Is it inherited? Why do we encounter inherited problems in purebred dogs and how best we can deal with them in a breeding program; first article in a series by Noa Safra.

Modern dog breeds represent closed populations under selection. This may increase the frequency of rare genes, making them widespread. For example: the dilute gene is “fixed” in Weimaraners meaning it is homozygous in all Weimaraners; Grey or Blue, Shorthaired or Longcoated. The dilute gene is a rare gene in other dog populations i.e. Pariah (wild) dogs. Some of the formerly rare, now more frequently seen genes in purebred dogs may have deleterious effects and cause disease.

These genes are not under selection by breeders so why are they becoming common too? There are two main events related to the development of a breed that contribute to this phenomena; population bottleneck and founder effect. Population bottleneck is the reduction in number of individuals contributing genes to the next generation. One cause of population bottleneck is importation; each time a breed is introduced to a new country (i.e. when Weimaraners first came from Germany to the US in the 1930’s) only the genes carried by these imported individuals are going to be found in the newly formed population. This can change the frequency of deleterious genes from rare in the original population to frequent in the new population.

Founder effect is the large influence that few individuals have on a certain population. This is the role played by the first imported Weimaraners in the US but also the influence of popular sires, past and present, on the breed. Since any single dog can only contribute one out of two alleles of any existing gene to its offspring, the frequency of genes carried by popular sires is going to rapidly increase. Some of these genes are desirable (these genes are under selection and probably the reason for the dogs’ popularity) but others, hidden ones, may be deleterious.

Most breeders are going to experience negative traits produced in a litter. These traits could be health, temperament or conformation related. Often times the appearance of these unwanted traits is the result of the rise in frequency of deleterious genes as was discussed above. However, other times the problems observed are not inherited but are the result of environmental factors or spontaneous mutations. For example: Dilated Cardiomyopathy (DCM) is inherited in Portuguese Water Dog, Irish Wolfhound, Doberman Pinscher and several other breeds. The genetic background of DCM in some breeds led veterinarians and breeders to assume that DCM is inherited in other breeds such as the Dalmatian. But Dalmatians (as well as other dogs) that were fed certain diets were presented with clinical DCM as the result of Taurine (an essential amino-acid) deficiency. This type of DCM can be reversed upon the addition of Taurine to the diet and does not have an inherited basis. Another example is cataracts in dogs. Whereas most cataracts in dogs (especially in young to middle-aged purebreds) have a genetic basis, some cataracts are secondary to non-genetic causes. These causes include trauma, nutrition, toxins, radiation, hypocalcemia, and concurrent diseases such as diabetes mellitus. Since some of the non-genetic cataracts could be congenital (present at birth), it is important to rule out these causes before concluding a genetic basis.

How do we find out if a problem is inherited in our breed?

Unfortunately, there is no simple answer. Most phenotypical traits (what we see in an individual animal) are the result of both the genotype (the genetic makeup) and the environment. The fraction of a trait that is determined by genes is called heritability and is measured between 0-1. For example: gender is probably the trait with the highest heritability (determined by X / Y chromosomes) and its heritability value may be very close to 1. Any other trait will have a value lower than 1 but if it is higher than 0.5 it is considered to be greatly influenced by genes. The only way to verify whether or not a condition is inherited in a specific breed involves hard work. Breeders should cooperate with interested veterinarians and researchers to provide the professionals with plenty of high quality data and samples. Pedigrees need to be studied in depth (parents and grandparents) as well as in breadth (full-sibs) and complete medical records must be collected and stored for all the individuals involved.

When in doubt, the best breeding strategy is to use normal dogs from mostly normal litters. The storage of frozen semen is important to maintain the availability of quality dogs with unknown level of risk pedigrees. If in the future the disease is to be determined heritable, DNA tests may evolve that can differentiate carriers from normal dogs, and frozen semen offspring can be reintroduced into the gene pool. Both DNA (from blood or cheek swabs) and semen should be stored in this case. The other possibility, of the condition characterized as non-inherited, will free the stored semen for wide use.

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