* Calico cats are female: Calico coloration is a result of the X chromosome. Female cats have two X chromosomes (XX), while males have one X and one Y (XY). The genes for black, orange, and white fur are all located on the X chromosome.
* The Dad's Role: Your orange tabby dad cat carried the orange fur gene on his X chromosome. Since he's male, he only has one X chromosome.
* The Mom's Role: Your dark tabby mom likely carries both the black fur gene and the orange fur gene on her two X chromosomes. This is common, even if she doesn't visually appear calico.
* How Calico Kittens Happen: When your mom and dad had kittens, here's what could have happened:
* Each kitten received one X chromosome from their mom and one X chromosome from their dad.
* The calico kittens received an X chromosome with the black fur gene from their mom and an X chromosome with the orange fur gene from their dad.
* The white patches are a result of a separate gene that determines white spotting patterns.
In short, you got calico kittens because your mom carried both the black and orange fur genes, and your dad contributed the orange fur gene to his daughters.