Trisomy 21 is known as -

Correct — option (3), Down syndrome.

NCERT's Class 12 Biology chapter Principles of Inheritance and Variation states: "The cause of this genetic disorder is the presence of an additional copy of the chromosome number 21 (trisomy of 21)." The disorder it describes is Down's syndrome.

Trisomy means one chromosome is present in three copies instead of the usual pair. NCERT adds that with the extra copy of chromosome 21, the total number of chromosomes becomes 47 instead of 46.

So trisomy 21 is Down syndrome.

The idea to remember: trisomy 21 is Down syndrome; trisomy 18 is Edwards syndrome.

Why the others are wrong

• (1)
  Evans syndrome — Evans syndrome is autoimmune, not chromosomal. Wikipedia's article (September 2021 revision) describes a disease in which a person's immune system attacks their own red blood cells and platelets.

  It resembles a combination of autoimmune haemolytic anaemia and immune thrombocytopenic purpura. No extra chromosome is involved.

• (2)
  Edwards syndrome — Edwards syndrome is also a trisomy, but of chromosome 18. Wikipedia's article (October 2021 revision) calls it trisomy 18, caused by a third copy of all or part of chromosome 18.

  The same article calls it the second-most common condition due to a third chromosome at birth, after Down syndrome. The number, 18 or 21, is what separates the two.

• (4)
  Gray baby syndrome — Gray baby syndrome is a drug side effect, not a genetic disorder. Wikipedia's article (August 2021 revision) describes a rare but serious effect in newborns, especially premature babies, after the antibiotic chloramphenicol accumulates.

  One mechanism the article gives: in infants, especially premature ones, the liver enzyme system that processes the drug is not fully developed.

A normal human cell has 46 chromosomes in 23 pairs: 22 pairs of autosomes and one pair of sex chromosomes, NCERT explains.

If chromatids fail to separate during cell division, a cell can gain or lose a chromosome; NCERT calls this aneuploidy. An extra copy of one chromosome is a trisomy; a missing one is a monosomy.

NCERT's examples: Down's syndrome (extra chromosome 21), Klinefelter's syndrome (47, XXY) and Turner's syndrome (45 with XO).

Key facts

• NCERT: Down's syndrome is caused by an additional copy of chromosome 21 (trisomy of 21), making 47 chromosomes.
• NCERT: Down's syndrome was first described by Langdon Down in 1866.
• NCERT: Klinefelter's syndrome is 47, XXY; Turner's syndrome is 45 with XO, one X chromosome missing in females.
• Wikipedia: Edwards syndrome is trisomy 18, the second-most common trisomy at birth after Down syndrome.
• Wikipedia: Evans syndrome is an autoimmune attack on red blood cells and platelets; gray baby syndrome follows chloramphenicol build-up in newborns.

Common traps

• Trisomy 21 versus trisomy 18: Down syndrome is chromosome 21; Edwards syndrome is chromosome 18.
• Not every 'syndrome' is chromosomal: Evans syndrome is autoimmune and gray baby syndrome is a drug side effect.
• Extra autosome versus extra sex chromosome: Down's adds a chromosome 21, while Klinefelter's (47, XXY) adds an X.

UnlockIAS will link similar questions from RAS Pre 2018 here once that paper is published on this site.

• practice — not a real PYQ

  According to NCERT, the karyotype of a person with Klinefelter's syndrome is –

  • (a)45 with XO
  • (b)47, XXY
  • (c)47 with an extra chromosome 21
  • (d)46, XY
  Answer(2) — NCERT says Klinefelter's syndrome is caused by an additional copy of the X chromosome, giving a karyotype of 47, XXY. Option (1) is Turner's syndrome, option (3) is Down's syndrome, and option (4) has no extra or missing chromosome.
• practice — not a real PYQ

  Gray baby syndrome in newborns follows the accumulation of which antibiotic?

  • (a)Penicillin
  • (b)Chloramphenicol
  • (c)Streptomycin
  • (d)Tetracycline
  Answer(2) — Wikipedia describes gray baby syndrome as a side effect in newborns following the accumulation of chloramphenicol. Options (1), (3) and (4) are not the antibiotic the article names.