Question

down syndrome is due to an extra __________. a x chromosome b y chromosome c autosome, number 21 d x chromosome or an extra y chromosome

Explanation:

Down syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of an extra autosome (non - sex chromosome). Specifically, it is an extra copy of chromosome 21.

• Option A: An extra X chromosome is related to conditions like Turner syndrome (XO) or Klinefelter syndrome (XXY), not Down syndrome.
• Option B: An extra Y chromosome (XYY) is a different chromosomal abnormality and not associated with Down syndrome.
• Option C: Correct as Down syndrome is due to an extra chromosome 21 (an autosome).
• Option D: As explained above, neither an extra X nor an extra Y chromosome causes Down syndrome.

Answer:

C. autosome, number 21