(in response to a query on why dogs from some show dog breeders are often associated with health issues, while rats from show rat breeders are considered more healthy)
If you look at the issues in dogs, they tend to come from two different main areas:
1. Artificially small genepools. Purebreeding and pedigree rules demand that a breed of dog can’t be outcrossed to another breed, and that new blood can’t be brought in (any “outcrosses” are to other lines, who can be traced back to the same foundation stock). This means that once a genetic problem becomes apparent in a line, it often can’t be removed because there is simply no other animal to breed to that is free of that problem, and also in many breeds they simply are difficult to continue due to inbreeding depression (small litters, poor immune systems, etc). For example, flat coat retrievers are incredibly prone to histiocytic sarcoma, and over 50% of the breed dies of cancer – there is not the new blood available in the breed to “fix” this issue, because they are not allowed to breed to a mongrel or another breed in order to try and improve this due to breed purity rules.
2. Exaggerated conformation. In bulldogs, the flat face and exaggerated skin leads to breathing issues (among other things). This isn’t due to inbreeding as such – in theory, you could cross two entirely unrelated flat faced dogs and produce flat faced puppies – but it’s selection for an extreme phenotype. In calling for a body structure that is inherently unnatural and extreme, an animal that is unhealthy is produced. This is a problem both with the breed standard and with the look that is exhibited by the breeders and rewarded by the judges.
Neither of these issues are something we need to worry about in rats. We don’t have “breeds” as such – all of our different colours, markings, and coat types can be bred to each other. While people do maintain lines where we tend to mate similar types together, if we encounter an issue or notice a shortcoming in some aspect we can – and are encouraged to – cross out to a different line or even an entirely different variety to improve the rats. While the majority of the time breeders do only breed from lines that are “known good”, it is not uncommon for breeders to find a rat of an unknown background that has something to offer to the fancy and incorporate them into a careful breeding program of their own known lines, increasing and improving the available genepool.
Secondly, all rats are bred to the same conformation standard, and this is a very moderate standard. If you read the standard carefully, and go along to shows to see how it is interpreted, you will see that it rewards what is essentially a well sized, healthy, well muscled, fit, healthy rat with overall moderate features, and specifically disallows for shortened faces (which are something that humans seem to naturally be drawn to, despite them being unhealthy for the animal). A conformationally correct rat is a thing of great beauty, a work of art in some ways – but it’s also essentially an animal that is the epitome of health and good conditioning.