Final answer:
In tortoiseshell cats, coat color is determined by X-linked traits. When a heterozygote tortoiseshell cat mates with a black cat, the offspring will have a calico phenotype, with black and yellow patches.
Step-by-step explanation:
Tortoiseshell cats have a unique coat color pattern that is determined by X-linked traits. Females heterozygous for an X-linked coat color gene will express different coat colors in different regions of their bodies, based on which X chromosome is inactivated. So, for a heterozygote tortoiseshell cat with a black cat, the black cat would contribute one allele for coat color and the other allele would come from the tortoiseshell cat. The resulting offspring would have a calico phenotype, with black and yellow patches in their coat.