Chromosomes: What the 23 Pairs Carry and What Happens When They Change

Chromosomes are packages of DNA. Most human cells contain 46 chromosomes arranged as 23 pairs: 22 pairs of autosomes and one pair of sex chromosomes. One chromosome in each pair is usually inherited from the egg and the other from the sperm.

Each chromosome carries many genes, and those genes contribute to different tissues, pathways and stages of development. So chromosome 7 is not “the lung chromosome” and chromosome 11 is not “the blood chromosome”. A single chromosome may contain genes involved in the nervous system, immunity, metabolism, growth and cancer protection at the same time.

That is the most important point to hold onto before looking at the pairs individually. The table later in this article gives representative examples of important genes and clinical associations. It is not a complete map of what each chromosome does.

First, what are you actually looking at when you see a chromosome?

The familiar X-shaped chromosome is usually shown because it is easy to teach, but a chromosome does not spend its whole life looking like an X.

Before a cell divides, its DNA is copied. Each replicated chromosome then consists of two identical sister chromatids joined at the centromere. The short arm is called the p arm and the long arm the q arm. The ends are protected by telomeres.

Chromosome locations are written using this structure. For example, 22q11.2 means chromosome 22, long arm q, region 1, band 1, sub-band 2. That is why conditions such as 22q11.2 deletion syndrome have what initially looks like a complicated name. It is really an address.

A karyotype is an arranged image of a person's chromosomes. The chromosomes are grouped by size, banding pattern and centromere position so large changes in chromosome number or structure can be seen.

The 23 pairs: what is carried on each chromosome?

Every chromosome contains hundreds or thousands of genes, so the examples below are deliberately selective. They show the breadth of what is carried on each pair and one representative condition associated with a change involving that chromosome.

