A calico cat is defined by its tri-color coat, which includes patches of white, orange, and black or their dilute variations. Yes, calico cats can have blue eyes, but this combination is rare. The presence of blue eyes is not related to the coat pattern itself but results from inheriting a specific, independently acting gene. This striking pairing requires the cat to possess the sex-linked genes for tri-color fur and an autosomal gene that overrides pigment production in the iris.
The Genetics Behind Calico Coloration
The distinctive patchwork coat of a calico cat is a visible manifestation of X-chromosome inactivation, or Lyonization. The gene responsible for orange and black pigments is located on the X chromosome. Since female cats possess two X chromosomes, they carry two different color alleles. Early in development, one X chromosome in each cell is randomly silenced, creating large, distinct patches of orange or black fur as the cells divide.
Because males typically have only one X chromosome, calico cats are almost exclusively female. The third color, white, is caused by the separate White Spotting gene. This gene inhibits the migration of pigment-producing cells (melanocytes) to certain areas of the skin, resulting in the white patches.
The Mechanisms of Blue Eye Color in Cats
Blue eyes in cats are an optical effect caused by a lack of melanin in the iris. When light enters an iris lacking pigment, it scatters, making the eyes appear blue. This pigment deficiency can be caused by several different genetic pathways, none of which are related to the X-linked calico gene.
Genetic Pathways for Blue Eyes
The Dominant White gene (W) acts as a master switch, masking all other coat colors and often resulting in blue eyes. The White Spotting gene (S), when expressed at a high-grade level, can also affect eye pigmentation. Both the Dominant White and high-grade White Spotting genes inhibit the movement of melanocytes from the neural crest to the developing eyes. If these pigment cells do not successfully colonize the iris, the eyes remain blue.
The Specific Link: When Calico and Blue Eyes Coincide
A calico cat can only have blue eyes if she possesses the Dominant White gene or the White Spotting gene expressed at a level that affects eye color. Since calico coloration requires the White Spotting gene for the white patches, high expression of this gene can simultaneously inhibit pigment in the iris.
The Dominant White gene is a more potent cause of blue eyes and is responsible for the rarest calicos with this trait. This gene inhibits melanocyte migration to the eyes and the inner ears, explaining the association between blue-eyed, white-spotted cats and congenital deafness.
Cats with two blue eyes due to the dominant white or high-grade white spotting genes have a 65% to 85% chance of being deaf, either partially or fully. This deafness results from the same melanocyte deficiency that causes blue eyes, as these pigment cells are necessary for the proper function of sensory hair cells in the inner ear.