This article is from the Fall 2002 AFRMA Rat & Mouse Tales news-magazine.

By Nichole Royer

Pamela Martin, Tinley Park, IL
Q My son and I have been working on two projects for a Science Fair.

1. Distribution Factor: Trying to create a black and white Dutch marked from a black and white mixed marked by selectively breeding to produce similar color distribution. We have been breeding father to daughter and mother to son. Results so far are very pleasing. I’m having trouble getting rid of the left mid-body spot. I’ve tried brother and sister mating but I’m not pleased with the results.
2. Mendelian Law: We have two purebred tan-bellied blacks who differ in one characteristic (short vs. long hair). Our plan is to selectively breed individuals so that long hair becomes an inheritable characteristic at a better ratio than 3 to 1.

We would appreciate any information you may have that might be helpful with our projects. Thanks in advance for your assistance.

A Distribution Factor: Attempting to create Dutch Marked mice is one of the more challenging projects out there when you do not have a group of Dutch mice to start with. It sounds like you are going about it the right way. Very close inbreeding (mother/son and father/daughter) of like marked mice will set the pattern faster than any other system.

What happens with very close inbreeding, is that over several generations, the number of possible combinations of genes available in your breeding group is narrowed considerably. This creates a breeding colony of very similar animals. In this case, you are breeding to set a specific pattern. By eliminating the mice who do not fit that pattern and continuing to inbreed the mice who do, you see a much smaller variation in pattern with each generation. This will be true for any characteristic or group of characteristics you choose to select for (eye size, color, pattern, type, ear shape and placement, health, etc.) Even when bred with an unrelated animal, you will still see a consistency in what a closely inbred animal produces, simply because you have narrowed down the possibilities of what it can pass along.

This, of course, makes this kind of inbreeding a very valuable tool. There is a downside however. If not done very carefully, you are just as likely to set undesirable traits (faults). This can be anything from small ears to poor health, bad temperament, and even a high incidence of tumors. It’s a balance—close inbreeding causes the doubling up on of whatever characteristics are present in those mice. The challenge for a breeder is to use it as a tool to double up on the good characteristics while eliminating the bad.

This is not an easy task, and it is the reason most folks do not breed too closely or for too many generations. Many reduce the risks by using line breeding (breeding of less related animals i.e. cousins, uncles/nieces, etc.). This poses far less risk, but also produces far slower results. Used correctly and carefully by a responsible breeder, however, close inbreeding is a very valuable and potent tool.

What you are facing is one of the downsides of inbreeding. You have unknowingly set an unwanted trait (that extra spot) along with the ones you were aiming for. Essentially all of your mice are now programmed to produce that left mid-body spot.

Ideally, you would get a fluke where a particular mouse would be born out of your group who did not have that spot and you could then inbreed on that particular mouse. It does not sound like that is likely to happen in your case however.

Since all of your mice are programmed to produce this characteristic, you will need to bring in a mouse from some other line which does not produce that trait. Given an ideal world you would be able to go out and find a closely related line of mice selected for the same characteristics as yours but which does not produce that unwanted spot. This would give you a line breeding which would introduce the “lack of spot” characteristic, without adding other unwanted genes.

The chances of finding just such a mouse are likely to be slim to none however. The only time that works is with groups of fanciers who are all breeding their own lines of mice, but whose mice share a common ancestry. Since this is the case, your best bet is to go on a mouse hunt for a marked mouse who does not have a spot in the unwanted area, and whose markings are not too far off from what you are aiming for. You may have to settle for very poor markings to get that illusive “non spot.”

You will be doing what is called “introducing an outcross.” That is, bringing a totally unrelated animal into your line to “introduce” the characteristic you want (or in this case don’t want) into your mice. Like close inbreeding, outcrossing has its risks. If you had the option to outcross into another line of inbred mice, their breeder would be able to tell you all about the common faults and weaknesses within the line. Once again, however, you would need another fancier breeding for similar objectives and goals.

As it is, you will have to take a shot in the dark. You will need to locate a marked mouse from a responsible source which does not have the spot you are trying to eliminate. If at all possible, you will want to choose a male from a breeding group of mice which do not have such a spot. Please carefully quarantine him before introducing him to your animals. This is one way many mouseries and ratteries have introduced devastating diseases.

Take this mouse and breed him to the best marked of your female mice. What they produce will be disappointing. Be prepared for it . . . the first generation out of an outcross produces a mish-mash of characteristics. It will give you a basis from which to work however. Keep the best marked animals (most likely they will be very poorly marked) who do not have that unwanted spot. Breed them to each other and back to their mothers. This generation you will see a big improvement in consistency over the last one, and you should have mice who do not have that spot. Continue inbreeding, selecting for the correct pattern and the lack of the left mid-body spot. It may take 3 or 4 generations to get back to the consistency of markings you had before, but you will be producing some animals who lack that spot.

Mendelian Law: This is a fun and easy project. The long haired characteristic is a simple recessive trait. This means that a mouse has to get one long haired gene from each parent in order to show the characteristic themselves.

We will assume (just to make things easier) that your short haired mouse did not inherit the long haired gene from either of its parents.

Breed your long haired mouse with your short haired mouse. All the resulting babies will be short haired since only one parent was able to give them the long haired gene (and they have to have two in order to be long haired themselves).

Take these babies and breed them back to their long haired parent. Approximately three-quarters of the babies should be long haired. I say “should,” simply because genetics are always a roll of the dice. If you “rolled the dice” (i.e. bred the mice) a large number of times, your results would be about three-quarters long haired. However, just like rolling dice, sometimes genetics do odd things. Just as it is possible (though not probable) to throw snake eyes five times in a row, it is possible to get an unusual number of short or long haired babies in a particular litter.

If you then take the long haired babies and breed them together, you will produce litters of babies who all have long hair.

Back to top