Genetics
There are lots of great resources that explain in depth genetics but the best place to start is with Bleuming Tails Rattery's explanation. They break it down in a great way and they even have a pdf with everything.
Bleuming Tails Rattery - Rat Genetics
If you are just more curious to see genetics in action, we have rescued the old genetics engine from RatBreeding.com after they shut down. It's very outdated as it only has a limited number of variations included but it gives an idea of how things work.
Genetics Engine
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