by Vickie Halstead RN, CVNS, CCRN, CEN, CLNC
Prior to breeding, pedigrees must be studied for two reasons, one being to determine phenotype (observable characteristics) and the other to determine genotype (genetic constitution). Predicting whether a mating will produce offspring either affected with or a carrier of the Bichon genetic cataracts is challenging, considering many carriers exist in pedigrees that are unidentified. Tracking the incidence of cataracts in your line is recommended, and sharing that information with other breeders will aid in making wise breeding decisions.
Although the defective gene was not identified, Dr. Kirk Gelatt’s research determined that the Bichon juvenile cataracts carry an incidence of 11.5%, the most common age of onset is 2-8 years, and the mode of inheritance is autosomal recessive. If cataracts begin to develop beyond the age of 8 years they are most likely geriatric cataracts. However, an ophthalmologist exam is necessary to differentiate the characteristics of juvenile versus geriatric cataracts. For more information read these articles on the bichon health web site: www.bichonhealth.org/HealthInfo/CataractInheritance.htm (Dr. Gelatt’s article) and www.bichonhealth.org/HealthInfo/EyeDisease.htm.
As Dr. Padgett states in his genetics book, knowledge of the mode of inheritance is essential to control the incidence of genetic diseases. To identify the carriers of Bichon cataracts in pedigrees, first obtain a list of those Bichons who were diagnosed with juvenile cataracts. Next, determine the status of the dams and sires of those dogs—affected or not. If those parents are not affected (passed their CERF exam), they are both carriers.
Autosomal recessive inheritance means that each parent donates one defective gene that causes offspring to be affected with cataracts or to carry the defective gene. This list demonstrates the effects of various combinations of gene distributions:
A major goal of breeding to prevent offspring with the genetic cataracts is to identify which Bichons are clear of the defective gene, meaning they are not affected and they are not carriers. To accomplish this, one would have to breed a dog to a known carrier. If no affected offspring result by the age of 8 years, you can assume that dog is clear of the defective gene, but 50% of the litter will be carriers (#5 above) because of the carrier genes passed down from the other parent.
To better understand the information in the above paragraph, the dogs involved are indicated with a letter:
If you mate dog A with dog B, and dog A is truly clear of the cataract gene, half of the litter will be carriers, half will be clear of the cataract gene, and cataracts will not develop in any dog from this litter. However, if 25% of the dogs from this litter develop genetic cataracts by the age of 8 years, dog A is a carrier.
If you have determined that dog A is clear of the cataract gene, then dog C has a 50% chance of being a carrier since the other parent (dog B) is a carrier. If you breed dog C to a dog that is proven clear of the cataract gene (like dog A), you will get a litter that is clear of the cataract gene only if dog C is not a carrier.
One other factor to consider is that any Bichons who are carriers or affected that are present in the pedigree behind the dog that is proven clear of the defective gene are negated. In other words, those dogs have no influence on genetic cataract patterns as long as they are present in the pedigree behind the clear Bichon.
The ultimate goal is to breed only dogs that are clear of the defective gene, which in time could eradicate the Bichon juvenile cataracts. This goal is only attainable if breeders share the health data from their Bichons with other breeders.
Reference: Control of Canine Genetic Diseases by George A. Padgett, 1998