Here's why:
* Different species: Deer mice (Peromyscus maniculatus) and fancy mice (Mus musculus domesticus) belong to different species and have different genetic makeup.
* Different genera: Deer mice belong to the genus *Peromyscus* while fancy mice belong to the genus *Mus*. This further emphasizes their distinct evolutionary paths.
* Reproductive isolation: Even if they were to mate, their offspring would be sterile or unable to reproduce. This is a common biological barrier between different species.
Therefore, it's not possible to intentionally breed deer mice and fancy mice, and it's important to understand that these animals are distinct species with their own unique characteristics.