My apologies if I am repeating what many of you here already know, but for some it may be new (and hopefully, interesting) information. I am not a Vet, but genetics is part of my field.
Down’s Syndrome (named after John Langdon Down, a British physician who first described the condition in 1866) is caused by a chromosomal mutation. The condition is relatively common in human beings, affecting one in 1000 people.
Most human beings have 46 chromosomes, arranged in 23 pairs, but a significant number of people have 45, 47 or more chromosomes. The way the chromosome pattern is expressed in writing is 46,XX or 46, XY or 47, XXY or 47, XYY etc. One of the chromosome pairs is referred to as ‘sex chromosomes’ since they define whether a foetus develops in to male, female or a blend of the two. Male development is usually influenced by the Y chromosome of an XY pair of whereas female development continues (since female is the ‘default’ life form for human beings) under the influence of an XX pair. However, it is possible for a man to have XX chromosomes and a woman to have a XY chromosomes (if you want to know how this happens, just ask
).
So-called 'domestic' Cats have 38 chromosomes, (they would have to be different wouldn’t they/!) but as with humans, felines have a pair of sex chromosomes which, for the majority, define male and female as in humans i.e. XX for female and XY for males. The Y chromosome carries relatively few genes, whereas the X chromosome has over 1,000. Many inherited conditions that have a potentially lethal effect are carried on the X chromosome (these conditions are referred to as ‘X-linked’ inherited).
Because most females have two X chromosomes, both humans, cats and other mammals are therefore potentially exposed to double the number of such inherited conditions. However, nature has provided for this in that mammalian females have a means to nullify one of the X chromosomes by a process known as ‘dosage compensation’. Briefly, this means that females can ‘switch off’ the second X chromosome - this switch can either be at-random or ‘imprinted’ i.e. fixed (this is usually the one inherited from the father, but don’t tell women, they already think they are superior
)
This ability of mammals with two XX chromosomes to switch off one X chromosome provides a degree of protection from the most lethal types of genetically inherited conditions. For instance, muscular dystrophy in human males is a life-limiting condition, whereas in females it is not.
One of the genes on the X chromosome in cats is the fur pigmentation gene (so female cats have two of these genes). Each of these X-linked genes is mapped to the fur colour orange, the other to black. This is why most tortoiseshell/calico cats are female (male cats with two X chromosomes (those who are 39,XXY) can also have the same fur pigmentation).
The reason for this preamble is to illustrate that humans and cats, as mammals are very similar genetically. So what about other genetically inherited conditions such as Down’s Syndrome?
The medical term for sex chromosomes is ‘allosomes’ and non-sex chromosomes are ‘autosomes’. A primary cause of Down’s Syndrome in humans is when autosome 21 has an extra (third) fragment - hence the term ‘trisomy 21’. When this happens, every cell in the body contains this three-part autosome and it is this extra fragment that contains the genetic code for the features that add up to Down’s Syndrome. Another variation of Down’s is known as ‘Mosaicism’ in which the trisomy affects some of the body cells whilst the rest have the typical number of chromosomes. The external features of Down’s syndrome include variation in shape of face, eyes, nose, ears, hands, feet, enlarged tongue, coarse hair texture, overall body shape, poor abdominal muscle tone and most importantly, brain development. Brain development affects cognitive abilities i.e the ability to learn, remember detail and apply knowledge to new situations. I have spent a large portion of my professional life with people who have Down’s Syndrome and apart from being some of the most engaging human beings I know, many if them are far more intelligent than they are given credit for. They *can* learn, and apply what they know, the secret is knowing how they learn and meeting this with appropriate, structured programmes. Down’s Syndrome also affects neurological function such as balance and co-ordination. Not everyone with Down’s Syndrome presents with all the features on the list so the most accurate way to diagnose the condition is a ‘karyotype’ test that reveals a map of the chromosomes taken from a blood sample. Under the microscope, you can clearly see the chromosomes arranged in pairs and if trisomy is present it can be seen which autosome is affected. If it is 21 which has an extra fragment then this an indicator of Down’s Syndrome.
Research has discovered that mice can present with the equivalent of trisomy 21, but the extra fragment is on autosome 16. The problem with identifying conditions that are a non-human model for conditions which have already been identified in humans, such as Down’s Syndrome, is because of the variation in the total number of chromosomes across mammalian species and the fact that the research has not been done. Trisomy in mice was discovered because mice are a favourite laboratory test species. I have discussed this with veterinarians and whilst some say that conditions such as Down’s Syndrome cannot occur in Cats, others say that it can. My attitude is that if the type of trisomy that causes Down’s Syndrome occurs in humans and mice it can also occur in other mammals. The problem that cat breeders/keepers/staff have is interpreting feline behaviour to the point that we can link it to behaviour typical of someone with Down’s Syndrome. I have spoken with a number of human parents who were sent home from hospital with reassurances from staff that their baby was “fine” only to be told some weeks later that their baby had Down’s Syndrome. If we still have problems diagnosing Down’s Syndrome in human beings until a karyotype test is done, it is extremely difficult to arrive at a diagnosis, in the absence of knowing what to look for in a karyotype test for a creature that behaves somewhat differently from humans, looks different and has a different number of chromosomes to us.
Those of us who spend more time than most with cats, know that one way to identify differences that may be linked to a developmental/medical condition is to compare what we feel is ‘typical’ behaviour with what seems to be ‘different’ or ‘odd’. The same goes for appearance and other features such as intelligence, balance, co-ordination, body shape and sociability. However, even here one has to be careful. Before we decided to actually adopt a cat, I sponsored a gorgeous, tiny scrap of a tabby girl who had been found in a ditch as a four week old kitten (what’s wrong with that, doesn’t everyone throw four week old kittens into a ditch .....?
). Following rescue she was found to have motor co-ordination difficulties which was later diagnosed as cerebral palsy. She resides permanently with a foster mum where she has access to the best of care (if she didn’t, we would have adopted her immediately). With regard to Down’s Syndrome, as with humans, other features would have had to be present in her appearance and behaviour.
So ..... if anyone tells you that ‘Down’s Syndrome does not affect cats”, just ask them how they know this with certainty
