Autosomal Recessive Transmission with Incomplete Penetrance:

 

What does this mean?
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An autosomal recessive disease is caused by a mutation located on a non-sex chromosome (autosome). For a dog to develop the disease, it must inherit two mutated copies of the gene (one from each parent)

In general, dogs carrying only one mutated copy (heterozygotes) do not show symptoms but can pass the mutation on to their offspring.
 

Incomplete penetrance adds complexity: even if a dog has two mutated copies (homozygous genotype), it will not always develop the disease or may show only variable signs.

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Key points to understand this mode of transmission:​

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Recessive nature: Two copies of the mutation are required for the disease to be expressed.
 

Silent carriers: Dogs carrying only one mutated copy are usually healthy but can transmit the mutation.

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Incomplete penetrance: Not all homozygous mutated dogs will necessarily express the disease, or not in the same way.

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Clinical variability: Some dogs may show severe symptoms, others mild symptoms, and some may remain asymptomatic for life.

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Concrete example: Degenerative Myelopathy (DM)

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Degenerative Myelopathy in dogs is a typical example of an autosomal recessive disease with incomplete penetrance.

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•  N/N dogs (no mutation) are not affected.

•  N/M dogs (carriers) generally do not develop the disease but can pass on the mutation.

•  M/M dogs (homozygous mutated) are at risk, but not all will necessarily develop the disease. Some may show progressive paralysis later in life, while others may remain free of clinical signs.

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Implications for breeding
 

​This mode of transmission makes management more challenging:

• DNA testing helps identify carriers and at-risk dogs.

• Carriers (N/M) can be used for breeding, but only with tested clear partners (N/N), to avoid producing at-risk puppies (M/M).

• Even if a dog is genotyped as “at risk” (M/M), it may never develop the disease. Breeding selection must therefore take into account both genetics and clinical observations.
   

In summary:

An autosomal recessive disease with incomplete penetrance means that the mutation must be inherited from both parents, but its clinical expression is not guaranteed. Genetics provides a probability of risk, not an absolute certainty. This highlights the importance of genetic testing, combined with careful veterinary monitoring, to better manage canine health and guide breeding decisions.
 

Sources :

• Orthopedic Foundation for Animals (OFA)

• Veterinary Genetics Laboratory

• Orphanet