Because their life expectancy is short and breeding is prolific, mice make excellent subjects for observing Darwin's theories on evolution. While captive deer mice can live up to 5 years, in the wild only about 5% survive the first year and the oldest live about a year and a half. Generally only the healthiest, smartest, fastest, strongest and luckiest survive long enough to reproduce and pass on their genes. Even casual observers can quickly see the effects of raising these mice in a domestic environment.
Genetic weakening occurs with mice raised in a safe (captive) environment. When mice are protected from the natural culling of the wild, genetically "weak" mice survive long enough to reproduce and pass their genes on. Even in one generation you can start to pick out the mice that wouldn't survive in the wild. This isn't necessarily bad for pet mice that will never have to contend with the rigors of wild survival. Some of these domesticated mice will develop traits that make them better pets, like being less excitable and easier to handle. Breeders often select stock for these traits that make good pets.
If living in captivity can produce traits that make better pets, what's the problem with genetic weakening and what can be done about it? Well along with more domestic traits come things like reduced intelligence, lower resistance to disease and all the undesirable traits that are no longer culled out. Selective breeding can help reduce genetic weakening. When deciding on which mice to breed, the selection process should consider a diversity of desirable traits, not just one trait like color. Mice that show undesirable traits should be excluded from a breeding program.
Many breeders use inbreeding to create strains of mice with particular
characteristics. A responsible breeder using this technique will understand
genetics; know their goals and the proper steps to accomplish them.
They will also be aware of the negative traits they may be promoting
and take actions to minimize them. If you plan to breed mice, here are
some things you should consider.
Many mice purchased from a pet store are already inbred.
A male and female purchased together are often brother and sister.
Many pet store mice are produced in mass breeding programs with no consideration given to the problems of inbreeding.
Many mice owners return their unwanted babies to pet stores to be resold. Often these returned mice are the product of inbreeding.
It's clear that in the long run inbreeding is bad, but how bad is it really. My experience with inbreeding is limited, but striking. I acquired Squeek with the intention of breeding her, she was caught wild and arrived pregnant. She had 3 healthy babies. Gordo (male), Mr. Earl (male) and C.C. (female). I accidentally allowed Mr. Earl to breed with his sister C.C. resulting in a double litter of 2 babies each. These are the results:
Apollo (male): Very quick and agile mouse. Died suddenly from unknown illness at about 18 weeks, previously looked very healthy.
Athena (female): Very quick and smart mouse. Developed diabetes, she did well on a controlled diet for over a year. Died suddenly at about 15 months.
Spirit (female): Slow for a deer mouse. Died suddenly at about 3 months.
Ghost (female): Slow for a deer mouse and a bit over weight. So far she has no other problems at age 15 months.
These are the only deer mice that I've seen develop any health problems. I have one other inbred mouse, Pumpkin, that was the product of a mother-son breeding. Pumpkin is doing well and shows no signs of any health problems. I have 13 other deer mice from unrelated parents. They are all over a year old and healthy.
I recommend that you avoid inbreeding without a solid understanding of genetics and a good reason to do so. If you choose to inbreed, only use high quality healthy mice. Out-breeding will generally provide healthier, smarter babies and is a much better choice for the casual breeder.