Final answer:
The tortoiseshell phenotype in cats is caused by X inactivation during female embryonic development, resulting in heterozygous females expressing different coat colors in various body regions.
Step-by-step explanation:
The tortoiseshell phenotype in some female cats is due to the X chromosome carrying the genes for coat color. During the embryonic development of female cats, a process called X inactivation takes place, where one of the two X chromosomes is randomly inactivated in each cell.
If a female cat is heterozygous for an X-linked coat color gene, it will therefore exhibit different coat colors in different regions of her body, depending on which X chromosome remains active in the cells of that region. This process creates distinctive patches of color in tortoiseshell cats. Male cats, possessing only one X chromosome, do not show the tortoiseshell pattern as they cannot have two different alleles for coat color active.