There are only three autosomal trisomic conditions (Patau, Edward, and Down Syndrome) where a fetus will survive to birth. Why do you think this is so?

An autosomal trisomic condition is the deviation of the chromosome number where an extra copy of the chromosome present with the chromosome pair. The three trisomic conditions are Patau - extra copy at chromosome 13, Edward - extra copy of chromosome 18, and Down Syndrome - an extra copy of chromosome 21

Children or fetuses affected by these trisomies show the least life expectancy and develop as mentally week, other than these trisomies other trisomic conditions are highly lethal so we do not see the survivors and it is due to dosage sensitivity which means that increasing or decreasing their expression levels will reduce cell viability.

explanation:

the fossil record extends back to a dog-like ancestor some 55 million years ago, which gave rise to the first horse-like species 55 to 42 million years ago in the genus eohippus.