I heard that animals cannot get Down syndrome or trisomy. Is that true and why can't they get it? They also have chromosomes.
Animals can be generated with genetic defects similar to Down Syndrome, but not that exact condition, except in the case of great apes.
Down Syndrome is a kind of defect called a chromosomal abnormality, meaning that either there is an extra chromosome or an excessive repetition of the same genes on a particular chromosome. In the case of Down Syndrome, that repetition in on a region of the 21st chromosome. Similar abnormalities in other chromosomes are possible, such as the abnormality which causes Klinefelter Syndrome.
Although animals do have chromosomes, they have different chromosomes than human beings do. For example, a horse has 32 chromosomes whereas a human has 23. Apes are most similar to humans, having 24 chromosomes. Therefore, if trisomy is to be found, apes would be most likely to have it. Researchers have attempted to answer this exact question and there is a paper on the subject entitled "Conservation of the Down syndrome critical region in humans and great apes".
The result of this study was that the area on chromosome 21 in humans that is responsible for trisomy is found analogously on chromosome 22 in apes and that apes can, indeed, get Down Syndrome, if they have duplication of this region. There do appear to be apes that suffer from this condition. The band diagram for both chromosomes is shown below:
where "H" indicates human and "C" indicates chimpanzee.
It depends on the animal, some can get it, others cannot.
Down syndrome is the result of an extra copy of the twenty first human chromosome. So, it is a rather humanly genetic problem.
However, the closer an animal is to humans, the greater the chance of it being at risk of suffering from down syndrome.
There have been several chimpanzees found with Down Syndrome. As the syndrome is caused by an extra copy of a specific chromosome (chromosome 21 in humans) only animals closely related to humans have a similar syndrome. Extra copies of other chromosomes cause even more serious syndrome and most are not compatible with life. To study Down syndrome, scientists have created mice that have extra copies of the same genes found on human chromosome 21. These mice have some of the features of Down Syndrome.$^1$
There had been some uproar a couple of years ago around a tiger (they have 19 chromosomes) who supposedly had down-syndrome. The tiger "Kenny" was called the tiger with down syndrome, however that is not true. His appearance and mental retardation was due to aggressive inbreeding.
A great benefit of having two copies of each of our genes is that damaging recessive mutations are seldom expressed. However, where parents are closely related there is a high chance of them both carrying the same rare version of a gene, with a one in four chance their offspring will inherit it from both sides. With enough damaging alleles in common this becomes like a repetitive game of Russian Roulette.