___________ occurs when there is an extra chromosome in the 21^st pair of chromosomes. A) Down syndrome B) Sickle-cell anemia C) Listeria D) PKU

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Down syndrome, also known as trisomy 21, is caused by the presence of three copies of chromosome 21 instead of the normal two. This extra genetic material leads to developmental and intellectual delays, as well as distinctive physical characteristics. Sickle-cell anemia is caused by a mutation in a gene on chromosome 11, Listeria is a bacterial infection, and PKU is an inherited disorder related to metabolism.

___________ Occurs When There Is an Extra Chromosome in the 21st

___________ Occurs When There Is an Extra Chromosome in the 21^st