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Inbreeding, linebreeding and outbreeding – what is what?

For me it is easy too see that most rat breeders have no real understanding of these essential subjects, and no wonder – these things are not as straight forward as some people may think! One thing many breeders are not informed about is that when you look for information there are different views of what is inbreeding and what is not. I have noticed three different views on what is inbreeding and what is outbreeding

  • one is the view commonly held in the pet rat world by many pet rat breeders
  • the second view is the practical one held in the laboratory by many animal care-takers
  • and the third view is the one held by the geneticists, the scientific definition

To complicate things even more there is a third way of breeding called line-breeding, and the views on what line-breeding is varies even more!

  • in the pet rat world there are many differents views on what line-breeding is
  • line-breeding is not used in the laboratory
  • line-breeding has no scientific definition that I know of but has been used to breed a wide variety of animals for a long, long time

Pet and show rat breeders' view of inbreeding

A pet or show rat breeder often seem to think that if you breed siblings together or an offspring to one of it’s parents then that is inbreeding. And if the same rat shows up two or three times (or more) in a normal pedigree (showing three or four generations back) that is NOT inbreeding but linebreeding. If you use this pet-oriented version of definition of the word inbreeding few pet or show rats are inbred.

Pet and show rat breeders' view of linebreeding

The most common view of linebreeding in the pet rat world is that linebreeding is a gentler form of inbreeding, which is only partly true. (The fact is that linebreeding is a very special form of inbreeding.) Since inbreeding is considered a no-no in the rat breeding world most breeders who inbreed thinks and says that they linebreed. Which is untrue! It is very uncommon that a rat breeder uses linebreeding.

I have seen the strangest ideas of what linebreeding is, most seems basically to stem from a misunderstandning of the name linebreeding. Since the name has the word “line” in it, people seem to think that linebreeding must be all about breeding a line. I even read an article that tells the strange view that linebreeding is when you have divided you breeding stock into two lines!

In fact linebreeding is more about pedigrees, and without pedigrees you can not really linebreed, only inbreed.

Practical lab view of inbreeding

Then you have the famous laboratory rats. Many pet rat breeders say that lab rats are short-lived, unhealthy and have a nasty temperament. At the same time lab rats are bred to be able to produce as many healthy rat babies as possible, and to be able to be handled by unexperienced lab workers without risk of getting bitten. All my lab rats except one that I never bothered to tame has been perfect pet rats, and I have owned many a lab rat over the years. When most lab rats are bred the practice is to breed only siblings, not cousins, parent-offspring or anything else – just siblings in every generation, for generation after generation, year after year. This is done because the researchers wants loads and loads of as close to identical rats as possible. These rats are as completely inbred as a rat can be! This is the common definition of an inbred rat at the laboratory. Such inbred rats are often much more healthy than any inbred pet rats, simply because they have been bred past the the stage where all diseases pop up many many years ago, and all disease-causing genes are already gone…Then there are some outbred laboratory rats. The only difference I’ve seen between inbred laboratory rats and outbred laboratory rats is that the inbred laboratory rats have parents that are siblings from the same litter and the outbred laboratory rats have parents that are still closely related but not siblings from the same litter. This means that any outbred laboratory rat should be viewed the same as an inbred laboratory rat from the pet breeders point of view. A true outbred rat can also be used in a laboratory, but I’ve never seen one, outbred mice I have seen though, so I’ll just descried how they are produced instead. To get an outbred mouse you mate a mouse from one inbred strain to a mouse from another inbred strain. But you don’t call these mice that are produced this way outbred mice, instead they call them outcrossed mice. This is just what they are called at the laboratory…

The geneticist's view of inbreeding

From the geneticist’s view the genes are important, or rather how many of the genes in the animal that are homologous instead of heterologous. So the geneticist do not use the term inbreeding as much, because it is not measurable. Instead they use the degree of homologous gene-pairs, often measured in percent. If you use this definition you will understand that inbreeding pet rats for a few generations gives more or less the same result as linebreeding for many generations, technically speaking.

Definition of linebreeding

Linebreeding has long been used for breeding birds, dogs, cats, horses and most kinds of animals we keep in captivity. The only definition of linebreeding I have found is published in a book called “Practical inbreeding for the pedigree livestock breeder” written by W Watmough, first published 1955. My copy of that book is the seventh edition. This is a popular book, and it should be! It describes almost all you need to know in order to breed good animals! I recommend every breeder to buy this book, wheather you breed German Shepherd Dogsses or pedigree birds. My friend calls it the “Breeding for dummies-handbook”, because it contains almost everything you need to know in order to be a successful breeder.

There is no strict definition, but generally speaking linebreeding has one very important element, and that is the foundation animal (or foundation pair). The foundation animal is supposed to a very good individual with many good points and few weeknesses. When you breed together animals that all have the same foundation animal in their pedigrees, in the hope that the offspring this way will resemble that same foundation animal – then you are linebreeding! If you own the foundation animal and can use it directly in your breeding program you will get faster results than if you do not own it, or it is already to old to breed from or (even deseased). As long as you mainly use pedigrees to decide which animal to breed with which you are linebreeding. When you are looking the animals themselves to decide which animal to breed with which you have swiched to inbreeding. Both linebreeding and inbreeding are the main tools behind every successful line of small animals like rats, cavies, mice, hamsters and the like.


People often talk about lines when it comes to animals. What is a line then? A line of animals are a bunch of animals, all related, and all having many similar traits. The animals from one line all look similar, and that is the point. They share most of the same good points (and also many of the same weeknesses). The only way to establish a line is to use some form of inbreeding. A breeder that avoids inbreeding at all cost may have really good animals, but he/she do not have a line!

Written by Eva Johansson, February 2004.