I've known about line breeding for a while and it's benefits. I'm always stunned when other people go "blah blah it's bad". If only because even in humans inbreeding doesn't result in issues most of the time unless done chronically and repeatedly.

Essentially with a pure-bred animal you will know everything about it (behaviour, health, ect) but with a crossbred animal you have no idea what you are getting.

And cheers for trying to squash all those silly misconceptions, :D

Personally the only times I have found inbreeding to cause problems is with inexperienced 'breeders', mostly from too much breeding between direct siblings.

I don't completely agree with the argument that without inbreeding undesirable recessive effects might be carried on. It only applies when Mendelian inheritance works.
By Mendel's definition, recessive traits don't show up when the animal is heterozygous. With inbreeding, the probability of getting homozygous animals, which show the trait and can therefore be excluded from future breeding, is increased.

However, in the real world, not only Mendel's laws of inheritance exist. There is for example the possibility that gene defects can accumulate. Take Chorea-Huntington for example: The disease comes from a pice of DNA that is repeatet too often, making it fragile and causing neurons to degenerate. The longer the repeated part is, the earlier the sympthoms show up. In humans, this can be from very old down to the age of for.
Now to spread the disease to the next generation, none of the parents needs to be sick. It is enough for one of the parents to have a "premutation" with a allready quite long repeated part. It gets combined with the regular part of the other parents healthy part in the child and the sympthoms will show up. Like this, over the generations the repeating sequence can get longer and longer, making the symphtoms show up earlier and stonger.

There are other diseases with similar effects. If you cross two carriers, the disease will get worse. Through this kind of inheritance inbreeding CAN cause new diseases, because it speeds up an effect, that could take centuries otherwiese. If it's not a lethal disease, but for example a weaker immune system or a higher propability to develop cancer, you won't notice it until a generation is born that is extremely weakened. But then it's too late.

Mendel's laws are important in genetics, but there is much, much more in genetics. I don't know whether this effect I discribed is the cause of inbreeding depression, but it seems likely to me: Individuals gained through careful inbreeding show the desired traits, but nevertheless tend to be weak and die early. This is a reason why direct inbreeding is forbidden for many races of dogs.

One thing I forgot about the inbreeding depression: With cattle and dogs it is generally considered normal that animals have weaker health the more homozygous they are - even if they don't show any sing of inherited diseases.

Individuals gained through careful inbreeding show the desired traits, but nevertheless tend to be weak and die early. This is a reason why direct inbreeding is forbidden for many races of dogs.

This does not appear to be the case with rats.
The healthiest and longest lived rats in the UK are those from breeders who inbreed. Those who don't? Their rats don't even show up on the radar of 'great' rats in the UK.

Dogs are a bit of a different issue to rats. While I believe that inbreeding in dogs has benefits too, we've been messing with dog breeding for thousands of years. We've had a lot longer to mess it up. The rat fancy is not at that point yet.
With rats, the domesticated rat has only really been in existence for around 100 years. Good ethical breeders who work tirelessly to improve health and temperament and life span have only really been at it for about half that time (not to say there weren't ethical rat breeders prior to that, but now, it is VITAL to do things 'right' with rats, and anyone throwing two unknown rats together is very frowned upon)
The rat fancy is not a fraction as huge as the dog fancy; The truly committed, dedicated rat breeders in this country who really do things completely right and with the best morals are in double digits. With dogs, far higher.
Also, dogs have been bred in a different way to rats. Dogs have been bred a lot for their function as a working animal, and their appearance.
Dog breeders will sometimes breed from an animal that perhaps doesn't have the best health, but is a stunning visual example of the breed, ie, they'll omit health concerns in favour of a better looking animal.
Not all, of course. But certainly a fair few. The dog show business is huge, and lucrative if you do well.

With rats, as well as only having been bred properly for a very short time, they're not bred to anything like the standards dogs are.
Yes, rats do have conformational standards they should adhere to, just as dogs do. There are standards set out for how a certain variety of rat should look, just as with dogs, and standards for the general look of a rat: its not desirable to breed rats that are scrawny/small, have small eyes, have short tails, have noses that are too pointed OR too blunt.
These would be 'faults' on the show bench.

