Back-Crossing of Mouse Strains

Notes

• As a standard, when mice are back-crossed >8 times under standard conditions they care considered pure. At this point they can either be back-crossed further or intercrossed for maintenance. It is better to keep back-crossing as many times as necessary.
• Pure Strain: Make sure that every iteration of a back-cross is to a pure strain, preferably purchased (see Ordering Mice) or recently purchased from a vender who is careful to maintain the purity of their strains. Generally we will use C57BL6/J (see http://jaxmice.jax.org/strain/000664.html) as it is the most widely available and commonly used strain in diabetes research.
• Maintainence of Wild-Type Mice: Wild-type mice can be purchased from a vendor and carefully inter-crossed. Generations of these mice should be tracked, and after 4 or more generations, a new cohort of wild-type mice should be purchased to avoid any potential genetic drift.

Setting Up a Back-Cross

• Generally we use female mice, and cross against a wild-type make since female mice are generally not used for experiments. Furthermore, since we cross against a pure male, then cross female pups to another pure male, the Y chromosome is always from the pure strain, and the X-chromosome is preferentially segregated to be from the pure strain.
• When ordering mice order mice at 9 weeks of age, let them acclimatize to the facility for a week then set up the breeding cage at that point
• For the next cross, generally take the first cohort of pups, and back-cross them again. This will make the process go faster.