What is Down syndrome?

What is Down syndrome?

• A. Extra copy of chromosome 21 (Trisomy 21)
• B. Deletion of chromosome 21
• C. Duplication of chromosome 21
• D. Inversion of chromosome 21

Answer: (A)

Down syndrome occurs when a person has an extra copy of chromosome 21, most commonly as trisomy 21. This extra chromosome changes gene dosage and leads to the characteristic physical features and developmental differences. Most cases arise from nondisjunction during egg formation, so the embryo ends up with three copies of chromosome 21 in nearly all cells. Deletion of chromosome 21 would remove genetic material and is not how Down syndrome arises. Duplication of chromosome 21 would mean extra copies, but not the typical whole-chromosome trisomy described in classic Down syndrome. Inversion rearranges genetic material without changing the total number of chromosome 21 copies and likewise isn’t the usual cause. There are rarer forms, like mosaic Down syndrome or Down syndrome due to a translocation, but the defining hallmark is an extra copy of chromosome 21.

Down syndrome occurs when a person has an extra copy of chromosome 21, most commonly as trisomy 21. This extra chromosome changes gene dosage and leads to the characteristic physical features and developmental differences. Most cases arise from nondisjunction during egg formation, so the embryo ends up with three copies of chromosome 21 in nearly all cells. Deletion of chromosome 21 would remove genetic material and is not how Down syndrome arises. Duplication of chromosome 21 would mean extra copies, but not the typical whole-chromosome trisomy described in classic Down syndrome. Inversion rearranges genetic material without changing the total number of chromosome 21 copies and likewise isn’t the usual cause. There are rarer forms, like mosaic Down syndrome or Down syndrome due to a translocation, but the defining hallmark is an extra copy of chromosome 21.