Definition of a Chromosome: Structure, Karyotype and Abnormalities

A chromosome is an organised DNA–protein structure that packages and transmits genetic information. Human chromosomes carry thousands of genes and regulatory sequences. Their number, structure and behaviour during mitosis and meiosis explain normal growth, congenital syndromes, infertility, miscarriage, some cancers and emergency presentations in newborns and adults.

Learning outcomes

The learner should be able to define a chromosome, explain its parts and packaging, distinguish homologous chromosomes from sister chromatids, describe karyotype notation and cell-division behaviour, classify numerical and structural abnormalities, recognise common clinical patterns and select appropriate first-line investigations.

1. Definition and basic organisation

A chromosome is a single, very long DNA molecule packaged with histones and other chromosomal proteins. During interphase, DNA is distributed through the nucleus as chromatin; during cell division, it condenses into visible chromosomes. Each chromosome has a characteristic sequence, size, centromere position and banding pattern.

In a diploid human somatic cell, one homologous chromosome is inherited from each parent. Homologues carry the same genes at corresponding loci but can contain different alleles. They are not identical copies. Sister chromatids, by contrast, are near-identical copies produced when one chromosome replicates before mitosis or meiosis.

2. Parts of a chromosome

Part	Function	Clinical significance
DNA molecule	Carries genes, regulatory elements and non-coding sequences	Variants or rearrangements may alter dosage or expression
Histones/chromatin	Compact DNA and regulate accessibility	Epigenetic dysregulation can affect development and cancer
Centromere	Builds the kinetochore for spindle attachment	Errors cause chromosome mis-segregation
p arm	Short arm (p for petit)	Breakpoints are recorded using band notation
q arm	Long arm (q follows p)	Many clinical deletions and translocations are named by q band
Telomeres	Protect chromosome ends and prevent end-to-end fusion	Shortening and telomere disorders cause premature ageing or marrow failure
Sister chromatids	Replicated copies joined before separation	Non-disjunction or chromatid breakage can produce abnormal daughter cells

3. Chromatin and higher-order packaging

DNA winds around histone octamers to form nucleosomes. Nucleosomes fold into loops attached to a protein scaffold and occupy chromosome territories within the nucleus. Euchromatin is relatively open and often transcriptionally active; heterochromatin is compact and often less active. Chromosome conformation brings distant enhancers into contact with promoters, so a rearrangement can disturb gene regulation even when no coding sequence is deleted.

4. Human chromosome complement

4.1 Autosomes and sex chromosomes

Chromosomes 1–22 are autosomes. The 23rd pair comprises sex chromosomes, conventionally XX or XY, although variations such as XO, XXY and mosaic combinations occur. Sex development depends on chromosome content, gene dosage, gonadal pathways and hormone response; chromosome appearance alone does not define a person’s gender identity.

4.2 Haploid and diploid cells

• Diploid (2n): most somatic cells contain 46 chromosomes.
• Haploid (n): mature eggs and sperm contain 23 chromosomes.
• Fertilisation restores diploidy. Errors during gamete formation or early embryonic divisions can produce aneuploidy or mosaicism.

4.3 Karyotype notation

A normal female karyotype is written 46,XX and a normal male karyotype 46,XY. Examples include 47,XX,+21 (an extra chromosome 21), 45,X (Turner syndrome) and 46,XX,t(14;21) (a translocation involving chromosomes 14 and 21). Laboratory reports may include mosaic cell lines and detailed breakpoint notation.

5. Chromosome behaviour in the cell cycle

5.1 Replication

During S phase, each chromosome is copied. The two sister chromatids remain connected at the centromere until separation. DNA replication is semi-conservative and includes proofreading and repair; unrepaired breaks or replication errors can become structural variants.

5.2 Mitosis

1. Prophase/prometaphase: chromatin condenses, the nuclear envelope breaks down and spindle fibres attach to kinetochores.
2. Metaphase: chromosomes align at the equator.
3. Anaphase: sister chromatids separate to opposite poles.
4. Telophase and cytokinesis: nuclei reform and two genetically similar daughter cells result.

Mitotic errors in somatic cells can produce mosaicism or contribute to cancer. Checkpoint failure allows cells with damaged or abnormal chromosomes to continue dividing.

5.3 Meiosis

Meiosis produces haploid gametes through two divisions. Homologous chromosomes pair and exchange segments during prophase I; homologues separate in meiosis I and sister chromatids in meiosis II. Crossing-over increases diversity, but abnormal recombination can create deletions, duplications or translocations. Non-disjunction leaves a gamete with an extra or missing chromosome.

