Calico cat
A single X chromosome can have either a black allele of the coloration gene or an orange version, but not both. However, a female cat has two X chromosomes, so it can have both versions, black on one chromosome and orange on the other, making the cat a calico. A male cat usually has one X chromosome and one Y chromosome. A Y chromosome does not have a color gene, so the cat can have either an orange or a black gene, but not both. Calico cats are often used as an example in biology classes when discussing sex chromosomes.