A Calico cat is not a breed but a distinct coat pattern characterized by three colors: white, orange (or red), and black (or a diluted form like blue or cream). This striking tri-coloration is a genetic phenomenon that occurs almost exclusively in females due to the location of the color gene on the X chromosome. Female cats have two X chromosomes, allowing them to express both the orange and non-orange (black) color alleles through X-chromosome inactivation, or Lyonization. The presence of white patches is controlled by a separate, non-sex-linked gene.
Common Eye Colors in Calico Cats
For the vast majority of Calico cats, their distinctive coat pattern has no influence on the color of their eyes. The genes that determine the orange and black fur patches are distinct from the genes that control iris pigmentation. Therefore, Calicos display the full range of eye colors common to domestic cats. The most frequently observed eye colors are shades of yellow, gold, amber, and copper. These warm tones are the result of moderate to high concentrations of melanocytes, the pigment-producing cells, within the iris. Green eyes, which result from lower levels of melanocytes, are also a common occurrence.
The Influence of White Spotting on Eye Color
The exception to the typical golden or green eyes in Calico cats is the occurrence of blue eyes or heterochromia. This phenomenon is not caused by the orange and black coloration but is directly linked to the white component of the Calico pattern. The white patches are governed by the white spotting gene, which prevents pigment cells from migrating to certain areas of the skin and fur during embryonic development. When this white spotting gene is highly expressed, such as in cats with extensive white fur, it can suppress pigment production in the iris. This suppression prevents melanocytes from reaching the eye, resulting in a lack of pigment that makes the eye appear blue. If the suppression of melanocytes occurs in only one eye, the cat will develop complete heterochromia, commonly known as being “odd-eyed.” The blue eye will lack pigment, while the other eye will display a typical color like gold, green, or copper.
Why Calico Genetics Don’t Dictate Eye Pigment
The genetic mechanisms controlling the Calico coat pattern operate separately from those determining the final eye color. The orange and black patches are determined by the O gene (Orange gene) located on the X chromosome, which controls the type of pigment produced in hair follicles. This X-linked gene, responsible for the mosaic coat, does not influence the production or placement of melanin within the iris. Eye color in cats is determined by a separate set of genes, primarily governing the density of melanocytes in the eye’s iris. The only point of genetic overlap that can affect eye color is the high-level expression of the white spotting gene, which is an autosomal gene separate from the X-linked color gene.