Keep in mind that rat genetics aren't as clean cut as ball python genetics. Your berkshire rats could be carrying a whole slew of different coat markings. For example: I bred two black berkshires together, and out of that litter, I had 1.2 solid black females (nose to tail). I bred the male to the two females, and got two litters of mostly albinos, and one was solid blue, one was solid black. I bred the same three again, and have a litter of mostly solid blacks, one siamese point albino, and one blue.
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