In Humans, an Extra Copy of Chromosome 21 Typically Leads

In humans, an extra copy of chromosome 21 typically leads to moderate to severe mental retardation and sterility, as well as to a greater likelihood of heart defects and other problems. This disorder is known as ____.

A) Turner syndrome
B) Down syndrome
C) cri-du-chat
D) Triple-X syndrome
E) Klinefelter syndrome