Chromosome	Examples of genes or functions carried here	Example when chromosome dosage or structure changes
1	Includes RHD and RHCE, which contribute to the Rh blood group system, alongside genes involved in coagulation, development and cell signalling.	1p36 deletion syndrome: loss of material from the short arm can cause developmental delay, hypotonia, seizures, hearing or vision problems and congenital heart disease.
2	Contains genes including LCT for lactase, MSH2 for DNA mismatch repair and COL3A1 for connective tissue.	2q37 deletion syndrome: can affect development, growth, learning and skeletal development.
3	Includes VHL and MLH1, important in tumour suppression and DNA repair, and genes involved in immune signalling.	3q29 microdeletion syndrome: may be associated with developmental and learning difficulties and variable neurodevelopmental or psychiatric features.
4	Contains HTT, FGFR3 and PKD2, linking this chromosome with neurological function, skeletal growth and kidney biology.	Wolf-Hirschhorn syndrome: usually caused by a deletion of 4p and associated with growth restriction, developmental delay, seizures and characteristic craniofacial features.
5	Includes APC, SMN1 and NSD1, with roles in tumour suppression, motor-neurone survival and development.	Cri-du-chat syndrome: deletion of part of 5p can cause a characteristic high-pitched cry in infancy, developmental delay, hypotonia and other health needs.
6	Contains the major histocompatibility complex, including many HLA genes central to immune recognition. The 6q24 region also contains imprinted genes involved in glucose regulation.	6q24-related transient neonatal diabetes: altered imprinting, duplication or uniparental disomy can cause neonatal hyperglycaemia that often improves in infancy but may recur later.
7	Includes CFTR, ELN and BRAF, linking chromosome 7 with ion transport, connective tissue and cell signalling.	Williams syndrome: usually caused by a 7q11.23 deletion that includes ELN. Features can include cardiovascular disease, characteristic developmental differences and a distinctive cognitive profile.
8	Contains MYC and WRN, with important roles in cell growth, DNA maintenance and cancer biology.	8p deletion syndromes: deletions involving the short arm can be associated with developmental delay and congenital anomalies, including heart defects in some people.
9	Contains the ABO gene, which helps determine ABO blood group, as well as TSC1 and FXN.	Kleefstra syndrome: often results from a 9q34.3 deletion involving EHMT1 and is associated with developmental delay, hypotonia and characteristic behavioural and physical features.
10	Includes RET, PTEN and drug-metabolising genes such as CYP2C9.	10q26 deletion syndrome: loss of material near the end of the long arm can affect growth, development and several organ systems.
11	Contains HBB for beta-globin, INS for insulin and WT1, which is important in renal and gonadal development.	WAGR syndrome: a deletion at 11p13 can involve WT1 and PAX6, producing increased Wilms tumour risk, aniridia, genitourinary differences and developmental needs.
12	Includes VWF, PAH and KRAS, contributing to haemostasis, amino-acid metabolism and cell signalling.	Pallister-Killian mosaic syndrome: usually involves an extra isochromosome made from two copies of 12p in some cells, creating mosaic tetrasomy 12p.
13	Contains BRCA2 and RB1, both important in protecting cells from malignant transformation.	Patau syndrome: trisomy 13 causes major disruption of development and is associated with multiple congenital anomalies and significant health needs.
14	Contains the immunoglobulin heavy-chain locus, central to antibody production, and SERPINA1, which encodes alpha-1 antitrypsin.	Ring chromosome 14 syndrome: chromosome 14 forms a ring after breaks and rejoining. Epilepsy, developmental delay and learning disability are common.
15	Includes FBN1 and the imprinted 15q11-q13 region containing genes such as UBE3A and SNRPN.	Prader-Willi and Angelman syndromes: both involve the same broad chromosome region, but the phenotype differs because some genes there are imprinted and parent of origin matters.
16	Contains the alpha-globin genes HBA1 and HBA2, PKD1 and TSC2.	16p11.2 deletion syndrome: can be associated with developmental and speech delay, learning differences, autism-related features and variable growth patterns.
17	Includes TP53, BRCA1 and NF1, making chromosome 17 particularly important in tumour suppression, DNA repair and neurodevelopment.	Smith-Magenis syndrome: most often caused by a 17p11.2 deletion involving RAI1 and associated with developmental, behavioural and sleep-related features.
18	Contains SMAD4, BCL2 and TTR, with roles in signalling, apoptosis and protein transport.	Edwards syndrome: trisomy 18 is associated with restricted growth, congenital anomalies and substantial medical needs.
19	One of the most gene-dense chromosomes. It includes LDLR, APOE and NOTCH3, linking it to lipid metabolism and neurological disease.	19p13.13 deletion syndrome: a rare microdeletion syndrome associated with developmental delay, growth differences and variable congenital features.
20	Includes JAG1, PRNP and GNAS, which are involved in embryonic signalling, nervous-system biology and hormone signalling.	Ring chromosome 20 syndrome: an abnormal circular chromosome 20, often mosaic, is strongly associated with epilepsy and can affect learning and behaviour.
21	Contains genes including APP, RUNX1 and SOD1, involved in neuronal biology, blood-cell development and oxidative stress pathways.	Down syndrome: most commonly caused by trisomy 21. The additional chromosome changes gene dosage across chromosome 21 rather than altering one single gene.
22	Includes TBX1, BCR and NF2. Chromosome 22 is involved in several developmental and acquired disease processes.	22q11.2 deletion syndrome: a microdeletion involving multiple genes, including TBX1, can affect cardiac, immune, endocrine, palatal and neurodevelopmental systems.
X	The X chromosome carries many genes unrelated to sexual development, including DMD, F8, MECP2 and SHOX. In cells with more than one X chromosome, most of one X is usually inactivated.	Turner syndrome: most commonly associated with monosomy X, written 45,X, although mosaic and structural forms also occur.
Y	Contains SRY and regions important for testicular development and spermatogenesis. The Y chromosome is much smaller and carries far fewer genes than the X chromosome.	47,XYY: an additional Y chromosome can occur. Many people have few obvious physical features, although tall stature and variable learning or developmental differences may occur.

Notice that the examples are not all the same kind of change. Some involve an entire extra chromosome, some a missing segment, some altered imprinting and some a chromosome that has rearranged into a ring.

When chromosome number changes

A change in chromosome number is called aneuploidy. A cell with three copies of one chromosome has a trisomy. A cell with only one copy of a chromosome that would normally be paired has a monosomy.

The most familiar autosomal trisomies that can result in a live birth are trisomy 21, trisomy 18 and trisomy 13, causing Down, Edwards and Patau syndromes respectively. Full trisomies of most other autosomes usually cause such a large change in gene dosage that development cannot continue to term.

Sex-chromosome aneuploidies are often better tolerated because of mechanisms such as X-inactivation and because the Y chromosome contains relatively few genes. Examples include Turner syndrome, Klinefelter syndrome, trisomy X and 47,XYY.

How does an extra or missing chromosome happen?

One mechanism is nondisjunction, where chromosomes fail to separate correctly during cell division. If this happens during meiosis, an egg or sperm may receive an extra chromosome or may be missing one. After fertilisation, the resulting embryo may therefore have 47 or 45 chromosomes rather than the usual 46.

