First, let’s define what makes a cat calico. The term calico doesn’t refer to a cat breed but a color pattern. Calico cats and tortoise shell cats have coat colors that are variations of orange, black, and white. Calicos have their colors as distinct blotches of color while tortoise shells will be more mixed.
So why are they female? The answer is in the genetics. In its simplest sense, the coat colors are connected to the X gene. Females have two X genes and males have one X gene and one Y gene. Because the girls have two X genes, they can show orange, black, and white. The boys can have black/white or orange/white coats, but not all three colors combined.
Leave it to the ladies to be fashion conscious!