According to a report from the University of Helsinki which tested nearly 6,788 dogs of more than 233 different breeds for recessive genes linked to 93 genetic disorders, about one in six dogs are carriers of something, and about one in six disorder carriers were found in a dog breed where it had not been previously seen in evidence. The study, published in PLOS ONE, was led by Finnish company Genoscoper Ltd. in partnership with researchers from the University of Helsinki and Pennsylvania. and is considered the most comprehensive study on canine hereditary disorders so far.
What does this mean for dog breeders? Simple: the more you double down by line breeding and inbreeding, the more likely you are to see two recessive genes come together to create disease. Outcrossing does not guarantee the resulting litter will be free of disease -- you may hit the same recessive gene in the outcross -- but it lowers the chance considerably, to perhaps as little as one dog in 500.