NCERT grounding
The NCERT Class 12 chapter Principles of Inheritance and Variation divides genetic disorders into two categories — Mendelian disorders and chromosomal disorders. On the second category, the textbook states plainly that chromosomal disorders are caused due to absence or excess or abnormal arrangement of one or more chromosomes. It then identifies the underlying event: failure of segregation of chromatids during the cell division cycle results in the gain or loss of a chromosome, a condition called aneuploidy. Failure of cytokinesis after telophase instead increases a whole set of chromosomes, which is polyploidy.
NCERT names Down's syndrome, Turner's syndrome and Klinefelter's syndrome as the common examples, noting that a normal human cell has 46 chromosomes (23 pairs) — 22 pairs of autosomes and one pair of sex chromosomes. When an additional copy is included it is a trisomy, and when one of a pair is missing it is a monosomy. The NIOS supplement reinforces the same karyotypes: Down syndrome 2n = 47 with trisomy of chromosome 21, Klinefelter 2n = 47 with XXY, and Turner 2n = 45 with X0.
"Down's syndrome is due to trisomy of chromosome 21... In Turner's syndrome, one X chromosome is missing and the sex chromosome is as XO, and in Klinefelter's syndrome, the condition is XXY. These can be easily studied by analysis of Karyotypes." — NCERT Class 12 Biology, chapter summary.
Non-disjunction and the three syndromes
To understand chromosomal disorders, begin with the cell division that produces gametes. During meiosis, homologous chromosomes pair up and then separate, so that each gamete receives exactly one chromosome from every homologous pair. In meiosis I the two homologues of a pair move to opposite poles; in meiosis II the two sister chromatids of each chromosome separate. The end result of normal meiosis is a gamete with the haploid number — 23 chromosomes in humans. Fertilisation restores the diploid number of 46.
A chromosomal disorder begins when this clean separation fails. The event is called non-disjunction — the failure of homologous chromosomes (in meiosis I) or of sister chromatids (in meiosis II) to disjoin and move to opposite poles. When non-disjunction occurs, one daughter cell receives both members of a pair while the other receives none. The result is gametes with 24 chromosomes (one extra) and gametes with 22 chromosomes (one short).
From normal meiosis to an aneuploid zygote
-
Step 1
Homologues pair
During meiosis I, the two members of each homologous pair come together at the metaphase plate.
2n parent cell -
Step 2
Non-disjunction
A pair fails to separate — both homologues (or both chromatids) move to the same pole.
segregation failure -
Step 3
Abnormal gametes
One gamete carries n + 1 chromosomes (24); the partner gamete carries n − 1 (22).
unbalanced -
Step 4
Aneuploid zygote
Fertilisation with a normal gamete gives 47 chromosomes (trisomy) or 45 (monosomy).
disorder expressed
Aneuploidy versus polyploidy
NCERT draws a sharp line between two ways the chromosome number can change. Aneuploidy is the gain or loss of one or a few individual chromosomes — it follows from failure of segregation of chromatids during cell division. A trisomy (one extra chromosome) and a monosomy (one missing chromosome) are both aneuploidies. Polyploidy is entirely different: it is an increase in a whole set of chromosomes (3n, 4n and so on), and it follows from failure of cytokinesis after the telophase stage. Polyploidy is often seen in plants and is exploited in crop improvement, whereas the three human syndromes in this subtopic are all aneuploidies.
Aneuploidy
±1 chromosome
one or a few chromosomes added or lost
- Caused by failure of segregation of chromatids during cell division
- Gives trisomy (2n + 1) or monosomy (2n − 1)
- Down (47), Klinefelter (47), Turner (45)
- Usually harmful in humans
Polyploidy
×whole set
a complete extra set of chromosomes
- Caused by failure of cytokinesis after telophase
- Gives 3n, 4n and higher multiples of n
- Common in plants, used in crop breeding
- Not the cause of Down, Klinefelter or Turner
A trisomy may involve an autosome or a sex chromosome. Down syndrome is an autosomal trisomy — the affected chromosome is autosome number 21. Klinefelter and Turner syndromes are sex-chromosome aneuploidies — the imbalance lies in the X and Y pair. Recognising which chromosome is involved is the fastest way to tell the three apart in an exam.
Figure 1. A single non-disjunction event yields two unbalanced gametes. Fused with a normal gamete, the extra-chromosome gamete gives a trisomic zygote (47) and the missing-chromosome gamete gives a monosomic zygote (45).
Down syndrome — trisomy of chromosome 21
Down syndrome is the most familiar autosomal trisomy. Its cause is the presence of an additional copy of chromosome number 21, so the affected individual carries three copies of chromosome 21 instead of two. The total chromosome count therefore rises to 47. The disorder was first described by Langdon Down in 1866, and trisomy 21 is the term you should associate with it.
NCERT lists a precise set of characteristic features. The affected individual is short statured with a small round head. The tongue is furrowed and the mouth is partially open. The palm is broad and shows a single characteristic palm crease. Importantly, physical, psychomotor and mental development is retarded. These features are recurring NEET answer options — the single palm crease in particular has been tested directly.
Chromosomes in Down syndrome
A third copy of chromosome 21 raises the count from 46 to 47. The karyotype is 22 normal autosome pairs, three chromosome-21 copies, and a normal XX or XY pair.
