Why is Down syndrome the most common trisomy disorder seen among liveborn infants?
A)The extra alleles from trisomies of larger chromosomes are embryo lethal.
B)Nondisjunction occurs more frequently for chromosome 21 than for any other chromosome.
C)Trisomies resulting from nondisjunction express fewer alleles than trisomies that result from unbalanced translocations.
D)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.

Question

Quizplus · Accepted Answer

Trisomy as a result of nondisjunction can occur with any chromosome. Chromosome 21 contains few gene alleles. Trisomy of chromosomes containing more alleles is not compatible with life and is embryo lethal. Thus, the pregnancy does not progress beyond the first trimester.

Why Is Down Syndrome the Most Common Trisomy Disorder Seen