Also, discerning coloration patterns in cats is a lot more complex than just the dominant/recessive gene inheritance.

Many phenotypes actually consist of polygenes, kind of like eye color in humans. That's why two blue-eyed people can have brown-eyed offspring, even if the basic rule of recessivity is apparently violated. This is probably what happened with your cats.

Additionally, a lot of the 'solid' brown/tan cats actually come from siamese.

All I can say is that genetics is an interesting field, and one I prefer not to mess with.
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Bob Daisy Emily