But the difference is that a good rat breeder will ALWAYS choose health and temperament over appearance on every single occasion.
A good rat breeder can have a rat that matches the NFRS standards 100%, perfect type, markings, everything but if that rat shows a health issue like cancer or respiratory weakness, they will NOT breed from that rat, regardless of how visually amazing it is.
Dog breeders, even good ones, are not so universal on this.
If a rat breeder chose to breed looks over health and temperament, that's it, they'd be lynched by the rat fancy and may as well give up altogether.
But dog breeders can do this, and still be considered 'good' breeders.
One of the main reasons for this is that there is no money to be made in having great show rats. There is no money to be made in selling the offspring of your fantastic winning show rat. Therefore, rat shows, while very capable of being bitchy at times, are generally more about fun and rat PR. There isn't money to be made, so the emphasis is far more on breeding healthy rats rather than beautiful rats.
If you can do both, that's ideal. But health, temperament and longevity take precedent over looks every single time in good rat breeding. Not so for dogs, sadly.

The rat fancy is so small, compared to that for dogs and cats, that we are able to have much more control over what should and shouldn't be bred.
If you look at the state of the dog world: dogs who can't breathe properly, dogs who can't walk, dogs with deformities that are actually promoted, its horrible.
It would be a cold day in hell before the rat fancy ever became like that.
Any mutation produced in the rat fancy which is deemed harmful to the animal or that would lessen that animal's quality of life is shunned, and people try to nip it in the bud.
We have no short-legged rats, as we do with cats and dogs. We have no brachycephalic rats, as we do with dogs and cats. The main 'mutations' we have is the dumbo ear (which took a LONG time to become accepted and was seriously resisted when it first popped up as the rat fancy needed to be certain it was not harmful to the animal before it would support breeding) the rex coat (which causes no health issues nor is it linked to any nasty issues) and the hairless.
Hairless rats are still frowned upon in the rat fancy now, and you are not allowed to show this variety at NFRS shows or even bring one into the building of a show as it is considered 'promoting' the variety. Hairless rats are still a big bone of contention in the rat fancy as some of the genes that cause hairlessness in rats (and there is more than one gene) are linked with an inability to lactate, and a lowered immune system, among other issues. A lot of people want to see them phased out completely, others promote working with the variety to breed out the health issues it can have.
But they are the main 'mutations' you see in rats. A very small amount when compared to the huge amount dogs are allowed to have, and even encouraged to have.

You can also get tailless rats, and dwarf rats.
Both varieties are seriously frowned upon here in the UK, and you would be hunted down and ripped a new arsehole by every other rat fancier if you dared to breed these varieties.
There was talk of someone having bred a pug faced rat a few years but, but again, it was nipped in the bud instantly and nothing more came of it.

So really, the rat fancy is far more intimate and has far more control over the animal it promotes than dogs or cats would ever have.

Say a mutation for huge, long basset-hound ears appeared in cats. The likelihood is that this would be bred from and become the latest 'fad', even if it caused a decrease in welfare for the cat. In rats, it would be fought against and pushed out as soon as harm could be proven.

With breeding anything, even completely reputably and responsibly and knowledgeably, you're GOING to have sick or unhealthy animals at some point in your breeding life. Its unavoidable, particularly with a species as sickly as rats.
Inbreeding has no more likelihood of increasing this chance than continual outcrossing. Most reputable breeders believe it has less chance. This is mainly why rat breeders inbreed: the results speak for themselves.

As I said, the best, most robust, healthy, long lived rats that are consistently produced in the UK are coming solely from breeders who inbreed.
In countries where inbreeding is frowned upon, they have a lot of issues reproducing this same success, and have far more sickly, short lived rats.

