American Fancy Rat & Mouse Association

This article is from the Fall 1997 AFRMA Rat & Mouse Tales news-magazine.

Breeding & Stuff


By Nichole Royer

Mary Cameron, Hailifax Co. N.S.
Q I have been a rat owner for six months, and I enjoy it a lot. I think having a rat for a pet is really great, but there are some things that I am not sure of. Will a brother and sister rat mate and have babies? Will this kind of a breeding create babies with problems?

A Yes, a brother and sister rat will mate and have babies. Any female rat over 8 weeks of age can get bred (and sometimes even earlier). It is recommended that litters be separated, males from females, by the time they are five and a half weeks old. Often females are not fertile until they are 8 weeks old, but they have been known to become pregnant as early as 5 weeks. Males are also fertile when they are very young and will impregnate their mother and sisters if left with them for too long.

While accidental brother/sister breeding should be prevented, many breeders use this kind of close inbreeding to improve their animals. Contrary to popular belief, inbreeding (done carefully by serious breeders) is not in itself bad. Inbreeding serves to retain and pass on the good points in a particular family of animals, while eliminating the bad. When animals are inbred (particularly close brother/sister breeding), whatever recessive traits are present within the animals become visible. The animals which display good traits are retained for breeding, those which have undesirable traits are eliminated from the breeding pool. In this way all the bad traits are weeded out of the line.

Inbreeding can be a very useful tool; however, it is important to be very careful about not fixing problems into the line. In the process of selecting for good points, it is very easy to retain animals with bad ones. If these animals are used for breeding, these bad points can become cemented into the line. Along with conformation, markings, and color you must also select for temperament, health, and other features which are just as important.

Another use for inbreeding is to produce animals with desirable recessive traits. If you have an animal with a desirable recessive trait, the only way to make more of them is to inbreed. Take for instance a Blue rat. If you have a single Blue rat and want to make more of them, the easiest way is to breed to a Black. This will produce all Black babies; however, if you breed one of them back to its Blue parent, you will get more Blues.

Needless to say, brother/sister breeding is not the only kind of inbreeding. We typically consider close family breeding (brother/sister, mother/son, father/daughter) to be inbreeding. Another kind of inbreeding is called line-breeding. Related animals who are not as close (cousins, aunts, grandparents) are used. This method is less likely to show problems within a line; however, the results take much more time to accomplish.

Stories abound about inbreeding causing major deformities, horrible temperaments, and dramatic health problems. It can, but only because those traits were already present within the line, and by inbreeding, became visible. If you do not inbreed, those traits can be passed along unseen, to occasionally surface in individual animals. Judicious use of inbreeding can eliminate many of these problems; however, a responsible breeder will discontinue a line if they discover it produces a major defect.

This is not to say inbreeding is for everyone. It requires far more thought and dedication than just “putting two rats together.” Fanciers use inbreeding as a tool to improve a line of animals which they have been working on for many years. They know their animals, and know what they are likely to produce. *

Updated March 3, 2014