White cats represent a fascinating case study in feline genetics where a single mutation dictates their appearance. The true cause is a powerful genetic instruction that overrides all other color information, often mistakenly linked to conditions like albinism. This mechanism involves a complex interplay of cell migration during development, which also explains a potential connection to a particular health concern.
The Dominant White Gene: How Pigment is Erased
The all-white coat color is primarily caused by a single allele, referred to as the dominant white gene. This gene is classified as epistatic, meaning its presence overrides or masks the effects of other genes that code for different colors or patterns. If this dominant gene is present, the coat will appear completely white, even if the cat genetically carries instructions to be black, orange, or patterned.
The mechanism for this color suppression begins during embryonic development. All color in a cat’s coat comes from specialized cells called melanocytes, which originate in the neural crest near the developing spinal cord. To color the fur, these melanocytes must migrate from the neural crest to the skin and hair follicles. The dominant white gene interferes with this process, preventing the melanocytes from properly migrating across the body.
Because the pigment cells cannot reach the hair follicles, the cat’s fur grows without color, resulting in an entirely white coat. Only one copy of the dominant allele is needed for a cat to be completely white. If the cat does not inherit this dominant allele, the melanocytes are able to migrate, and the cat will display its underlying color.
White vs. White: Albinism and Spotting
The appearance of a solid white coat is not always due to the dominant white gene, as white coloration can arise from two other distinct genetic conditions. True albinism is one condition that results in a lack of pigment, caused by a different, recessive gene known as the C locus. Unlike the dominant white gene, which blocks the movement of pigment cells, albinism prevents the production of melanin itself.
True albino cats are rare and typically display pinkish or very pale blue eyes due to the complete absence of pigment in the iris. This condition also results in very light skin and heightened sensitivity to light. Importantly, albinism is not linked to the deafness often associated with the dominant white gene.
A third, more common cause of white is white spotting, or piebaldism, which is caused by an allele at the S locus. White spotting also involves a failure of melanocyte migration, but it is only partial, resulting in white patches or spots on a colored coat.
Although a cat with an extreme form of white spotting may appear nearly all white, the genetic instruction is for patches rather than the complete suppression seen with the dominant white gene. Consequently, cats with white spotting do not carry the same high risk for congenital deafness as those with the dominant white gene.
The Link Between White Fur and Hearing
The same genetic mechanism that prevents melanocytes from reaching the fur can also affect the development of the inner ear, creating a link between the white coat and congenital deafness. This phenomenon is an example of pleiotropy, where a single gene influences multiple traits. The problem stems from the shared embryonic origin of pigment cells and certain cells within the auditory system.
For proper function, the inner ear, specifically the stria vascularis, requires melanocytes. These pigment cells play a necessary role in maintaining the correct chemical balance of the fluid within the cochlea, the part of the inner ear responsible for hearing. When the dominant white gene disrupts the migration of melanocytes, these cells fail to reach the stria vascularis.
Without the necessary melanocytes, the stria vascularis degenerates shortly after the kitten’s birth, leading to sensory deafness. The risk of deafness is significantly higher in white cats with one or two blue eyes, a trait also linked to the gene’s effect on pigment in the iris. Studies suggest that the incidence of deafness among all dominant white cats is around 50%, rising to 70% in those with two blue eyes.
Deafness can be unilateral, affecting only one ear, or bilateral, affecting both. While the gene is fully penetrant for the white coat, its effect on hearing is less predictable, meaning not every white cat will be deaf. Deafness is less common in white cats that have amber or gold eyes, suggesting that the degree of melanocyte suppression in the ear is related to the degree of suppression in the iris.