Normally, only female cats have the tortoiseshell phenotype because

The answer to this question would be: a male inherits only one allele of the X-linked gene controlling hair color

Tortoiseshell phenotype appears when a cat has two different X-linked gene that controls hair color. This result as a combination of hair color. Male cat sex gene is XY. Normally male cat only has 1 X gene, thus only have 1 color and can't have tortoiseshell phenotype unless there is pathological multiplication in X gene.

Ithink the dna sequences may vary but the amino acid sequences are identical.