Here's why:
* The X chromosome: The genes responsible for fur color are located on the X chromosome. Female cats have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
* Coat pattern inheritance: The calico and tortoiseshell patterns result from random inactivation of one of the X chromosomes in each cell. This leads to patches of different colored fur.
* Rare exceptions: It's possible for a male cat to have the calico or tortoiseshell pattern, but it's extremely rare. This happens when a male cat inherits an extra X chromosome (XXY), resulting in Klinefelter syndrome.
So, while a calico or tortoiseshell cat is very likely female, it's not impossible for a male cat to have this coat pattern.