A cat showing patches of orange and more darkly pigmented fur in its coat. The sex-linked gene O is responsible for the conversion of eumelanin to phaeomelanin, which gives the fur an orange coloration. The O gene is epistatic to those autosomal genes, which give the coat a black or agouti color. Since the X chromosome is randomly inactivated in somatic cells during development, females heterozygous for O will show the tortoiseshell phenotype. The term calico is sometimes applied to tortoiseshell females that also have patches of white fur. Such females also contain the dominant spotting gene S.
Subjects: Genetics and Genomics.