[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [General pathology](https://durgical.com/en/general-pathology/) › [Genetic disease](https://durgical.com/en/general-pathology/#a-genetic) › Chromosome abnormalities: number and structure [General pathology](https://durgical.com/en/general-pathology/) · [Genetic disease](https://durgical.com/en/general-pathology/#a-genetic) # 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 ## Key points - The commonest cause of aneuploidy is nondisjunction in maternal meiosis I; its risk rises with maternal age. - Only trisomies 21, 18 and 13 and sex chromosome aneuploidies regularly survive to birth. - Robertsonian translocation: fusion of two acrocentric chromosomes (13, 14, 15, 21, 22); the carrier is healthy but may have children with Down syndrome. - Balanced reciprocal translocation: healthy carrier, recurrent miscarriage or children with an imbalance. - Microdeletions (22q11.2, 7q11.23, 5p) are invisible on a routine karyotype: they are sought with FISH or microarray. ## 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](https://durgical.com/en/conditions/down-syndrome/) | Trisomy | A whole extra chromosome 21 | 47,XX,+21 or 47,XY,+21 | | [Turner syndrome](https://durgical.com/en/conditions/turner-syndrome/) | Monosomy | One X chromosome is missing | 45,X | | [22q11.2 deletion syndrome](https://durgical.com/en/conditions/digeorge-syndrome/) | Microdeletion | A small segment of the long arm of 22 is lost | del(22)(q11.2) | | [Cri du chat syndrome](https://durgical.com/en/conditions/cri-du-chat-syndrome/) | Terminal deletion | The tip of the short arm of 5 is lost | del(5p) | | [Williams syndrome](https://durgical.com/en/conditions/williams-syndrome/) | Microdeletion | A segment of 7 including the elastin gene is lost | del(7)(q11.23) | | [Chronic myeloid leukaemia](https://durgical.com/en/conditions/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. ## Memory aid A whole chromosome too many or too few was born in meiosis; a missing sliver has to be hunted with FISH. ## 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. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/general-pathology/chromosome-abnormalities-number-and-structure/) ## Related - [Trisomies 21, 18 and 13, and the 22q11.2 deletion](https://durgical.com/en/general-pathology/trisomies-21-18-and-13-and-the-22q11-2-deletion/) - [Sex chromosome disorders: Klinefelter and Turner](https://durgical.com/en/general-pathology/sex-chromosome-disorders-klinefelter-and-turner/) - [Mendelian inheritance: autosomal dominant, recessive and X-linked](https://durgical.com/en/general-pathology/mendelian-inheritance-autosomal-dominant-recessive-and-x-linked/) - [Translocations, amplifications and epigenetic changes in cancer](https://durgical.com/en/general-pathology/translocations-amplifications-and-epigenetic-changes-in-cancer/) ## Related conditions - [Down syndrome (trisomy 21)](https://durgical.com/en/conditions/down-syndrome/) - [Turner syndrome](https://durgical.com/en/conditions/turner-syndrome/) - [DiGeorge syndrome (22q11.2 deletion syndrome)](https://durgical.com/en/conditions/digeorge-syndrome/) - [Klinefelter syndrome (47,XXY)](https://durgical.com/en/conditions/klinefelter-syndrome/) - [Trisomy 18 (Edwards) and trisomy 13 (Patau)](https://durgical.com/en/conditions/trisomy-18-and-trisomy-13/) - [Cri-du-chat syndrome (5p deletion)](https://durgical.com/en/conditions/cri-du-chat-syndrome/) - [Williams syndrome](https://durgical.com/en/conditions/williams-syndrome/) - [Chronic myeloid leukaemia](https://durgical.com/en/conditions/chronic-myeloid-leukaemia/) ## Elsewhere in DurgiPedia - [Meiosis and nondisjunction: where trisomies come from](https://durgical.com/en/embryology/meiosis-and-nondisjunction-where-trisomies-come-from/) - [Malformation, disruption, deformation, sequence, syndrome, association](https://durgical.com/en/embryology/malformation-disruption-deformation-sequence-syndrome-association/) - [Prenatal diagnosis: ultrasound, serum screening, cell-free DNA, CVS, amniocentesis](https://durgical.com/en/embryology/prenatal-diagnosis-ultrasound-serum-screening-cell-free-dna-cvs-amniocentesis/) - [FISH (fluorescence in situ hybridization)](https://durgical.com/en/biochemistry/topics/fish/) [← All of general pathology](https://durgical.com/en/general-pathology/) 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](https://www.genome.gov/about-genomics/fact-sheets/Chromosome-Abnormalities-Fact-Sheet) (National Human Genome Research Institute (NHGRI), 2026); [Can changes in the number of chromosomes affect health and development?: MedlinePlus Genetics](https://medlineplus.gov/genetics/understanding/mutationsanddisorders/chromosomalconditions/) (MedlinePlus, US National Library of Medicine, 2026); [22q11.2 deletion syndrome: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/22q112-deletion-syndrome/) (MedlinePlus, US National Library of Medicine, 2026). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/general-pathology/chromosome-abnormalities-number-and-structure/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/chromosome-abnormalities-number-and-structure/