Final answer:
The tortoiseshell fur pattern in cats is a result of sex-linked inheritance and codominance of the coat color gene located on the X chromosome. Due to X inactivation, female cats can express this trait, whereas male cats cannot.
Step-by-step explanation:
The tortoiseshell fur pattern in cats is a classic example of sex-linked inheritance because the gene for coat color is located on the X chromosome. In the embryonic development of female cats, one of the two X chromosomes is randomly inactivated in each cell, leading to the distinctive patches of color seen in tortoiseshell cats. This phenomenon is due to the effect of X inactivation, a process where one of the X chromosomes in a female becomes nonfunctional in each cell, creating the mosaic of fur colors. Since male cats have only a single X chromosome and lack a second X to inactivate, they cannot be tortoiseshell.
Codominance is also at play here, where the orange gene (XO) and the black gene (XB) are expressed equally, creating patches of black and orange fur. However, this can only be observed in females that are heterozygous for the X-linked coat color gene, as they will express different coat colors in different regions of their bodies depending on which X chromosome is inactivated in certain cells.