Trisomy 21, also known as Down’s Syndrome, is fairly common as these things go. Multisomy (is that a word?) of the X and Y chromosomes is also well-known. How come I’ve never heard of trisomy 22? I understand why trisomy 3 is unknown, but if trisomy 21 doesn’t keep a fetus from being unviable, trisomy 22 shouldn’t either. Are the symptoms small enough not to be noticed, just like some people with trisomy 23 can live their whole lives without knowing they have it? Is there such a thing as a person with trisomy 20?
Most multisomys are lethal. Trisomy 21 is unusually benign. You can read up on trisomy 22 and all the others here.
Oh, and 80% of all trisomy 21 conceptions end in stillbirth or miscarriage, so your premise is flawed as well.
There are a whole multitude of chromosomal abnormalities each presenting with different characteristics (‘Potter’s pathology of the fetus’ is a good reference guide). As a general rule of thumb, when thinking about mono and polysomys consider the effect of gene dosage to a developing fetus.
Large chromosomes (eg 1,2,3) contain more genes than small ones (eg,21,22). The global effect on a developing fetus will be greater if it is trisomic for the larger chromosomes simply by virtue of the involvement of more genes being present at aberrant levels.
Trisomies involving smaller chromosomes can give rise to viable gametes (due to fewer genes being present in triplicate) and these gametes can go on to be fertilized to produce liveborn trisomic babies.
The X chromosome is unique in that it undergoes inactivation (polysomic genes are switched off). As a result multiple copies of X are tolerated in developing cells (XXXXX and XXXXY genotypes are documented in adult patients).
Conventionally, human chromosomes are assigned numbers 1-22, X and Y.