Final answer:
In female cats, X-chromosome inactivation during embryonic development leads to the mottled coloring seen in calico cats.
Step-by-step explanation:
The mottled coloring of calico cats is due to X-chromosome inactivation. In female cats, one of the two X chromosomes randomly inactivates in each cell during embryonic development. This inactivation leads to a tortoiseshell pattern if the cat has two different alleles for coat color. Male cats, on the other hand, never exhibit a tortoiseshell coat color because they only have one X chromosome.