Down syndrome is associated with trisomy of which chromosome?

Down syndrome is associated with trisomy of which chromosome?

• A. 18
• B. 21
• C. 13
• D. 9

Answer: (B)

Down syndrome results from having an extra copy of chromosome 21, creating a gene dosage imbalance that alters development. This trisomy most often occurs when chromosome pairs fail to separate properly during egg formation (nondisjunction), so the fertilized egg ends up with three copies of chromosome 21 instead of two. The unique set with three copies leads to the characteristic features of Down syndrome. The other trisomies mentioned involve different chromosomes (such as an extra copy of chromosome 18 or 13), which correspond to other conditions and are not Down syndrome.

Down syndrome results from having an extra copy of chromosome 21, creating a gene dosage imbalance that alters development. This trisomy most often occurs when chromosome pairs fail to separate properly during egg formation (nondisjunction), so the fertilized egg ends up with three copies of chromosome 21 instead of two. The unique set with three copies leads to the characteristic features of Down syndrome. The other trisomies mentioned involve different chromosomes (such as an extra copy of chromosome 18 or 13), which correspond to other conditions and are not Down syndrome.