Question
Explain Down syndrome with reference to its cause, characteristic symptoms, and karyotype.
Solution — Step by Step
Down syndrome is caused by trisomy 21 — the presence of an extra copy of chromosome 21. Instead of the normal 2 copies, affected individuals have 3 copies of chromosome 21.
This extra chromosome arises due to non-disjunction during meiosis, where chromosome 21 fails to separate properly during gamete formation (usually in the mother’s egg cell during meiosis I or II).
The karyotype of a Down syndrome individual is written as 47, +21 (or ).
A normal human karyotype has 46 chromosomes (23 pairs). In Down syndrome, there are 47 chromosomes — 22 pairs of autosomes (normal) plus an extra chromosome 21, along with the sex chromosomes (XX or XY). Down syndrome affects both males and females equally.
Down syndrome presents with a recognisable set of physical and developmental features:
Physical features:
- Short stature with a stocky build
- Flat facial profile with upward-slanting eyes (epicanthal folds)
- Single crease across the palm (simian crease)
- Small ears and a protruding tongue
- Poor muscle tone (hypotonia) at birth
Developmental features:
- Mild to moderate intellectual disability
- Delayed speech and motor development
- Increased risk of congenital heart defects (~40–50% of cases)
- Higher susceptibility to respiratory infections
The extra chromosome 21 results in an overdose of the ~300 genes present on that chromosome. This disrupts normal development during embryogenesis, leading to the characteristic physical and cognitive features.
The risk of trisomy 21 increases significantly with maternal age — especially after 35 years, because older eggs are more prone to meiotic errors.
Why This Works
Non-disjunction is the key mechanism to understand here. During normal meiosis, homologous chromosomes separate so each gamete gets exactly one copy. When non-disjunction occurs, one gamete gets two copies of chromosome 21 (n+1 = 24 chromosomes), and when it fuses with a normal gamete (23 chromosomes), the zygote has 47 chromosomes.
This is an example of aneuploidy — an abnormal number of chromosomes. Trisomy (three copies) is more viable than monosomy (one copy) because having an extra gene copy is generally less harmful than missing an entire chromosome.
Common Mistake
Students often confuse Down syndrome with Turner syndrome or Klinefelter syndrome. Remember: Down syndrome involves autosome chromosome 21 (not sex chromosomes), and affects both sexes equally. Turner syndrome (45, X0) and Klinefelter syndrome (47, XXY) involve sex chromosomes and affect specific sexes. In board exams, the specific chromosome number and which chromosome is affected are always worth a mark — write “47, +21” explicitly.
For NEET: Down syndrome is the most common chromosomal disorder compatible with survival. The mnemonic “Down = 21 (trisomy of chromosome 21)” helps distinguish it from Patau syndrome (trisomy 13) and Edward’s syndrome (trisomy 18).