Klinefelter syndrome — 47, XXY
Klinefelter syndrome is a sex-chromosome aneuploidy. It is caused by the presence of an additional copy of the X chromosome, producing a karyotype of 47, XXY — that is 44 autosomes plus two X chromosomes and one Y chromosome. Because a Y chromosome is present, the individual has overall masculine development. However, the extra X also drives some feminine development: the breast develops, a condition called gynaecomastia. Such individuals are sterile.
NEET 2023 tested Klinefelter syndrome directly through a statement-matching question, and the examiner was careful to separate it from Down syndrome. The two correct statements were that the individual has overall masculine development with feminine development also expressed, and that such individuals are sterile. The statements about being described by Langdon Down, being short statured, and having retarded development belong to Down syndrome, not Klinefelter — a deliberate trap.
Turner syndrome — 45, X0
Turner syndrome is the mirror image of Klinefelter syndrome: instead of an extra sex chromosome, one is missing. It is caused by the absence of one of the X chromosomes, giving a karyotype of 45, X0 — 44 autosomes plus a single X and no second sex chromosome. This is a monosomy of the X chromosome. Because no Y is present, the individual is female.
The single X, however, is not enough for normal female development. Turner syndrome females are sterile because the ovaries remain rudimentary. They also lack secondary sexual characters, and the NIOS supplement adds that the individual is often short statured with a web-like skin on the neck. Turner is the only one of these three syndromes with 45 chromosomes — a count that should immediately flag it in an exam.
The three syndromes are distinguished by which chromosome is affected and whether it is gained or lost. Memorise the karyotype line of each.
Down syndrome
47
trisomy of chromosome 21
Type: autosomal trisomy
Features: short stature, small round head, furrowed tongue, palm crease, retarded development
NEET 2023 · NEET 2016Klinefelter syndrome
47, XXY
extra X chromosome
Type: sex-chromosome trisomy
Features: masculine build, gynaecomastia, sterile male
NEET 2023 · NEET 2019Turner syndrome
45, X0
one X chromosome missing
Type: sex-chromosome monosomy
Features: sterile female, rudimentary ovaries, no secondary sexual characters
NEET 2019 (option)The karyotype — how the disorders are seen
NCERT closes the chapter by noting that all three disorders "can be easily studied by analysis of Karyotypes". A karyotype is the arrangement of an individual's chromosomes, photographed at metaphase, stained and laid out in order of decreasing size with the sex chromosomes placed last. Because every chromosome is visible and counted, an extra chromosome or a missing one stands out at once. This is what makes chromosomal disorders fundamentally different from Mendelian disorders — the defect is large enough to see under a microscope, not a single base change hidden inside a gene.
Figure 2. A karyotype makes the disorder visible. Down and Klinefelter raise the count to 47 by trisomy; Turner lowers it to 45 by monosomy of the X chromosome.
Note that maternal age increases the chance of a non-disjunction event. The NIOS supplement records that the possibility of giving birth to a child with Down syndrome is far greater in pregnant mothers above the age of forty — a useful link between the cellular cause and an observable risk factor.
Worked examples
A child is born with 47 chromosomes. Karyotype analysis shows 22 normal autosome pairs, three copies of chromosome 21, and a normal XY pair. Name the disorder and the cellular event that caused it.
The presence of three copies of chromosome 21 is trisomy of 21, so the disorder is Down syndrome. The extra chromosome arose from non-disjunction — failure of homologous chromosomes or sister chromatids of pair 21 to separate during meiosis — which produced a gamete with an extra chromosome 21. Fertilisation of that gamete by a normal gamete gave 47 chromosomes.
Distinguish between Klinefelter and Turner syndrome on the basis of karyotype, sex, and fertility.
Klinefelter syndrome has the karyotype 47, XXY — an extra X. The individual is male (a Y is present) with masculine development, but shows gynaecomastia and is sterile. Turner syndrome has the karyotype 45, X0 — one X missing. The individual is a sterile female with rudimentary ovaries and absent secondary sexual characters. Both are sex-chromosome aneuploidies and both are sterile, but Klinefelter is a trisomy (47) and Turner is a monosomy (45).
A plant cell undergoes failure of cytokinesis after telophase and ends up with double the chromosome number. Is this aneuploidy or polyploidy, and could it cause Down syndrome?
Failure of cytokinesis after telophase increases a whole set of chromosomes, so this is polyploidy, not aneuploidy. It cannot cause Down syndrome: Down syndrome is an aneuploidy (gain of a single chromosome 21) caused by failure of segregation of chromatids, not by gain of an entire chromosome set. Polyploidy is common in plants and is unrelated to the three human syndromes.
Which clinical feature is shared between Down syndrome and Klinefelter syndrome, and which feature clearly separates them?
Both disorders raise the chromosome count to 47 and both arise from non-disjunction — that is the shared feature at the chromosomal level. The clearest separating feature is gynaecomastia: breast development occurs in Klinefelter syndrome but is not a feature of Down syndrome. In addition, Down syndrome shows a single palm crease, a furrowed tongue and retarded mental development, while a Klinefelter individual has overall masculine development and is sterile.
Common confusion & NEET traps
The single most common error is assigning a feature to the wrong syndrome. NEET examiners exploit this by writing statement-matching questions that mix a Down syndrome feature into a Klinefelter question, or attribute "described by Langdon Down" to the wrong disorder. Anchor each syndrome to its karyotype number first, then attach the features.