Gene expression can be define as a process that occur in every living cell of the organism. It is a process in which information carried by the gene is used for the synthesis of a functional gene product. These products are proteins, but from the non-coding regions of the gene products like transfer RNA (t-RNA) or small nuclear RNA (sn-RNA) can also be produced.
The black fur in mice is probably more the dominant-inherited trait. The white fur is probably a recessive gene so less of the baby mice will have white fur.