Final answer:
Female mammals heterozygous for X-linked genes show different cell patches expressing different alleles due to X inactivation, causing features such as varied fur colors in tortoiseshell cats.
Step-by-step explanation:
Female mammals that are heterozygous for an X-linked gene have patches of cells expressing one allele and patches expressing another due to a process known as X inactivation.
This phenomenon occurs when female mammalian embryos, consisting of a few thousand cells, inactivate one of their two X chromosomes in each cell.
This inactivation becomes a permanent feature in all cells derived from the initial cell, leading to mosaicism. In the example of tortoiseshell cats, which are typically female due to their two X chromosomes, heterozygosity for an X-linked coat color gene results in different patches of fur color. This is because each patch of fur corresponds to either the maternal or paternal X chromosome that is inactivated in that particular patch of cells.