According to the study titled Inbreeding Effects by Carol Beuchat, Ph. D. published in The Institute of Canine Biology, a dog lives on average 20.6 days less for every 1% increase in inbreeding. Inbred dogs are more likely to get health complications and behavioral problems than outbred dogs.
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.
Indeed, our results both at the individual level and in comparing purebred versus mixed breed dogs indicate that inbreeding can have a highly significant effect on lifespan. Mixed breed dogs lived 1.2 years longer, on average, than size-matched purebred dogs (consistent with findings from Patronek et al.
On top of health issues, you also run into potential birth defects. Inbreeding two closely related dogs can cause malfunctioning organs, cosmetic defects, and other abnormalities. While some congenital disabilities are manageable, others pose lifelong trouble for the dog.
Abstract. Because of their decreased overall fitness and genetic variability inbred individuals are expected to show reduced survival and lifespan under most environmental conditions as compared with outbred individuals.
Shorter Life Span
With all the complications we have mentioned so far, no wonder inbred dogs have shorter life spans as compared to outbred ones. Research conducted in the Bokyo Lab shows that there is a 6 to 10-month reduction in lifespan for every 10% increase in inbreeding.
Other negative effects of inbreeding documented include decreased immune system function, decreased viability, decreased reproductive ability and the loss of genetic diversity (i.e. decrease in genetic variation). Collectively, these adverse effects of inbreeding are known as inbreeding depression.
The highest level of inbreeding by far (> 80%) is for the Norwegian Lundehund. This breed suffers from extremely low fertility and high puppy mortality as well as an often lethal gastrointestinal disorder.
Inbreeding levels of 5-10% will have modest detrimental effects on the offspring. Inbreeding levels above 10% will have significant effects not just on the quality of the offspring, but there will also be detrimental effects on the breed. Do I still have to worry about COI if I am doing the health tests for my breed?
Advantages of inbreeding
Inbreeding leads to exposure and elimination of harmful recessive genes. Inbreeding helps in the accumulation of superior genes. Through selection, less desirable genes can be eliminated, resulting in an increase in the productivity of the inbred population.
Inbred dogs tend to be less intelligent than other members of their breed, and while it's not always the case, inbreeding can lead to higher rates of behavioral issues. Aggressive behaviors, such as rough play, snapping at or biting people or other animals, and even serious attacks are all more common in inbred dogs.
They found that on average, the more inbred an animal was (using its “true COI” measurement), the smaller the litter size. The least inbred dogs had an average litter size of 8 puppies, while the most inbred dogs had an average litter size of 6-7 puppies.
There isn't an absolute scientific formula for determining the average lifespan of a dog, but most pups are cuddly companions for about a decade. According to the Guinness World Book of Records, the longest-living dog ever recorded was Bluey, an Australian cattle dog, who lived nearly 30 years!
Inbreeding affects health
The average inbreeding based on genetic analysis across 227 breeds was close to 25%, or the equivalent of sharing the same genetic material with a full sibling.
The smaller breeds of dogs tend to live the longest. Yorkshire terriers, Chihuahuas, Dachshunds, Toy Poodles and Lhasa Apsos are the breeds who typically live the longest with these averaging a lifespan of up to 20 years.
25% = the genetic equivalent of a dog produced from a father to daughter mating, or the mating of full-brother/sister. More than 25% - inbreeding is accumulative, so if it has occurred to a significant degree over several generations, the inbreeding coefficient may exceed 25%
Conclusion. Inbreeding is a double-edged sword. On the one hand a certain amount of inbreeding can fix and improve type to produce excellent quality animals. On the other hand, excessive inbreeding can limit the gene pool so that the breed loses vigor.
Risks of Breeding Father to Daughter in Dogs. As a general accord, it is often highly advised NOT to breed father and daughter dogs because of the number of risks associated with it. In fact, this type of breeding has been banned by The Kennel Club.
Many inbred dogs are less intelligent than other members of their breed. Unfortunately, this also accounts for higher-than-normal rates of behavioral issues.
Canine health: a mixed tale
Not all dog breeds are alike in terms of health, however. Tamaskan Dogs, Barbets, Australian Labradoodles, Danish-Swedish Farmdogs, Mudis, and Koolies were some of the least inbred, and they had rates of vet care comparable to mixed-breed dogs.
Although every dog's health is unique to themselves, over the years, many people have discovered that mixed breed dogs are often healthier than purebred dogs. Because purebred dogs have a limited gene pool, genetic disorders that arise continue from generation to generation.
The most obvious effects of inbreeding are poorer reproductive efficiency including higher mortality rates, lower growth rates and a higher frequency of hereditary abnormalities. This has been shown by numerous studies with cattle, horses, sheep, swine and laboratory animals.
Smaller populations become inbred more quickly, so the simplest way to reduce the rate that inbreeding is to maintain a larger population of breeding animals. The easiest way to do this without producing an oversupply of puppies is to increase the number of different sires being used in breeding.