..... >.> The dog fancy is not so black and white actually. There are some people in some breeds who don't health test, who are just ribbon winners....and you know....every responsible breeder will tell you to stay away from them and exactly why. Responsible breeders are the ones donating their time, money, and effort into getting their dogs health tested, to take them to have samples drawn for the study of genetic diseases...driving hundreds of miles. At least here in the states in most breeds. The fancy in the UK seems very.....disjointed. -.- Considering that The Kennel Club bought into that stupid 'documentary' without even asking the breeders in detail and decided to make all these weird changes. You'd think a kennel club would...you know...KNOW dogs better than that and not listen to propaganda and lies. Considering they don't even recognize one of their own native breeds even though there isn't a 'population is lacking' excuse, I really shouldn't be surprised.

Most of what you say is surely right, especially when I take a look at the dog breeders around here. But there's always a fight between the "beauty" breeders and the "hunting performance" breeders. The hunting performance breeders want to ban beauty breeders from crossing their dogs with the hunting dogs because they claim that those dogs are sickly, have a bad character and can't be trained at all. For some smaller breeds, like fox terriers, this means only inbreeding remains.

But still, defects like getting special forms of cancer or weaker immune systems are common in all kinds of animals when they are inbred for a longer time. Maybe you are right and rats are simply not bred long enough to show those effects or their lifes are generally to short. But there's also a probability that defects show up after the animal has allready been bred and passed them on to the next generation.

In any case, it is surely not true that inbreeding doesn't raise the chance of getting of getting genetic defects. What it does is creating more and more homozygous animals. If there is a recessive defect in the line, the breeder will notice and not select those animals for breeding. Generally, inbreeding splits the next generation in very healthy and very sick animals. Breed the very healthy ones, and you get a more suitable animal. But this only works for some generations until things like the accumulation I wrote about show up. In the end it definitely increases the chance of genetic defects.

Like you said, if you try to breed special looks, like big ears, short legs and so on, this is not a porbability, but it is safe to say that you'll get other genetic defects in no time. If this is rarely done in rats, it might be another reason why it doesn't happen that often.

I'm going to ask a biologist how they do that with their knock-out rats. They are inbred over generations to show special genetic defects, but they have to remain stable, so they can be bred on in large numbers. So they have to do something against the accumulation of other genetic defects.

It's an interesting subject, even though I don't even own rats. I researched a bit and found German breeders who claim that inbred rats are smaller and more sickly. Huh. I think I rathere believe you there, because there is no reason why they should be generally more sickly.

Genetics are a complex matter and inbreeding isn't necessarily a bad thing, so I'm not opposed to it if the breeder knows what he's doing. Colby's line of pit bulls have been linebreed since the 1800's and they are tough-as-nails working dogs, not frail and weak creatures as many showbred lines are.

My only real issue with inbreeding is that it makes the inbred group more vulnerable to diseases. Meaning that if a new kind of virus hits the population, it affects the entire population because the individuals are so similar in their genetic structure. I can imagine that this would be especially sad for rat breeders, who keep their animals in large groups.

Thanks for this, it was actually pretty enlightening. Over here in North America, I know inbreeding gets a pretty bad rap (regarding dogs) because unscrupulous breeders do it without regard for improving the health of the dog, but rather that they can breed more dogs that way, therefore sell more puppies. So the end result is a lot of dogs that have some pretty horrendous issues, not just limited to physical health. A pretty good example of this is white Doberman pinschers. If you're not familiar, it's a good example of what poorly done inbreeding results in.

However, I didn't know that done right it can actually improve the health of the animal you're breeding, so that was pretty cool to find out.

White dobes are just tragic. Im getting a dobe next year, so its a breed I've researched extensively.
The white animals.....visually.....very stunning. But there is no way in hell I'd sacrifice health and welfare for an animal that just looked cool. I might be wrong but I don't think we have a population of white dobes in the UK, or at least, not a significant one. I've never seen one, and most sites that mention them seem to be USA based.
We must have a few here though, somewhere.

