Practical Genetic Counseling in Pure-Bred Population
By : Jerold S. Bell, DVM,
Tufts University School of Veterinary Medicine
Genetic defects are controlled by single, or a handful of genes, compared with the estimated 40,000 to 100,000 genes
in the dog genome. Prudent breeding practices dictate that you do not throw the puppy out with the bath water in
genetic disease control. With the development of gene probe identification of defective genes and tests for carriers
of defective genes, practical genetic counseling can be provided to dog breeders.
A recommendation to eliminate all carriers of a defective gene from a gene pool may result in a significant loss of
genetic diversity. Additional, previously unknown defective genes could be concentrated through genetic
bottlenecking. A recommendation to breed heterozygous carriers to homozygous normal dogs prevents affected dogs,
and keeps the gene pool diverse. However, it does not provide selective pressure to decrease the frequency of the
The goal of genetic counseling is to control the spread of defective genes effectively, while preserving the health and
genetic diversity of the purebred dog population. There are different breeding program recommendations based on
several factors, including;
Basic protocols for genetic counseling and breeding management of genetic disorders can be based on the known mode
of inheritance, and the availability of genetic tests:
Recessive disorders with a test for carriers: Testable disorders allow breeders to use all breeding stock, and should
result in no loss of breeding lines or genetic diversity. Quality individuals who test as carriers should be bred to
normal testing individuals, preventing additional affected individuals. The offspring should be tested, and the carrier
parent should be replaced in the breeding program with a quality, normal offspring. Additional carrier offspring should
not be placed in breeding homes; as the goal is to reduce the frequency of the defective gene in the population.
As each breeder tests and replaces carriers with normal testing individuals, the problem for the breed as a whole
Recessive disorders without a test for carriers: The problem with these disorders is the propagation and
dissemination of unapparent carriers in the gene pool. Relative risk pedigree analysis can provide objective risk
assessment for prospective breeding animals and planned matings. This requires knowledge of the carrier or affected
status of close relatives in the pedigree; which is best accomplished through a breed club supported open health
registry. By determining the average carrier risk in the population, breeders can be counseled to attempt matings
with risk factors lower than the breed average. They should lower the carrier risk of their breeding stock with each
generation, by replacing higher risk individuals with a quality, lower risk offspring. Breeding an individual once and
replacing it with an offspring allows breeders to improve their chance of moving away from a defective gene.
The number of offspring placed in breeding homes should be limited, as the goal is to lose the defective gene, and not
increase the chance of propagating it. A negative aspect of pedigree analysis is that it selects against families,
regardless of an individual’s normal or carrier status. On the other hand, it allows for the objective risk assessment
and continuation of lines that might otherwise be abandoned due to high carrier-risk.
Autosomal dominant disorders: Managing dominant disorders is usually straightforward, as all individuals carrying the
defective gene are affected. Selecting a normal sibling, or parent for future breeding maintains the breeding line.
If the disorder shows incomplete penetrance and there is not a genetic test, relative risk analysis and breadth of
pedigree analysis (below) can identify individuals with high carrier risk.
Polygenic disorders, or those without a known mode of inheritance: These disorders require knowledge of the affected
or normal status of full-sibs to prospective breeding animals. Individuals whose siblings are normal, and whose
parents’ siblings are normal have the greatest chance of carrying a low genetic load for the condition. This breadth of
pedigree analysis is more important than normalcy in the depth of pedigree (parents and grandparents only.) Affected
individuals can be replaced with a normal sib or parent, and bred to a low-liability mate. Breeders can replace the
higher risk parent with a quality, lower risk offspring, and repeat the process.
It is distressing to breeders when a genetic disorder is confirmed. Positive and practical genetic counseling
recommendations can be offered to maintain breed lines and genetic diversity, and improve the overall health of