Outcrossing, Linebreeding and Inbreeding in Rats (An Introduction)

By Elisabeth Brooks, © 2005 Spoiled Ratten Rattery

Breeding pretty much any type of animal to improve it will involve a combination of breeding styles. Choosing excellent animals to begin with and using inbreeding, linebreeding and occasional outcrosses while concentrating on your goals will help you to produce healthy, friendly, beautiful rats. . .what breeders all strive for, the whole rat!

Outcrossing

An outcross breeding is the breeding of two animals that are not related to each other. Generally, this will mean that they do not have the same or related animals in their pedigrees for four generations or more. Outcrossing is done to introduce new traits that are missing from a line, for instance, it might be done to improve eyes or in hopes of improving the size and/or length of the tails. It might be done to improve vigor in a line that has been inbred or linebred for many generations so that the litters tend to be small or the pups no longer have the strong health (vigor) or size that is normally seen in that line. It can also be done in an effort to "breed out" a particular trait such as a tendency toward tumors or diabetes. Considering that a great many issues are rooted in genetics, breeding an outcross does not eliminate a problem. It just spreads it out. It may eliminate it in time if the correct animals are chosen for breeding, the ones that somehow did not get the recessive gene. Often, though, it just hides that recessive so that it can show up later in the line (and in others with rats from that breeding).

When doing an outcross, think very carefully about which animal to bring in, why you are bringing him in and if he will complement the rats you are working with. You want to improve those few traits with the outcross, but you do not want to ruin the work you have already done. While you may want to improve eyes, you are also bringing in every problem that is in that rat's line and adding it to your rats and whatever weaknesses are in their line. You do not want to bring in something that is going to give you a whole new problem to "breed out." Know the rat you choose, and do not breed him to every rat you have. Choose a couple of mates from that line, breed him to them, choose the best offspring, one or two of each sex from each litter. When the time comes to breed from them, take time to breed only the best back to a rat or two from your original line. Test for problems. Look for them so that when they do show up, you can decide if the outcross was a success or if you will not be able to continue with those from the new rat. Do not put the rats from the outcross back to the rest of your original line until you are very sure the outcross was a success. If you move too fast, you could ruin your entire line.

If a rat seems to be nice in the area you are looking for but will be a step down for your rats in too many other areas, you will want to hold out for a better choice for an outcross. Remember the reason you are doing it, and if it cannot achieve what you need for your rats, there is no reason to do it. Every outcross can potentially bring in all sorts of problems. Know the rats you are working with, and get to know the potential new ones before bringing them in.

It seems a common practice in some parts of the fancy to outcross repeatedly (generation after generation) without testing for health issues and without stopping to evaluate the rats, without attempting to set desireable traits, etc. These are definitely not the rats to use to outcross to your own. They are not consistent, and they are almost as much a mystery health-wise as a rat you pick up from a pet store. Even when you do find a "sport" (a rat that is an exception to others in its family, usually a nice example but that will not be able to reproduce himself because he was basically a fluke) from a totally outcrossed line, he isn't going to be one to stake any hopes on. He is pretty much a roll of the dice, especially when it comes to health (each outcross could have brought in any number of things that were never checked since the breeder always outcrosses), and while he will be an outcross to your own, he could potentially bring in so many problems over the next several generations that it is not worth it to take the chance ruining your years of work. . . unless you have the time and space to devote to a whole new line, for the time being.

Linebreeding

This is breeding related rats and may involve inbreeding in order to work toward specific traits and consistency in rats. They may not be closely related (such as uncle x niece), but are often a concentration of a particular animal or family of animals (for instance, two or three siblings). In linebreeding, that particular exceptional animal might be chosen for a breeding, then his daughter bred back to him, a granddaughter from that breeding bred back to him or his brother, etc. Basically, it is concentration of related rats to make a line, to set traits the breeder wants in their rats. In the process of doing those breedings recessives carried in the line, good and bad, will most likely come to the fore so that the breeder learns exactly what is in the line. While linebreeding, the breeder will also set certain traits (again, good and bad), so he must be sure not to breed two animals together that have the same fault. One must also not breed related rats together just because of the pedigree. Look at the rats when making your choices taking into consideration health traits, temperament and physical features, and choose the correct rats for each other, not just because they are related and you really loved their grandmother.

Strengths and faults in the line will come out, so this is not a method to use unless the line (or rat) is strong to begin with. Rats can be linebred (or inbred) for many generations provided they are healthy and the breeder started with strong animals. There will come a time for an outcross to bring in traits that do not exist in the line (say, to improve size that is lacking in the line or to improve the arch of the back and hind end). When choosing an outcross, the breeder will look for an animal that is similar to the ones he already has, their strengths, but that has the line or size, for example, he is looking to improve. He will also check that there are not major health or temperament issues in the new rat's line and that any that are there are traits he is comfortable working with.

