Most cases of Down syndrome are the result of
A) paternal age.
B) maternal age.
C) multiple codon repeats on the X chromosome.
D) the inability to metabolize phenylalanine.

Question

Quizplus · Accepted Answer

Most cases of Down syndrome are caused by a third copy of chromosome 21, which usually occurs due to an error in the egg cell division during meiosis. This error is more likely to occur as women age, so maternal age is the primary factor in the occurrence of Down syndrome.

Most Cases of Down Syndrome Are the Result of