective gene and
one copy of the normal gene - called carriers - show no symptoms but can pass
the defective gene onto their offspring. When two apparently healthy carriers
are crossed, 25% (on average) of the offspring will be affected by the disease,
25% will be clear and the remaining 50% will themselves be carriers.
The mutation responsible
for the recently been identified at the Animal Health Trust. Using the
information from this research, have developed a DNA test for the disease. This
test not only diagnoses dogs affected with the disease but can also detect
those dogs which are carriers, displaying no symptoms of the disease but able
to produce affected pups. Under most circumstances, there will be a much
greater number of carriers than affected animals in a population. It is
important to eliminate such carriers from a breeding population since they
represent a hidden reservoir of the disease that can produce affected dogs at
any time.
The test is available now
and information on submitting samples is given below.
Breeders will be sent
results identifying their dog as belonging to one of three categories:
CLEAR: the dog has 2 copies of
the normal gene and will neither develop Hereditary Cataract, nor pass a copy
of the Hereditary Cataract gene to any of its offspring
CARRIER: the dog has 1 copy of the
normal gene and 1 copy of the mutant gene that causes Hereditary Cataract. It
will not develop Hereditary Cataract but will pass on the Hereditary Cataract
to 50% (on average) of its offspring
AFFECTED:
the
dog has 2 copies of the Hereditary Cataract mutation and is affected with
Hereditary Cataract. It will develop Hereditary Cataract at some stage during its
lifetime, assuming it lives to an appropriate age
Carriers can still be
bred to clear dogs. On average, 50% of such a litter will be clear and 50%
carriers; there can be no AFFECTED dogs produced from such a mating. Pups which
will be used for breeding can themselves be DNA tested to determine whether
they are clear or carrier
Testing can take up to 6
weeks for results to come back so make sure you have submitted blood samples
long before you plan your mating
If you want like to
download a screening form for HC (click here)