Inbreeding

Inbreeding is the breeding together (or continued breeding together) of closely related animals such as siblings or offspring back to a parent. Inbreeding does not put bad traits into a line; it reveals them, if they are there. It is also a quick way to set desired (and undesired) traits in a line.

A classic example of poor inbreeding is the standard feeder rat which is purchased from a pet store and is sick within days. That rat is not sick because of inbreeding. That rat is sick because there is poor resistance to myco running in his family, and the breeder chose to inbreed on them anyway. They chose to set that poor trait, one that already existed in the line.

Inbred rats, after a few generations, will be the most consistent in health, temperament and type, so if there are many negative traits in the rats, think carefully before inbreeding on them. If there are too many negatives, do not breed them at all, but then, that goes without saying with any animal or breeding style. Once these traits are set, it will be difficult to "breed them out." You want to set desireable traits while eliminating the less desireable ones. In laboratories, though, there are rats that have been inbred 40 generations or more and still have the size and vigor they had when they started. The lab chose the right rats to start from for the traits they needed. Those are not, necessarily, going to be the same traits a breeder looks for toward contributing to the fancy, but the premise is the same.

Choosing Animals to Breed

When choosing animals to breed, choose for the traits you want toward your club's goals. Look for the temperament that you prefer in your rats and that is acceptable in the show ring. Some prefer playful rats, some prefer docile ones, but either should be fine to show as long as the judge can handle them with no difficulty. No lines are perfect, but look for rats with little or no health issues, and whatever there is, be sure it is something you are comfortable dealing with. Every breeder has different comfort levels when it comes to potential health issues. Look for rats that look the way you like them to look but that still fit within your club's standards. It's not just about pedigrees; an animal is much more than the paper. Just because a breeder (or judge) likes their bucks to look a certain way does not mean you have to like it, too. As long as he is within the club's standards for type and does not have a health issue because of his look (such as a very short nose or problems with the spine), then he could be fine for a breeder to work with toward what the breeder likes and toward the club's standards.

It is good that breeders do not all breed the same thing or for the same exact look. It will help keep diversity in the fancy if breeders do not all go with the latest fad or think they have to breed the latest BIS buck (or someone related to him just because he won, no matter what they look like).

When choosing any breeding, sentiment needs to be put aside, as hard as it may be, in order that the breeder can honestly evaluate each animal, see his strengths and his weaknesses. When he looks at his animals, he may see that some should not be bred at all, but he will also see which have strengths that he can capitalize on with specific other rats while, hopefully, eliminating the weaknesses of both. A breeding is to improve on both parents.

The fancy is new in relation to other animals, but it is not so new, at this point, that we must still bring in pet store animals or wild or laboratory animals in order that we may have unrelated rats for outcrosses. There is much genetic diversity in the fancy. We can find quality outcrosses with serious breeders throughout the country and overseas. Further, breeders have been tracking the health and temperament of their rats for many generations, and most of the hobby breeders concerned with these things are registering their animals and tracking the information publicly so that others may find them when they need information about their rats or for when they are looking for an outcross to their own.

When working with a new line (less than four generations, still pretty new even at just four), do not spread them out too much right off by selling pups to lots of other breeders. If a problem arises in the new project, a breeder will be thankful that the rats are not already being bred all over the region or the country. The breeder also remembers that each time the rats go to someone new for breeding, new information will come up when they are bred (especially when they are outcrossed) there.

In rats, it is possible to improve them by "breeding up" (breeding to an animal much nicer, usually in type, than the other), however, it is a long process of doing that outcross, then working to fix all the negatives, then breeding up again. These breedings will sometimes produce "sports" in the beginning, but do not put too much into them. Work from them, if you like, but do not count on that particular rat as your basis. Be patient if this is the way you have chosen to go rather than getting exceptional, compatible mates from the start. It is going to take a long time to get consistently nice rats.

Putting it Together

Historically, and in more species than rats, successful breeders use a combination of linebreeding, inbreeding and occasional outcrosses to achieve their goals. Research, set your goals, start with the best rats you can find, make a plan for reaching those goals, and do not do a breeding unless it will help you reach them. Most of all, remember, it is about the rats, improving the rats, and it does not just stop with producing them. Take excellent care of them, and enjoy them as your pets while you work toward those goals, and you could have a "winning" combination.

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Last updated 11/2005

"Outcrossing, Linebreeding and Inbreeding in Rats" article and photos © 2005, E. Brooks of Spoiled Ratten Rattery. All rights reserved.