Practical Genetic Counseling in Purebred Populations
2003 AKCCHF Health Conference presentation summary
Jerold S. Bell, DVM | Tufts University School of Veterinary Medicine
Subject:.. 2003 AKCCHF Health Conference Practical Genetic Counse.
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 bathwater 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
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 defective gene.
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:
1] Populous breeds versus rare breeds: Genetic selection in a populous
breed does not tend to restrict genetic diversity. Recommendations
should be geared so that a defective gene does not become widespread in
the breeding population. In rare breeds, genetic selection should not be
so severe to further restrict genetic diversity in a small gene pool.
2] Widely dispersed versus recently mutated defective genes: Strict
control should be instituted against recent mutations, to not allow them
to become widespread. Selection against a widely dispersed gene depends
on its frequency.
3] High frequency versus low frequency defective genes: High frequency
genes require a long-term control program that will diminish the
frequency without altering the dynamics of the gene pool. Tests for
carriers would be helpful, so that genetically normal dogs will not be
selected against. Selection against a low frequency gene focuses on
strict control when observed.
4] Single gene versus polygenic disorders: Selection against
polygenically controlled disorders must focus on the affected or normal
status of the full sibs [littermates] of the breeding dogs and their
parents. Knowledge of breadth of pedigree gives selection information on
the possible genetic variation in the individual breeding animals.
5] Disorders with tests for carriers, versus no test for carriers: With
a direct gene test, breeders only have to know the results of the dogs
they plan on breeding. With phenotypic tests or no tests for carriers,
the knowledge of the carrier or affected status of related dogs is
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
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 no genetic test, relative risk analysis and breadth of
pedigree analysis 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 that 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 breeds.