Nondisjunction can also occur after fertilisation during mitotic cell division. In that situation, only some cells may carry the chromosome change. This is called mosaicism.

Mosaicism is one reason chromosome syndromes can vary so widely. A person with mosaic trisomy 21, for example, has some cell lines with three copies of chromosome 21 and others with two. The proportion and distribution of those cell lines can influence the phenotype, although it does not allow an exact prediction of clinical severity.

Structural chromosome changes do not alter the chromosome count in the same way

A chromosome can also have the expected number of copies while part of its structure has changed.

A deletion removes genetic material. Cri-du-chat syndrome and 22q11.2 deletion syndrome are examples.

A duplication creates an extra copy of a chromosome segment. This increases the dosage of the genes in that region.

An inversion occurs when a segment breaks, rotates and reinserts in the opposite orientation.

A translocation moves genetic material to another chromosome or another position. If there is no overall gain or loss of material, the translocation is described as balanced. A person carrying a balanced translocation may have no health effects themselves, although the rearrangement can affect reproductive outcomes because eggs or sperm may receive an unbalanced set.

If material is gained or lost, the rearrangement is unbalanced. The clinical effect then depends on which chromosome regions and genes are involved.

Structural chromosome changes also occur in acquired disease. The Philadelphia chromosome in chronic myeloid leukaemia is created by a translocation between chromosomes 9 and 22, producing the BCR::ABL1 fusion gene. That is a somatic change in leukaemic cells rather than a chromosome syndrome present throughout the body.

Why Down, Edwards and Patau syndromes are taught together

Down syndrome is usually caused by an additional copy of chromosome 21. Edwards syndrome involves chromosome 18 and Patau syndrome chromosome 13. These are the three autosomal trisomies most commonly associated with pregnancies that can progress to term.

The extra chromosome affects many genes simultaneously, which is why each syndrome influences several organ systems rather than causing one isolated problem.

Down syndrome is associated with a recognisable pattern of development and an increased frequency of certain health conditions, including congenital heart disease, thyroid disease, hearing and vision problems and some haematological conditions. Disability varies between individuals.

Edwards syndrome is associated with restricted growth and multiple congenital anomalies, often involving the heart, kidneys, musculoskeletal system and other organs. Medical needs are often significant.

Patau syndrome can affect development of the brain, face, heart and other organs and is also associated with major medical complexity.

It is important not to turn a chromosome result into a prediction of one exact life course. Full trisomy, mosaic trisomy and partial trisomy are genetically different, and individual outcomes vary.

Sex chromosome syndromes

The 23rd chromosome pair is usually XX or XY, but human sex development involves more than the presence of one chromosome alone. Genes across the genome, hormone production, hormone receptors and embryological development all contribute.

Turner syndrome most often involves one X chromosome rather than two, written 45,X. Short stature and ovarian insufficiency are common, and congenital cardiac or renal anomalies can occur. Mosaic forms are well recognised.

Klinefelter syndrome usually involves 47,XXY. The additional X chromosome can affect testicular development and testosterone production. Infertility, tall stature and variable learning or psychosocial needs may occur, although some people remain undiagnosed until adulthood.

47,XYY and trisomy X can also occur. Many people have mild features, so these chromosome patterns may be identified incidentally.

Deletions can be tiny compared with a whole chromosome

Not every chromosome syndrome is visible as an obviously missing chromosome on a standard karyotype.

22q11.2 deletion syndrome is caused by loss of a small region on chromosome 22. It can affect several systems, including congenital heart disease, palatal function, immune function, calcium regulation, hearing and neurodevelopment. The phenotype is highly variable, even between relatives carrying the same deletion.

Cri-du-chat syndrome involves deletion of part of chromosome 5p. The amount of material lost varies, which contributes to variation in the clinical picture.

Prader-Willi syndrome and Angelman syndrome add another layer. Both involve the 15q11-q13 region, but the result depends on which parent's genetic contribution is absent or inactive because this region contains imprinted genes. The chromosome address can therefore be similar while the phenotype is very different.

How chromosome changes are detected

The right genomic test depends on what clinicians are looking for.

Karyotyping examines chromosome number and large structural rearrangements. It can identify aneuploidies such as trisomy 21 or large translocations, but it cannot reliably detect very small deletions or duplications.

Chromosomal microarray can detect much smaller losses and gains of DNA, known as copy-number variants. It is useful for microdeletion and microduplication syndromes, but it generally does not detect balanced translocations because no DNA has been gained or lost.

Fluorescence in situ hybridisation (FISH) uses fluorescent probes to investigate specific chromosome regions and can help clarify deletions, duplications or structural rearrangements.

