General pathology · Genetic disease
Chromosome abnormalities: number and structure !
- Topic
An extra chromosome, a missing one, or a piece lost or moved alters many genes at once and causes syndromes with several malformations.
Updated 2026-11-01
What it is
Chromosome abnormalities are numerical (aneuploidies such as trisomy or monosomy) or structural (deletions, duplications, translocations, inversions, isochromosomes). They are a major cause of miscarriage, congenital malformations and intellectual disability, and some acquired translocations drive cancers.
Step by step
- In meiosis I homologous chromosomes fail to separate (nondisjunction), or in meiosis II the chromatids fail to separate.
- One gamete gets an extra chromosome and another gets one too few.
- Fertilisation gives a trisomic (47 chromosomes) or monosomic (45) zygote.
- Most of these conceptions are lost as early miscarriages: about half of first-trimester miscarriages carry an abnormal chromosome complement.
- If nondisjunction happens after fertilisation, the embryo is a mosaic, with normal and abnormal cell lines and a milder picture.
Types of chromosome abnormality
| Example | Type | What happens | Notation |
|---|---|---|---|
| Down syndrome | Trisomy | A whole extra chromosome 21 | 47,XX,+21 or 47,XY,+21 |
| Turner syndrome | Monosomy | One X chromosome is missing | 45,X |
| 22q11.2 deletion syndrome | Microdeletion | A small segment of the long arm of 22 is lost | del(22)(q11.2) |
| Cri du chat syndrome | Terminal deletion | The tip of the short arm of 5 is lost | del(5p) |
| Williams syndrome | Microdeletion | A segment of 7 including the elastin gene is lost | del(7)(q11.23) |
| Chronic myeloid leukaemia | Acquired translocation | Exchange between chromosomes 9 and 22 forming the BCR-ABL1 fusion gene | t(9;22) |
| Robertsonian translocation | Balanced structural | Fusion of two acrocentrics; healthy carrier with risk for offspring | e.g. der(14;21) |
| Isochromosome X | Structural | An X made of two long arms; one cause of Turner syndrome | i(Xq) |
In the lab
- G-banded karyotype: chromosomes in metaphase; detects aneuploidy and large structural changes.
- FISH: fluorescent probes for a specific region; useful for microdeletions and rapid diagnosis.
- Chromosomal microarray: detects small gains and losses across the genome, but not balanced translocations.
- Prenatal screening with cell-free fetal DNA in maternal blood; diagnosis is confirmed by chorionic villus sampling or amniocentesis.
Test yourself
Think of the answer, then tap the question to see it.
What is the commonest mechanism of trisomy 21?
Nondisjunction in maternal meiosis I, whose risk rises with the mother's age.
Why is a carrier of a 14;21 Robertsonian translocation healthy?
Because all essential genetic material is kept; only acrocentric short arms without indispensable genes are lost.
Which test detects a 22q11.2 microdeletion that a routine karyotype misses?
Fluorescence in situ hybridisation (FISH) or a chromosomal microarray.
What is chromosomal mosaicism?
Having two or more cell lines with different chromosome make-up, because of a division error after fertilisation.
Related
- Trisomies 21, 18 and 13, and the 22q11.2 deletion
- Sex chromosome disorders: Klinefelter and Turner
- Mendelian inheritance: autosomal dominant, recessive and X-linked
- Translocations, amplifications and epigenetic changes in cancer
Related conditions
- Down syndrome (trisomy 21)
- Turner syndrome
- DiGeorge syndrome (22q11.2 deletion syndrome)
- Klinefelter syndrome (47,XXY)
- Trisomy 18 (Edwards) and trisomy 13 (Patau)
- Cri-du-chat syndrome (5p deletion)
- Williams syndrome
- Chronic myeloid leukaemia
Elsewhere in DurgiPedia
- Meiosis and nondisjunction: where trisomies come from
- Malformation, disruption, deformation, sequence, syndrome, association
- Prenatal diagnosis: ultrasound, serum screening, cell-free DNA, CVS, amniocentesis
- FISH (fluorescence in situ hybridization)
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Chromosome Abnormalities Fact Sheet (National Human Genome Research Institute (NHGRI), 2026); Can changes in the number of chromosomes affect health and development?: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); 22q11.2 deletion syndrome: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026).
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Representative model, for educational purposes only.