Answer:
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.
If the expression of a trait requires only one copy of a gene (one allele), that trait is considered dominant. If the expression of a trait requires 2 copies of a gene (2 alleles), that trait is considered recessive. One exception is X-linked disorders.
