Breeding Mice - The Methods


The word 'inbreeding' has unpleasant connotations to many breeders. It is often thought of as being scary and something to condemn; a practice that only produces unfit individuals. Such prejudice often originates in the fallacious belief that inbreeding is simply the random mating of close relatives. This, however, is simply not the case. Mating siblings, or a parent and offspring together is only one form of inbreeding (the closest one). It is possible to divide inbreeding into three different grades -- close, moderate, or wide -- each with a different degree of proximity to the gene pool of the animal being bred.

Close Inbreeding

The closest possible inbreeding is to mate a parent and its offspring together (mother and son, or father and daughter). This way the mated animals share half of their genetic make-up as offspring inherit half of their genes from each parent. Almost as close a form of inbreeding is to breed siblings together, as they will have a similar, but not identical, genetic make-up.

Close inbreeding is used to fix desired qualities in the strain; for example, new colors or coat varieties, desired type or size. Although close inbreeding is an easy way to get uniform siblings with the desired qualities, it is also a way to fix unwanted qualities - for example lowered fertility and hereditary diseases. That is why close inbreeding is not a highly recommended breeding method in the long run.

Using close inbreeding is like putting all of your eggs in one basket. As long as the animals are healthy and pass on the desired qualities, all is well. However, if and when they start to pass on unwanted qualities, the whole strain can be ruined. If you decide to use close inbreeding, you must always carefully select the animals used for breeding and look for possible hereditary diseases or bad temperament. You should never use unfit animals for breeding.

Moderate Inbreeding (the "harem" system)

Much more moderate than close inbreeding is using one buck and several does, so that the offspring have a common father but different mothers. The doe and buck mated are thus always from a different dam.

Strain breeding (a.k.a. Line breeding)

One form of moderate inbreeding is strain breeding, where the breeder focuses attention on certain exceptional animals, usually bucks. This means that the founding buck of a strain will appear as many times as possible in that strain. Thus the same buck is the forefather of both parents several times in the pedigree of the offspring. Followed strictly, this can be a very misleading method which will not give the desired results. It is easy to forget that the offspring inherit half their genes from the dam. If the breeder does not monitor the quality of the females at all, the result is of mixed quality; the offspring may be good but they may be very poor as well.

Strain breeding combined with strict monitoring of the does, usually produces the best results. This means that only the best does are mated with the best bucks, so that the offspring have the greatest possibility of inheriting all of the good features of the strain.

Colour Strain breeding

One form of strain breeding is colour strain breeding, where in addition to the normal strain breeding the animals mated are of the same colour or genetically compatible. With rodents in particular, where great attention is paid to the coloration and markings of animals, colour strain breeding combined with appropriate out crosses is the most recommended breeding method.

Wide Inbreeding

When several bucks and does are used while forming a strain, it is called wide inbreeding. These are mated together, avoiding brother-sister matings and limiting the use of half-brother to half-sister matings.

Out crosses

An outcross is the mating of two animals from completely different strains. Breeding based entirely on out crosses doesn't often lead to good results; the quality of offspring can be vary varied even with the best animals. The main problem with using out crossing as a primary form of breeding is that an unreasonably large number of breeding animals are needed and often after a few generations the breeder is in a situation where many of her animals are related to each other to some extent (thus she is no longer out crossing).

Most breeders find strain breeding, improved with thoughtful out crosses, to be the most rewarding method. This means that the animal used to improve the breeder's own line (the outcross) must be of extremely high quality or must have some desired trait such as a new colour or coat variety. One must always be careful with out crosses, as along with desired qualities the strain may get unwanted problems such as inherited diseases or bad temperament.

A Breeder should never outcross randomly simply to see "what might happen" with a particular combination or "just to bring in new blood". Generally, if the outcross did not give the desired results, you should not use the offspring in your own strain.

About Forming a Strain

It is impossible to form a strain without at least moderate inbreeding, as this is the only way to come up with animals of even quality. A strain can be described as being:

" ... a group of animals, carrying similar qualities and producing similar offspring."

There are good and bad strains; some strains develop for the better some for the worse. The aim of a breeder is to maintain the highest level achieved or, better yet, to further develop and improve it. Many strains can be developed further into different branches, where the qualities of the original strain are maintained with a number of new qualities added.

It is easier for a rodent breeder to form her own strain, than for breeders of other livestock; rodents breed faster and take up less space than, say, horses or dogs. Where a bigger animal has had one generation, a rodent may have given birth to dozens. There is a risk in this, as using poor breeding methods can cause a major catastrophe over generations and ruin an entire strain. If this happens, the breeder seldom has the desire to breed any more.

- T. Cooke: "Exhibition and Pet Mice"
- R.Robinson, "Colour Inheritance in Small Livestock"

Text by Minna Koivu.
English translation by Satu Karhumaa.
Translation proof-read and corrected by Antonia Swierzy.
Published earlier: SMG, "Aavikkorumpu2/94"; SNL, "Haisulit 2/3-95"