Final answer:
The tortoiseshell phenotype in female cats is due to X inactivation, where each cell randomly inactivates one X chromosome, resulting in variegated coat color depending on the active allele. This phenomenon only occurs in females since males have a single X chromosome.
Step-by-step explanation:
The tortoiseshell phenotype in some female cats is the result of X inactivation, a process that occurs during the embryonic development of female mammals. For cats, the gene for coat color is located on the X chromosome. In tortoiseshell cats, this gene has two different alleles that influence coat color. Each individual cell will inactivate one of its two X chromosomes randomly, which means that the active X chromosome may carry either allele for coat color. This results in patches of different colors across the cat's fur, which is the signature tortoiseshell pattern. This can only be seen in female cats, as they have two X chromosomes. Male cats, having only one X chromosome, cannot express this phenotype, except in rare cases of genetic abnormalities where the male cat may have an extra X chromosome.