Question
What are Down syndrome, Turner syndrome, and Klinefelter syndrome? Explain the chromosomal basis of each disorder and list their characteristic features.
(NCERT Class 12, Chapter 5 — Principles of Inheritance and Variation)
Solution — Step by Step
Chromosomal disorders arise due to aneuploidy — an abnormal number of chromosomes. This happens because of non-disjunction during meiosis, where homologous chromosomes (or sister chromatids) fail to separate properly. The result is gametes with one extra or one missing chromosome.
The individual has 47 chromosomes instead of 46 — there are three copies of chromosome 21.
Karyotype: 2n = 47 (44 + XX or XY, with an extra chromosome 21)
Features:
- Short stature, round face with a flat nasal bridge
- Broad palm with a characteristic simian crease
- Partially open mouth, furrowed tongue
- Intellectual disability (varying degrees)
- Epicanthal folds over the eyes
The risk increases significantly with maternal age above 35 years.
Affects females only. One X chromosome is missing — the individual has 45 chromosomes.
Karyotype: 2n = 45 (44 + X0)
Features:
- Short stature, webbed neck
- Shield-shaped chest with widely spaced nipples
- Ovaries are rudimentary — the person is sterile
- Lack of secondary sexual characteristics at puberty
- No menstruation (primary amenorrhoea)
Affects males only. There is an extra X chromosome — the individual has 47 chromosomes.
Karyotype: 2n = 47 (44 + XXY)
Features:
- Tall stature, long limbs
- Gynaecomastia (development of breast tissue)
- Sparse facial and body hair
- Testes are small and the person is sterile
- Mild intellectual impairment in some cases
Why This Works
All three disorders trace back to non-disjunction — the failure of chromosomes to separate during anaphase of meiosis I or meiosis II. When a gamete with an extra chromosome fuses with a normal gamete, the resulting zygote is trisomic (2n + 1). When a gamete missing a chromosome fuses with a normal one, the zygote is monosomic (2n - 1).
Down syndrome is an autosomal aneuploidy (chromosome 21), while Turner and Klinefelter syndromes are sex chromosome aneuploidies. This distinction matters in NEET — questions often ask you to classify disorders as autosomal vs. sex-chromosomal.
Alternative Method — Quick Comparison Table
| Disorder | Karyotype | Chromosomes | Sex | Key Feature |
|---|---|---|---|---|
| Down syndrome | 44 + XX/XY + 21 | 47 | Both | Trisomy 21, simian crease |
| Turner syndrome | 44 + X0 | 45 | Female | Monosomy X, webbed neck |
| Klinefelter syndrome | 44 + XXY | 47 | Male | XXY, gynaecomastia |
NEET loves matching-type questions on chromosomal disorders. Memorise the karyotype and one standout feature for each: Down = simian crease, Turner = webbed neck, Klinefelter = gynaecomastia. That’s usually enough to crack the MCQ.
Common Mistake
Students confuse Turner (X0, female, 45 chromosomes) with Klinefelter (XXY, male, 47 chromosomes). The trick: Turner has LESS (monosomy, one X missing) and Klinefelter has MORE (trisomy, one extra X). Also remember — Turner affects females and Klinefelter affects males. Never mix them up.