Back in April 15, a member of TheCatSite.com started a thread about a kitten she had helped birth. The kitten had "offset eyes" and a strange forehead, she said. Her friend claimed it had Down Syndrome but she wasn't sure. She brought the question to our community and asked:
Can cats have Down Syndrome?
It's not just that kitten, of course. In recent years, social media seem to be bringing forward cats with unique facial features.
Grumpy Cat, aka Tardar Sauce, has reached unprecedented global fame thanks to her unusual face which her owners say is a form of dwarfism.
Lil Bub is another cat whose abnormal visage has led to stardom, including a huge social media following. Her lower jaw is too small for her face and she is toothless. In her case, it's believed to be an effect of a bone disorder, which also caused degeneration of the legs and difficulty walking.
Monty is a sweet friendly cat who has almost 300 thousand Facebook followers. He's a special needs cat with a unique look. According to his owners, the lack of nasal bridge bone and other facial deformities are caused by a chromosomal abnormality.
It's not just domestic cats that draw the media's attention. Kenny was a captive white tiger with a deformed face caused by unscrupulous breeding of white tigers for profit. The inbreeding caused Kenny and his littermates to suffer severe genetic problems.
Could these cats have Down Syndrome though?First, we need to understand what Down Syndrome in humans actually is. I'm afraid things are about to get just a little bit technical here...
All living things pass on genetic material from one generation to the next. The genetic material of all plants and animals is arranged in pairs of structures called chromosomes.
Each species carries its own number of chromosomes, usually (but not always) arranged in pairs. For example, alligators carry 16 pairs of chromosomes in each cell and so do cherries. Ducks have 20 pairs, while sweet potatoes have no fewer than 45 pairs of chromosomes! We humans have 23 pairs of chromosomes.
As genetic material passes on from one generation to the next within the same species, the number of chromosomes stays the same. Each parent contributes one chromosome of each pair so that their offspring once again has the total number of chromosomes its parents had, neatly arranged into pairs. Sometimes, something goes wrong and the offspring gets a total of three chromosomes, instead of two. This is called a trisomy and it almost always creates some form of genetic disorder.
In human beings such a trisomy can sometimes happen in chromosome #21. The result is the genetic disorder we call Down Syndrome. People with Down Syndrome usually have mental disability and distinct facial features, including slanted eyes, a relatively flat nasal bridge and a slightly protruding tongue.
The claim that some cats have Down Syndrome probably stems from the fact that there are cats born with unusual features, including slanted eyes, like Kenny the tiger had, a flat nasal bridge, such as Monty's or a protruding tongue just like Lil Bub's.
So, Do These Cats Have Some Form Of Down Syndrome?Down Syndrome is a trisomy of chromosome pair #21. Cats have only 18 pairs so they don't have a pair numbered as 21. Technically speaking, cats cannot have trisomy 21.
Can they have the equivalent though? Perhaps the equivalent genes are found in another pair of chromosomes and a trisomy of that pair could cause a feline version of the very same syndrome?
I asked Prof. Leslie Lyons, an associate professor at the University of Missouri School of Veterinary Medicine and Surgery. Prof. Lyons is a leading expert on comparative genetics and the head of the Feline and Comparative Genetics Laboratory. According to her, the answer is no. The genes on human chromosome 21 are found in a group as part of a larger chromosome in cats called C2. Because this chromosome is so large and has more genes than the smaller human chromosome 21, we do not have an equivalent of Down's Syndrome in cats," she said.
If a feline embryo were to have three copies of the C2 chromosome - the one containing the same genes which in humans are found in chromosome 21 - there would be far too many genetic problems for that embryo to survive. The C2 chromosome is too large for the body to sustain a chromosomal abnormality in that pair.
Prof. Lyons mentioned one trisomy that is well-known and well-documented in cats. It affects the pair of XY chromosomes. When a cat carries three sex chromosomes instead of a pair - XXY, instead of XY or XX - it is born as a sterile male. These rare males are sometimes born with a coat pattern containing both black and orange, such as a calico or a tortoiseshell. To have both colors in one cat requires two copies of the X chromosome, so calico cats are almost always female, except for the rare male carrying this trisomy. Other than the sex chromosome trisomy, Prof. Lyons could not point out any specific trisomies known in cats.
So, what's the bottom line?
There is no such thing as Feline Down Syndrome. The specific genes which cause Down Syndrome in humans simply cannot have a third copy in a living cat.
Cats - like any other organism - can have genetic disorders and these can cause a variety of internal and external abnormalities. Facial abnormalities displayed by some kittens and cats may be somehow visually similar to those seen in Down syndrome but the two are entirely unrelated.
Picture of Lil Bub courtesy of www.lilbub.com