Rapid tests such as quantitative fluorescence polymerase chain reaction can be used for common aneuploidies in appropriate prenatal diagnostic pathways. Sequencing and whole-genome approaches can identify additional genomic changes that are too small or too complex for traditional chromosome analysis.

Screening and diagnosis are not the same thing. For example, non-invasive prenatal testing can estimate the chance of some chromosome conditions, but a higher-chance screening result is not itself a definitive diagnosis. Diagnostic testing and genomic counselling are used when confirmation is required.

What this means in nursing practice

Chromosome results often appear in areas of nursing that do not initially feel like “genetics”: maternity, neonatal care, paediatrics, cardiology, endocrinology, haematology, oncology, fertility services and learning-disability care.

The notation can look intimidating, but it usually becomes manageable when you identify three things: which chromosome is involved, whether material has been gained or lost, and whether the change is present in all cells or only some.

It is also worth being careful with language. A chromosome syndrome does not tell you everything about the individual. The same genomic change can produce a wide range of clinical features, and some people have far fewer health problems than a textbook list might suggest.

For nurses, the purpose of understanding chromosomes is not to memorise 23 separate lists. It is to understand why a change in gene dosage can affect several organ systems at once, why two chromosome conditions can look very different, and why genomic testing needs to be interpreted alongside the person rather than in isolation.

Summary

Most human cells contain 46 chromosomes arranged as 23 pairs. Pairs 1 to 22 are autosomes, while the remaining pair contains the sex chromosomes. Every chromosome carries many genes with functions across multiple tissues and systems.

Chromosome conditions can result from changes in number, such as trisomy or monosomy, or changes in structure, including deletions, duplications, inversions, translocations and ring chromosomes. Mosaicism occurs when the chromosome change is present in only some cell lines.

Down syndrome, Edwards syndrome, Patau syndrome, Turner syndrome, Klinefelter syndrome, 22q11.2 deletion syndrome, Cri-du-chat syndrome and several rarer conditions all illustrate different ways chromosome dosage or structure can alter development and health.

The chromosome number is only the beginning. What matters clinically is which genetic material has changed, how much has changed, when the change occurred and which tissues carry it.

References

Genomics Education Programme (2026) Chromosomal conditions. Available at: https://www.genomicseducation.hee.nhs.uk/genotes/knowledge-hub/chromosomal-conditions/

Genomics Education Programme (2026) Karyotype. Available at: https://www.genomicseducation.hee.nhs.uk/genotes/knowledge-hub/karyotype/

Genomics Education Programme (2026) 22q11.2 deletion syndrome. Available at: https://www.genomicseducation.hee.nhs.uk/genotes/knowledge-hub/22q11-2-deletion-syndrome/

Genomics Education Programme (2026) Turner syndrome. Available at: https://www.genomicseducation.hee.nhs.uk/genotes/knowledge-hub/turner-syndrome/

National Human Genome Research Institute (2026) Chromosome. Available at: https://www.genome.gov/genetics-glossary/Chromosome

National Human Genome Research Institute (2026) Chromosome Abnormalities Fact Sheet. Available at: https://www.genome.gov/about-genomics/fact-sheets/Chromosome-Abnormalities-Fact-Sheet

MedlinePlus Genetics (2026) Chromosomes & mtDNA. Available at: https://medlineplus.gov/genetics/chromosome/

MedlinePlus Genetics (2026) 6q24-related transient neonatal diabetes mellitus. Available at: https://medlineplus.gov/genetics/condition/6q24-related-transient-neonatal-diabetes-mellitus/

MedlinePlus Genetics (2026) Pallister-Killian mosaic syndrome. Available at: https://medlineplus.gov/genetics/condition/pallister-killian-mosaic-syndrome/

MedlinePlus Genetics (2026) Ring chromosome 14 syndrome. Available at: https://medlineplus.gov/genetics/condition/ring-chromosome-14-syndrome/

MedlinePlus Genetics (2026) Ring chromosome 20 syndrome. Available at: https://medlineplus.gov/genetics/condition/ring-chromosome-20-syndrome/

NHS (2025) Edwards' syndrome. Available at: https://www.nhs.uk/conditions/edwards-syndrome/

NHS (n.d.) Patau's syndrome. Available at: https://www.nhs.uk/conditions/pataus-syndrome/

NHS (n.d.) Screening for Down's syndrome, Edwards' syndrome and Patau's syndrome. Available at: https://www.nhs.uk/pregnancy/your-pregnancy-care/screening-for-downs-edwards-pataus-syndrome/