Which disorder in humans is caused by trisomy of chromosome number 21?
- (a)Turner syndrome
- (b)Down syndrome
- (c)Super-female syndrome
- (d)Klinefelter syndrome
Correct — B, Down syndrome. It is caused by trisomy of chromosome 21: three copies instead of the normal two, giving 47 chromosomes (karyotype 47,+21). It usually arises from meiotic non-disjunction, and its frequency rises with maternal age.
- (a)Turner syndrome — A sex-chromosome disorder — monosomy X (45,X) in females — not a chromosome-21 trisomy.
- (c)Super-female syndrome — Triple-X (47,XXX): an extra X, again a sex-chromosome aneuploidy, not chromosome 21.
- (d)Klinefelter syndrome — 47,XXY in males — an extra X on the sex chromosomes, not a trisomy of chromosome 21.
Aneuploidy means having an abnormal number of chromosomes, usually from non-disjunction during cell division. It divides into autosomal trisomies (chromosomes 1–22) and sex-chromosome aneuploidies (X, Y). Down syndrome is the classic autosomal trisomy; Turner, Klinefelter and Triple-X are sex-chromosome conditions.
Only one option involves an autosome (chromosome 21); the other three all involve the sex chromosomes — that contrast is the fastest way to eliminate them.
- Down syndrome = trisomy 21 (47,+21)
- Edwards syndrome = trisomy 18; Patau syndrome = trisomy 13
- Turner = 45,X; Klinefelter = 47,XXY; Triple-X = 47,XXX
- Risk of trisomy 21 rises with maternal age
Only Down syndrome involves an autosome (chr 21); the rest are sex-chromosome aneuploidies.
- Confusing an autosomal trisomy (Down) with sex-chromosome disorders (Turner/Klinefelter)
- Mixing up trisomy 21 (Down) with trisomy 18 (Edwards) / 13 (Patau)
Both MPPSC and UPSC test the chromosome number behind a named disorder, or give a karyotype (e.g. 45,X) and ask the syndrome. Memorise the number–syndrome pairs.
No directly related past PYQ was found (the nearest genetics questions test a different concept).
- practice — not a real PYQ
Edwards syndrome in humans is caused by trisomy of which chromosome?
- (a)13
- (b)18
- (c)21
- (d)X
Answer(b) 18 — trisomy 18.
- practice — not a real PYQ
The karyotype 45,X is associated with which condition?
- (a)Down syndrome
- (b)Klinefelter syndrome
- (c)Turner syndrome
- (d)Patau syndrome
Answer(c) Turner syndrome.