What is down syndrome?
Also known as trisomy 21, down syndrome is a genetic condition which is caused by an extra or partial copy of chromosome 21. Chromosomes are threadlike structures which are located within the nucleus of every cell and contain deoxyribonucleic acid (DNA). DNA carries genetic instructions for the development, functioning, growth and reproduction.
Usually, there are two pairs of chromosomes, each parent contributes one chromosome in each pair. Trisomy is a genetic disorder in which the organism has three chromosomes instead of two which causes development delays and congenital defects.
Trisomy often occurs because of errors during meiosis, when gametes (eggs and sperm) are formed. The replicated chromosomes are sorted into daughter cells in two steps, called meiosis I and meiosis II. Each gamete has one set of chromosomes (during fertilisation, each cell and sperm combine to form 19 pairs of chromosomes). If chromosome pairs fail to separate properly during cell division, the gamete may end up with a second copy of chromosomes.
Can cats have down syndrome?
Humans have 23 pairs of chromosomes, 22 are autosomes and the 23rd pair are the sex chromosomes. Cats have 19 pairs of chromosomes, 18 autosomes and the sex chromosome pair. The sex chromosomes determine gender, XX is female and XY is male. Cats can’t have down syndrome because they only have 19 pairs of chromosomes and down syndrome occurs on chromosome 21.
Cats can, however, have other trisomy disorders, the most well known is Klinefelter syndrome which is characterised by an extra sex chromosome, so instead of the cat being XX or XY, it is XXY or XYY. Male cats with an extra X chromosome are usually sterile, with longer limbs and more effeminate than XY (male) cats, however, in most cases, this genetic abnormality won’t be noticed by veterinarians or pet owners.
One exception to this rule is if the male is tortoiseshell or calico. Most cat lovers know that calico and tortie cats are almost always female. This is because calico and tortoiseshell cats must have two X chromosomes for the colour to occur as the O (orange) gene is located on the X chromosome. In the normal calico or tortie female, she carries one O gene which contributes orange and one non-orange gene, which contributes the black, grey or brown colour. It is not possible for an XY (male) to be calico or tortie as he only has one X chromosome, so he will either be red or non-red. The exception is if he has Klinefelter syndrome and therefore has a second X chromosome (XXY).
When trisomies occur on any of the autosomes (chromosomes 1 to 18), a range of physical and developmental abnormalities which may include a short/broad nose, wide-set eyes, vision problems, low muscle tone, small ears and learning disabilities.
Why are cats with trisomy rare?
It is estimated that 1 in 1,100 babies born in Australia has down syndrome, which is not common but not rare. Trisomy in people can occur on any chromosome, but all except trisomies 21, 18, 13 and the sex chromosomes are lethal.
So where are all the cats with trisomy? Many cats with chromosome abnormalities die in utero and are reabsorbed, others may die shortly after birth, are killed by their mother or be euthanised. Animal shelters are overflowing with healthy kittens and adult cats, so the chances of a cat with trisomy in an animal shelter are low.