What should I consider if I’m thinking about adding a purebred dog or cat to my family?
If you are interested in a particular breed of dog or cat, be aware that different breeds are predisposed to different inherited disorders, and health and welfare problems. Breeders also have different practices; for example, some practice close inbreeding while others do not. The breed you choose and the breeder and their practices will significantly influence your future companion animal’s health and welfare.
We recommend that you:
[1] Janes M et al (2020). The usage of Mate Select, a web-based selection tool for pedigree dogs for promoting sustainable breeding. Canine Medicine and Genetics 7(1): 14–14.
[2] Thomas, D.L. (2020) Inbreeding and Inbreeding Depression. In Molecular and quantitative animal genetics (1st ed.; H. Khatib, Ed.). Hoboken, New Jersey: Wiley Blackwell: 15–24.
[3] Leroy, G. (2011). Genetic diversity, inbreeding and breeding practices in dogs: Results from pedigree analyses. The Veterinary Journal 189(2): 177–182.
[4] Yordy at al (2020) Body size, inbreeding, and lifespan in domestic dogs Conservation Genetics 21(1): 137–148.
https://kb.rspca.org.au/knowledge-base/what-do-the-terms-inbreeding-and-linebreeding-mean/
Expression of Deleterious Recessive Alleles
Dogs have over 600 genetic diseases. A cow has a little over 400, a cat over 300, and a goat less than 100. In wild animals, the comparisons become even more shocking. A gray wolf has six genetic disorders, and a coyote only three. The reason for this is because of the closed gene pool and the impact it has had in causing tremendous homozygosity in the genes of dogs.
Dogs frequently will have two copies of an allele that is identical for a trait. Some very bad diseases in dogs are expressed only when a sire that has a recessive gene is mated with a dame that also is a carrier or when a dam is a carrier of an X-linked recessive gene. The impact on the litter of puppies can be tragic.
For example, in the rare disease of Severe Combined Immunodeficiency, a dam will be perfectly healthy but will pass on to fifty percent of her puppies the X gene with the recessive gene for the disease. Male puppies with the recessive gene will be listless (i.e. having or showing little or no interest in anything), have diarrhea, and most likely die. Female puppies will be fine, but fifty percent of them will likely to be carriers, so this goes on and on over dozens of future generations.
A serious cardiac disease in which there are four defects in the heart present is Tetralogy of Fallot and it occurs when a puppy gets one recessive gene from the dam and one from the sire. The disease causes the puppy to suffer from a continual lack of oxygen.
For some diseases, it is possible to genetic test for before mating. Most notably degenerative myelopathy, progressive retinal atrophy, and von Willebrand’s disease have genetic testing available. Currently, there is no commercial genetics test for severe immunodeficiency disease. Other fatal diseases in dogs, especially forms of cancer, are suspected to be genetically inherited but the mechanisms are not yet understood.
Pedigree-based COI
These estimates are based on the relatedness of individuals in a pedigree. 25% is the value from a mother-son or full-sibling mating; 12.5% being the value from a grandparent-grandchild or half-sibling mating; and 6.25% being the value from a first cousin mating. These values accumulate. Logically, all individuals have COIs between 0% (completely outbred) and 100% (completely inbred). So three generations of full-sibling matings would lead to a COI of 50%.
Ideally, the pedigree is complete all the way back to the founding of the breed. However, in reality, most pedigrees only go back maybe 5 to 10 generations. Most COI calculators assume that the original ancestors in the pedigree are unrelated. Therefore, a COI calculated from a 5-generation pedigree could be much lower than that calculated from a 10-generation pedigree. This is likely much lower than the true COI if the complete pedigree back to the breed founders was known. For this reason, there’s no one answer for what a “good” COI is; It all depends on how complete the pedigree is. Furthermore, because of the principle of segregation, two individuals with identical expected COIs from a pedigree may have very different levels of inbreeding. This depends on which individuals inherit which chromosomal segments.
Is inbreeding dogs safe?
For millenia, dog breeders have intentionally mated relatives as a way to fix traits in a lineage, recognizing that there is a reduction of fitness in offspring of close relatives. However, this certainly wasn’t always intentional as one can see from this post on the Hapsburg lip. A century ago, Sewall Wright devised the coefficient of inbreeding (COI) as a way to measure inbreeding, a statistic still popular today. Conveniently, with Embark’s dog DNA test, we automatically report the COI for every dog, so let’s explore what it means!
Like humans, dogs tend to be 99.8-99.9% genetically similar to other members of their species. Even other species can exhibit similarities—dogs and humans are 64% similar at the base pair level. But genetic variation is the spice of life, and the 0.1-0.2% of the genome that differs encodes a myriad of variation. Some of these we have intentionally perpetuated like body shape, coat color, or behavior. Unfortunately, other less desirable variants confer potentially harmful effects on health, longevity, and reproductive success.
Harmful mutations come in three main varieties: recessive, dominant, and additive. These harmful dominant and additive mutations are quickly weeded out in large outbred populations. This occurs because the individual carrying these mutations has a reduced fitness. Recessive mutations, on the other hand, are different. A harmful recessive mutation might “break” a gene. This has little or no consequence if an individual has a working copy of the gene from his or her other parent. However, this can have disastrous consequences when an individual inherits two broken copies. Outbred individuals almost never inherit two broken copies. Therefore, natural selection or breeders cannot effectively select against them unless there is a genetic test for the mutation. For example, if a mutation is at 1% frequency in an outbred population, any given dog has a 0.01% chance of inheriting two copies of the mutation—clearly a very small chance.
As such, every dog population—or in the context of purebred dogs, every dog breed—contains an abundance of rare recessive mutations that were either present in a founder individual or arose spontaneously in the dog population sometime afterwards. These rare mutations are hardly ever problematic for outbred individuals because they almost always inherit at least one working copy; however, they can cause real problems for inbred individuals—animals that arise from the mating of closely related parents.
Let’s consider what happens with dogs in a mother-son mating. A mother passes along 50% of her genome to each pup, so each rare (<1% frequency) recessive mutation carried by the mother has a 50% chance of being transmitted to a son. Offspring from a mother-son mating would, therefore, have a 25% chance of inheriting two bad copies of the mutations that have been passed down to the son. This is a greater than 100-fold risk compared to an outbred dog!
Inbreeding in dogs has real consequences. Research in the Boyko Lab has shown that a 10% increase in inbreeding can lead to a 6% reduction in adult size (poor growth) and a six- to ten-month reduction in lifespan. Reduced litter size and fertility are also likely. These risks occur from both classical inbreeding and from drift in small populations where every individual is a not-so-distant relative. Assessing these risks depends on accurately quantifying the likelihood that mutations will be identical-by-descent, or inherited from the same ancestor.
There are three ways to quantify the coefficient of inbreeding (COI): (1) Using a pedigree, (2) Trying a small set of polymorphic markers, or (3) Testing a genome-wide marker panel. How do you easily find out?