Down syndrome is associated with a mutation on which chromosome?

• A. Chromosome 21
• B. Chromosome 18
• C. Chromosome 13
• D. Chromosome 11

Answer: (A)

Down syndrome is linked to having an extra copy of chromosome 21, so there are three copies instead of two in most of the body's cells. This trisomy 21 means there’s more genetic material from that chromosome, which disrupts normal development and leads to the characteristic features and health issues of the condition. The extra chromosome 21 is usually caused by nondisjunction during the formation of the egg. In other chromosomal disorders, different chromosomes are involved—for example, conditions associated with chromosome 18 or chromosome 13—so those are not the chromosome linked to Down syndrome. A small number of Down syndrome cases come from a Robertsonian translocation involving chromosome 21, but the common association remains chromosome 21.