Final answer:
Tortoiseshell/calico cats arise due to X chromosome inactivation in female cats with two different coat color alleles on their X chromosomes, resulting in a variety of color patches. This pattern does not occur in male cats as they have only one X chromosome.
Step-by-step explanation:
The unique coat of tortoiseshell or calico cats is a result of X chromosome inactivation during the embryonic development of female cats. In these cats, the gene for coat color resides on the X chromosome. As female cats have two X chromosomes, if they possess two different alleles for coat color, one of the X chromosomes will be randomly inactivated in each cell. This random inactivation leads to patches of fur expressing one allele or the other, resulting in a mosaic of coloration typical of tortoiseshell or calico patterns.
Male cats, having only a single X chromosome, do not exhibit the tortoiseshell or calico phenotype. This is because they do not undergo X inactivation, which requires the presence of two X chromosomes.
In terms of inheritance, coat color in cats is an example of X-linked inheritance. Since the genes for black and yellow coat colors are on the X chromosome, and males have only one X, they can only be black or yellow, not calico, which requires both colors to be expressed.