Although in humans there are 22 pairs of autosomal chromosomes, only three different chromosomal trisomies are commonly seen in newborns. of the remaining 19 autosomes, many trisomies have not been seen in newborns. why not?

The reason why only 3 out of the 22 possible autosomal trisomies are seen in newborns, is because the rest of them are chromosomal abnormalities not compatible with life and thus an spontaneous abortion occurs. The embryos suffering from these trisomies do not survive for more than a few weeks in the uterus, so the miscarriage often goes unnoticed for the mother as she may not have realized she was pregnant.

The only trisomies that are allow fetuses to be born are: Patau Syndrome (trisomy 13), Edwards Syndrome (trisomy 18), and Down Syndrome (trisomy 21). Out of these three, people with Down Syndrome have the largest life expectancy.