There is a difference between RESPONSIBLE inbreeding and IRRESPONSIBLE inbreeding. I think that is where people get their facts mixed up. Too much inbreeding, especially over a very long time, with absolutely NO outcrossing will create problems. By too much inbreeding I mean CONSTANTLY breeding siblings or other close family members to each other, over and over and over again. This tends to happen with irresponsible breeders since they don't watch who is producing what offspring and tend to just let it happen. This is generally true for pet store and feeder rats since they just let males and females stay in the same cage, no matter what until the doe gets pregnant. This can lead to many many siblings repeatedly inbreeding.

This is completely ignoring the GOOD aspects of inbreeding though. If done carefully, as you said, it can help eradicate issues and seal in good ones. Most people when they hear inbreeding automatically just to what I described in my previous paragraph.

I think this is exactly the problem.
And its a problem with the concept of 'breeders' in general. So many people just automatically get on their high horse and decide to be 'anti-breeder' because they lump all breeders into the same category.
There is a world of difference between a rodent mill set up where animals are bred back to back, without any knowledge or concern as to who is breeding to who or how often, or what is being bred in, and a good reputable breeder who has information on their lines going back many generations, who only breeds a few litters a year, who is working purely to better health and temperament, and who breeds rats for the love of the species and a desire to improve their welfare.
Some people seem to be so ignorant about the world of difference between the two. In the rat fancy, we call the first example a BYB, to differentiate.
But until people actually meet a good reputable breeder, they would probably assume they're all like that.
But the breeders I know, and have had rats off, are truly amazing people with so much passion and knowledge for these animals, and who create rats that live good, long lives without health problems. When you compare the average rat from one of these breeders with your average pet shop or BYB rat, the difference is staggering.

Being involved in rescue, I run into a lot of pro-rescue nazis who are absolutely incapable of ever accepting the idea of breeding, and insist all breeders are the same just because they've never met a good one, or haven't bothered to look for one.

I think people also get funny about breeding because they instantly relate everything back to dogs. I don't know why, but whatever species you're talking about, someone will relate it back to dog breeding eventually. And as I said in reply to levthan, the world of dog breeding is SO far removed from the world of rat breeding that there is virtually no comparison to be made.

THANK YOU

Pisses me off when people don't understand genetics.

In my anthropology class this was a much talked about topic where a majority of my class was all HURR DURR INBREEDING MAKES YOU RETARDED.

Augh. ><

I can't say Im an expert on genetics; some of the things some of these rat breeders come out with may as well be in Japanese to me! :P
BUT despite not being an expert myself, I read a hell of a lot of articles and posts from the top breeders in the UK and most of them word things in a way I can understand. To me, the biggest proof to me that inbreeding was beneficial was the number of breeders I highly respect, and know personally to be as devoted to rat welfare as I am, who are inbreeding to some capacity. And these people know infinitely more about genetics and breeding than I ever will. I know these people would never do anything that wasn't beneficial to the goal of breeding healthier, longer lived rats so if they support it so fiercely, it really can't be wrong.

Nah. They inbreed to get pure bred dogs that have the least amount of diseases and medical problems possible. It works the same way with most (if not all) animals. Inbreeding with genetics you KNOW are better than a gamble with those you don't.

Furthermore, it's suggested that every animal(including humans) and plants, carry about 3 lethal genes. So, chances are...there's always a risk no matter what, everyone/thing is a carrier of something.

Thank you for the article.

Out of curiosity, I've heard lab rats (I'm guessing control group) have longer lifespans, but weaker immune systems. Is this true? Has there been any interest in outcrossing lab lines with pet lines for longevity (if at all even possible)?

I'd like to get into rat breeding, but probably would not be able to for a long while if at all, but I don't know a whole lot about it.

I'd have liked to breed my Nick when he was alive because he seemed long lived, but he died over three and half years old of cancer that didn't show up till the last two months of his life, at which point I felt horrible for wanting to breed him at all. No respiratory illness though. How do rat breeders take into account later illnesses?

How long do rats live on average in the UK? I know in the US 2-3years is average.

I know this is a lot of off-topic questions, but I'm just curious. I'd love to see the day the average rat lifespan reaches somewhere around 5 years.