Data from the Morris Animal Foundation Golden Retriever Lifetime Study show that inbreeding depression, the result of breeding closely-related individuals, reduces litter sizes in purebred golden retrievers.

The study, conducted by Morris Animal Foundation research partners at Embark Veterinary Inc., was one of the first to examine genetic measures of inbreeding in domestic dogs rather than using pedigree-based estimates. The team recently published their results in the journal Mammalian Genome.

"This scientifically proves something we've known anecdotally for a few years; that fecundity, or the measure of how successfully a dog can reproduce, is threatened by inbreeding," said Dr. Erin Chu, Senior Veterinary Geneticist at Embark, a Boston-based dog DNA testing company.

"Breeders need to ensure that the dogs they choose to mate maintain diversity in their lineages to preserve healthy and successful breeds."

Since most purebred dogs are descended from a handful of ancestors, the degree of relatedness between mating pairs is often unknown, but likely closely related.

Researchers sought to genetically identify whether breeding closeness is associated with factors such as adult body size or litter size among female dogs used for breeding.

For the study, the team examined DNA and phenotype data from 93 female golden retrievers enrolled in the Foundation's Golden Retriever Lifetime Study. All the dams were reproductively intact and had been bred at least once.

In addition to the dams' basic biological information, the team analyzed data that captured every aspect relating to the dams' reproduction, such as the timing of their heats, successful conception rates and how many puppies survived to weaning. The researchers evaluated the associations of all these data points against a genomic coefficient of inbreeding, which measures how closely related a dam and sire are. All but one of the associations was statistically insignificant.

The team discovered that the degree to which a dog was inbred influenced the number of puppies it birthed. They found that, on average, a dam that is 10% more inbred than another will produce one less puppy per litter.

Dr. Chu said this work sets the stage for larger analyses to investigate genomic regions associated with fecundity and other measures of fitness, such as negative behavior, mortality and longevity.

"There are definite repercussions to being more inbred with every generation and we want to minimize those as much as possible," said Dr. Janet Patterson-Kane, Morris Animal Foundation Chief Scientific Officer. "This is something to keep in mind to ensure we have healthy breed populations for years to come."

The Golden Retriever Lifetime Study is the most extensive, prospective study ever undertaken in veterinary medicine. Launched in 2012, and reaching full enrollment in 2015, it gathers information on more than 3,000 golden retrievers from across the United States, throughout their lives, to identify the nutritional, environmental, lifestyle, and genetic risk factors for cancer and other diseases in dogs.