By Carol Beuchat PhD
This is an amazing and balanced compilation of studies on linebreeding and high COI with a focus on dogs. It’s a great and pretty easy read but just in case you don’t want to read it (and I’d highly recommend that you do!!!) I’ve highlighted some key quotes and graphics below.
Linebreeding effects puppy mortality and litter size
“For these breeds, those slopes are about 0.1, which means that an increase in inbreeding of 10% reduces litter size by about 1. If normal litter size is about 6, a COI of 30% - which is not uncommon in many breeds - would reduce the litter size by HALF.”
“(The litters) are likely to be smaller, less vigorous, have more birth defects and higher mortality, grow more slowing, have shorter lifespan, and of course increased incidence of genetic disorders caused by recessive mutations.”
“As inbreeding increases above 25%, the mortality increases, to about 30% for dogs with COI between 25-50%, and higher still as COI increases.”
Linebreeding also effects adult dogs’ longevity
Although this graphic is for Bernese mountain dogs specifically, “in Standard Poodles, dogs with inbreeding less than 6% live 4 years longer than those with higher COI, and the risk of bloat is roughly proportional to the increase in COI - a 10% increase in COI elevates the risk of bloat by about the same amount.”
“In Bernese Mountain Dogs, each 10% increase in COI reduces lifespan by 200 days. For a dog with COI of 30%, that's a reduction in lifespan of almost two years.”
Summary
“As fertility goes down and litters get smaller, there are fewer animals born each generation, and those that are have an increased risk of genetic disorders, shorter lifespans, and general lack of vigor.”
“This becomes a negative feedback loop that is called "the extinction vortex", and once a population heads down this path it can be very difficult to stop. Note again, that threshold for this vortex phenomenon is about COI = 10%.”
“Evidence of inbreeding depression and the other consequences of inbreeding are very evident - singleton litters, high rates of cancer in young dogs, allergies, and many other issues that seem to be "normal" in the breeding of dogs these days.
Advantages of Linebreeding
increased uniformity
“fixing” of desired traits and breed type
increased prepotency (ability to pass on traits to offspring)
Disadvantages of Linebreeding
lower fertility
lower “vigor”
birth defects
smaller size
fewer offspring
slower growth
higher offspring mortality
shorter lifespan
increase in genetic diseases
reduced “genetic potential” (ability to improve a trait)
I especially found the extinction vortex concept interesting!! Linebreeding doesn’t happen in the wild since there isn’t enough of an advantage to homogenization of that degree. A general rule of evolutionary bio is that if an animal has a deleterious trait that prevents them from reproducing, such as neural tube defects, they won’t have the chance to pass on their genetics and that deleterious trait will be removed from the gene pool, while deleterious traits that present at the end of life, such as cancer, stay in the gene pool. The combination of negative effects at birth prevent this from occurring naturally and the negative effects at maturity should prevent humans from choosing to linebreed, especially when studies like these are available.
I know this is so long and I’m sorry but I hope this makes sense and someone else finds it interesting!
