Let’s talk about inheritance. You can go back to Gregor Mendel, who did a lot of work with Pea plants learning how genes are passed on, but lets keep it simple. Let’s use the gene for eye color as an example. Ignoring Ruby eyes for now, mice have either black or pink eyes. A gene is made up of two parts, and the mouse (and everyone else) gets one part from Mom and one from Dad. These parts are called Alleles, and some alleles are what is called “dominant” to other alleles.
Imagine you’re playing that card game called “War”, where two people each turn over a card, and the higher card wins. The higher card is dominant, so it wins. The allele for black eyes is like an Ace, so if it’s there at all, it wins and the mouse has black eyes no matter what the other card is. It could be another black eyed allele, or it could be a pink-eyed one. The only way to get pink eyes is with two pink eyed alleles. Recessive genes are shy, they hide behind the dominant ones if they can.
So let’s imagine Mom and Dad both have black eyes. Mom could have two alleles for black eyes, or one for black eyes and one for pink eyes. Same for Dad. The babies each get one of Moms alleles and one of Dads. So they’ll all have black eyes unless both Mom and Dad have a shy recessive allele for pink eyes, and then about ¼ of the babies will randomly get that double pink-eyed combination and turn out with pink eyes. Freaky! If the mouse has pink eyes, though, you always know it has two pink eyed alleles, but if it has black eyes, all you know for sure just by looking is that it has at least one black eyed allele.
If you know what mom and dad looked like, though, you can make some logical deductions. If Mom has black eyes and Dad has pink eyes, what can you predict about the babies? Well, any black eyed babies had to get a black eyed allele from Mom (because they can’t get it from Dad—with pink eyes he doesn’t have one) and a pink-eyed allele from dad. So we know those babies all have that shy pink-eyed gene hiding in there that could show up in their children or grandchildren. You can also look at the babies and make some guesses about whether Mom has that shy pink eyed allele. If there were any pink-eyed babies, you know they got pink-eyed alleles from both Mom and Dad, so she has it. If you don’t get any pink-eyed babies, well, you can’t ever really be sure in this case that it wasn’t just chance, but the odds are, if you got 12 black-eyed babies and none with pink eyes, that Mom doesn’t have that allele.
The same idea can be applied to the brown and blue genes, too. Brown and Blue are both recessive, like the pink eyed allele. So the mice can have them without being brown or blue, and these colors can show up in the babies as a surprise. If you keep track of what the moms and dads looked like for a few generations, you can start to make some good guesses about what color babies are possible, and plan for it.