Common trisomy disorder

Why is Down syndrome the most common trisomy disorder seen among live-born infants? The extra alleles from trisomies of larger chromosomes are embryo lethal.

Nondisjunction occurs more frequently for chromosome 21 than for any other chromosome.

Trisomies resulting from nondisjunction express fewer alleles than trisomies that result from unbalanced translocations.

The incidence may not be truly higher but is recognized more often because the features of a baby with trisomy 21 are more obvious and not missed as often as are the more subtle features of other trisomies.