“Red cats are always male”, “White cats are deaf”, “Calico cats are always female”, myth or not? Last week we looked at how certain coat colours in cats are linked to being female and male. This week, we will have a closer look at the reason why white cats are more often deaf than cats with other coat colours.
White coat, blue eyes and hereditary deafness
White cats are more often deaf than cats with other coat colours, but it’s not just the white coat that might indicate an increased risk of hereditary deafness. Research has shown that in white cats, 17% to 22% are born deaf if they have two brown eyes. Once a cat has one blue eye and one brown eye (odd-eyed), which is called heterochromia, the percentage of white cats that are born deaf rises to 40%. Cats with two blue eyes have a 65% to 85% chance of being deaf. Cats that are deaf in one ear usually will not show any indications of their hearing loss, which might result in it going unnoticed by cat owners.
Genetics of hereditary deafness in white cats
Last week we already gave a basic introduction to genetics. We talked about what alleles are, how they can be dominant and recessive, and what chromosomes are. For this article, it will be useful to know what Melanocytes are. Melanocytes are cells found in the skin, eyes, ears and several other parts of the body and they contain Melanin. Melanin produces hair, eye and skin pigmentation. It’s also useful to know what a locus (plural:loci) is. A locus is the fixed position on a chromosome where a specific particular gene or a genetic marker is located.
When it comes to the white coat colour in cats, this can either be caused by locus C or Locus W. On the C locus you will find the C gene and/or the alleles for siamese, burmese and complete albino, making the allelic series for the C locus: Full colour(C) – Burmese(bs) – Siamese(cs) – complete albino(c).
On the W locus you will find the W gene and/or its mutations. The allelic series for the W locus is: Full white (W), much white spotting (wm), little white spotting (wl), and a normal coat colour (n). Both spotting alleles are often designated together as (Ws) white spotting.
The full white (W) gene is a masking gene which suppresses the replication and migration of melanocytes. This lack of Melanocytes means there will be a lack of Melanin and thus a lack of pigmentation resulting in a white coat.The (W) gene is called a masking gene because even though a cat might carry the genes for a specific colour and pattern, these won’t be visible from the outside as they are “masked” by the white coat. They can however pass this colour and pattern on to their offspring.
The gene for full white coat colour(W) is dominant, meaning that both cats that are homozygous (WW) or heterozygous (W/n – W/Ws) for this gene will have a white coat. Cats with genotype (Ws) will have white spotting which can range from a few white hairs to an almost fully white coat.. Cats that lack this dominant masking gene (n) will show the original colour and pattern of their coat , either with or without white spotting depending on their genotype. Possible genotypes for the W locus are:
- (W/W): Cat will have a white coat and most likely will have some form of hearing impairment. Research showed that nearly 75% of cats with two copies of the W gene(homozygous) were deaf and the rest had at least some form of hearing loss.
- (W/Ws): Cat will have a white coat, may have normal hearing or have some form of hearing impairment. Research showed that in cats with this genotype 1/3 had normal hearing, 1/3 had hearing impairment and 1/3 were deaf.
- (W/n): Cat will have a white coat, may have normal hearing or have hearing impairment. Research showed that in cats with this genotype 60% had normal hearing, 17% had hearing impairment and 21% were deaf.
- (Ws/Ws): Cat will have a normal coat colour with white spotting, there is no hearing impairment or deafness.
- (Ws/n): Cat will have a normal coat colour with white spotting, there is no hearing impairment or deafness
- (n/n): Cat will have a full coloured coat (unless Albino, which is determined on the C Locus) with no white spotting and will have normal hearing.
So how is the white fur and blue eyes related to being deaf?
As described above, Melanocytes are cells which contain melanin. They are found in the skin, eyes, ears and several other locations, and produce the substance that gives pigmentation. The dominant white gene (W) fully disrupts the replication and migration of melanocytes into the skin, resulting in the white fur as it will lack pigmentation.
The dominant white gene (W) can also fully, or partially, disrupt the replication of the migration of melanocytes in the eyes and ears. The lack of melanocytes in the eye(s), and in turn the lack of melanin, will turn them blue. In odd-eyed coloured cats the disruption only happened in one eye. Cats with a homozygous genotype for dominant white (WW), and cats that carry the dominant white gene (W) and the allele for white spotting (W/Ws) tend to have blue or odd eyes more frequently than those that have the genotype (W/n). And as can be seen above, those are also the genotypes that most often lead to some form of hearing impairment or deafness.
The hearing impairment and deafness happen because when the white gene disrupts the melanocytes in the ear, the inner ear will degenerate. This will lead to unilateral (one sided), or bilateral (two sided), full or partial hearing loss.
So what happens when a cat gets it white coat colour (and blue eyes) through Albinism? As the white coat of an Albino cat is acquired differently than cats that are white because of the White masking gene, they are at no more risk of being deaf than coloured cats. It’s true that Albino cats also lack Melanin, but the reason they lack this is of a different reason than with the (W) gene. In Albinism the melanocytes are present, but they lack one or more enzymes that are responsible for the melanin production.
My cat is deaf, do I need to take any special precautions?
If you suspect your cat might be deaf in one, or both ears, a vet can do a test to check. Cats that are only deaf in one ear, usually will show no signs and adapt without an issue. If your cat is deaf in both ears, they can still lead happy and fulfilling lives, and the only real precaution one should take is keeping them safe in situations where their safety depends on their ability to hear. This mainly means to not let them outside as they won’t be able to hear danger approaching. Deaf cats might startle easier as they won’t hear it when approached and will often depend more on vibrations to stay aware of their surroundings. When approaching a deaf cat, try to do this when in their line of vision. If this isn’t possible, for example while they are sleeping, try to alert them of your presence by using vibrations. Cats can also learn to understand hand signals to communicate.
Hereditary deafness and hearing impairment are connected to the dominant white gene (W), and through this article we hope to have explained the genetics behind it. Depending on how a cat acquired its white colour, and their genotype, they may or may not be more at risk of being deaf.