Question

why is down syndrome called trisomy 21?

a) the syndrome is caused by having 21 pairs of autosomes.
b) the person has 21 pairs of chromosomes instead of 23.
c) the person has an abnormal gene on chromosome 21.
d) the syndrome results from an extra chromosome 21.

Explanation:

• Option A: Humans normally have 22 pairs of autosomes. Down syndrome is not about 21 pairs of autosomes.
• Option B: Humans normally have 23 pairs of chromosomes. Down syndrome is not due to having 21 pairs instead of 23.
• Option C: Down syndrome is a chromosomal - level disorder (aneuploidy), not a single - gene (abnormal gene on chromosome 21) disorder in the traditional sense.
• Option D: Trisomy means having three copies of a chromosome. In Down syndrome (trisomy 21), there is an extra copy of chromosome 21.

Answer:

D. The syndrome results from an extra chromosome 21.