Biology · Principles of Inheritance and Variation · NEET
It is trisomy of chromosome 21, NOT 23. The affected person has three copies of chromosome number 21 instead of the normal two copies. The number 23 sometimes confuses students because humans have 23 pairs of chromosomes in total, but Down's syndrome affects only pair number 21. Remember: Down's = chromosome 21.
A person with Down's syndrome has 47 chromosomes, not the normal 46. Normal humans have 46 chromosomes (23 pairs). Because one extra copy of chromosome 21 is present, the total goes up by one to 47. This is why it is called trisomy (tri = three copies of chromosome 21).
It is caused by an EXTRA chromosome, not a missing one. There is an additional (extra) copy of chromosome 21. This is the opposite of Turner's syndrome, where one X chromosome is missing (45, X0). A very common NEET trap swaps these two — do not mix them up. Extra chromosome 21 = Down's; missing X = Turner's.
Chromosome 21 is an autosome (a body chromosome), not a sex chromosome. So Down's syndrome is caused by autosomal non-disjunction. This is different from Klinefelter's (extra X, a sex chromosome) and Turner's (missing X, a sex chromosome), which involve sex chromosomes. Because it is an autosome, Down's syndrome affects both boys and girls equally.
The extra chromosome comes from non-disjunction. Non-disjunction means the two copies of chromosome 21 fail to separate properly during meiosis (gamete formation). So one gamete (egg or sperm) ends up with an extra chromosome 21. When this gamete joins a normal one at fertilisation, the child gets three copies of chromosome 21. This is called aneuploidy (gain or loss of a chromosome).
Down's syndrome was first described by Langdon Down in the year 1866. NEET has directly asked this fact, and it is also used as a trap in Klinefelter's questions (where option A wrongly says Klinefelter's was described by Landon Down). Remember: Langdon Down, 1866, chromosome 21.
Key NCERT features: the person is short statured with a small round head, a furrowed tongue and a partially open mouth. The palm is broad with a characteristic (single) palm crease. There is also physical, psychomotor and mental development that is delayed (retarded). NEET has asked directly about the broad palm with single palm crease.
A disease caused by an autosomal primary non-disjunction is
Broad palm with single palm crease is visible in a person suffering from
Statement I: Down's syndrome is caused by the absence of one of the X-chromosomes. Statement II: Turner's syndrome is caused by the presence of an additional copy of the chromosomes. Choose the correct answer:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The karyotype has 47 chromosomes with three copies of chromosome 21. It is often written as 47, and specifically as trisomy 21. The extra chromosome 21 is the only change; all other chromosome pairs are normal.
It is a chromosomal disorder, not a Mendelian (single-gene) disorder. It is caused by a whole extra chromosome (aneuploidy from non-disjunction), not by a change in one gene. This makes it different from haemophilia or sickle-cell anaemia, which are single-gene disorders.
Yes. It can be detected before birth by amniocentesis, a test that studies fetal cells and substances in the amniotic fluid. NEET has noted that amniocentesis can detect Down syndrome (but cannot detect a structural defect like cleft palate).
Yes, because chromosome 21 is an autosome (not a sex chromosome), Down's syndrome can occur in both males and females equally. This is unlike Klinefelter's and Turner's, which involve sex chromosomes and affect one sex.
'Tri' means three and 'somy' refers to chromosome copies. So trisomy 21 means three copies of chromosome number 21 instead of the normal two. This single extra chromosome causes all the features of Down's syndrome.
