I’ve found it quite hard to find information on rat genetics that’s relevant to New Zealand. We have a very limited number of colours and markings compared to other countries due to import laws so often the information online is confusing as it talks about genes that we don’t have here. The genetics of the three most common marking types – self, berkshire, and hooded – is straight forward though.
A self rat (HH) has no distinct white markings. Often their undersides are a lighter shade than their backs but they are basically all one colour. Berkshire rats (Hh) have white paws, white makings on their undersides, and they usually have white tail tips. Hooded rats (hh) have a coloured head and shoulders and a coloured stripe or blotches down their spine but the rest of their body is white. If a self is breed to a self they should produce get 100% self babies. Similarly, two hooded rats together should produce a whole litter of hooded babies. Two berkshire parents however should produce 25% self, 25% hood, and 50% berkshire. Punnett squares for these three crosses look like this:
There are three other pairings possible: hooded and berkshire, hooded and self, and self and berkshire. A hooded rat paired with a berkshire should produce 50% berkshire and 50% hooded babies, a hood and a self should produce a full litter of berkshires, and a berkshire and a self should produce 50% self and 50% berkshire babies. The Punnett squares look like this:
It’s not really that simple, and more in depth research suggests that the genetics denoting markings are not as pure as as they would seem from the Punnett squares, but they serve as a good estimate for what will come out of a pairing.