6. Numerical chromosome abnormalities

6.1 Aneuploidy

Aneuploidy is loss or gain of one or more individual chromosomes. Trisomy means three copies; monosomy means one copy. Most autosomal monosomies are incompatible with survival, while trisomy 21, 18 and 13 may survive with characteristic clinical patterns. Sex-chromosome aneuploidies often have milder or variable features because of X-inactivation and lower gene density.

6.2 Polyploidy

Triploidy (three complete sets) and tetraploidy (four sets) affect every chromosome and usually cause severe developmental abnormalities or early pregnancy loss. Mechanisms include fertilisation by two sperm, failure of meiotic division or failure of early mitotic cytokinesis.

6.3 Mosaicism

Mosaicism occurs when a post-zygotic mitotic error creates genetically different cell lines. The phenotype depends on which tissues carry the abnormal line and the proportion of affected cells. A blood karyotype can therefore be normal while another tissue is affected.

7. Structural chromosome abnormalities

Abnormality	Mechanism	Typical consequence
Deletion	Segment is lost	Haploinsufficiency, contiguous-gene syndrome or cancer driver
Duplication	Segment is repeated	Increased gene dosage or altered reading frame
Inversion	Segment excises, reverses and reinserts	Often balanced, but may disrupt a gene or impair meiosis
Reciprocal translocation	Segments exchange between non-homologous chromosomes	Balanced carrier may be well; offspring may inherit an unbalanced complement
Robertsonian translocation	Long arms of acrocentric chromosomes fuse	Carrier has 45 chromosomes but may be phenotypically normal; reproductive risk varies
Ring chromosome	Both ends break and join in a ring	Loss of terminal material and mitotic instability
Isochromosome	One arm is duplicated and the other lost	Gene dosage imbalance, for example some Turner variants
Dicentric/acentric chromosome	Two centromeres or no centromere	Segregation failure and cell loss

7.1 Balanced versus unbalanced

A balanced rearrangement contains the same overall DNA amount, although gene disruption or altered regulation can still cause disease. An unbalanced rearrangement has extra or missing material and is more likely to produce congenital anomalies, developmental delay or pregnancy loss. A clinically healthy parent may carry a balanced translocation and have recurrent miscarriages or a child with an unbalanced karyotype.

8. Chromosome banding and nomenclature

Conventional karyotyping arrests dividing cells, stains condensed chromosomes and arranges them by size, centromere position and banding pattern. G-banding creates a reproducible map. A locus such as 7q31 means chromosome 7, long arm (q), region 3, band 1. Banding detects large changes but may miss small copy-number variants or single-gene variants.

9. Laboratory methods

Test	Best for	Important limitation
Karyotype	Large aneuploidy and structural rearrangements in dividing cells	Low resolution; may miss small or low-level mosaic changes
FISH	Rapid targeted detection of a known locus or aneuploidy	Only answers the probes selected; not a genome-wide test
Chromosomal microarray	Genome-wide deletions and duplications (copy-number changes)	Usually does not detect balanced translocations or low-level mosaicism reliably
QF-PCR or rapid aneuploidy testing	Selected common chromosome-number abnormalities	Limited scope and requires confirmatory planning
Genome/exome sequencing	Small variants and some structural changes	Interpretation, incidental findings and coverage limitations
Bone-marrow cytogenetics	Acquired chromosome changes in leukaemia and other malignancies	Somatic result does not automatically indicate inherited risk

Specimen choice matters. Blood is useful for constitutional testing, while chorionic villus, amniotic fluid, tumour tissue or bone marrow may be required for a specific clinical question. Mosaicism and tissue-limited abnormalities require careful interpretation.

10. Examples with clinical relevance

Trisomy 21

Down syndrome results from an extra copy of chromosome 21, usually due to meiotic non-disjunction, but translocation and mosaic forms occur. Emergency concerns include congenital heart disease, airway problems, atlanto-axial instability considerations, infection, thyroid disease and transient abnormal myelopoiesis. Treat the acute presentation first and use syndrome-aware assessment.

Trisomy 18 and trisomy 13

These conditions are associated with multiple congenital anomalies, severe developmental impairment and high infant mortality. Decisions should be individualised with multidisciplinary and family-centred care.

45,X and sex-chromosome variations

Turner syndrome may involve monosomy X or mosaic/structural X changes, with short stature, gonadal dysgenesis, cardiac and renal anomalies. Klinefelter syndrome (commonly 47,XXY) may present with tall stature, hypogonadism, infertility or learning differences. Phenotype varies, and the karyotype should not be used to stereotype the patient.

22q11.2 deletion and other microdeletions

Copy-number loss can affect heart development, palate, immunity, calcium regulation and neurodevelopment. An infant with seizures, hypocalcaemia, conotruncal heart disease or recurrent infections may need targeted or microarray evaluation.

Acquired chromosome abnormalities in cancer

Somatic translocations, deletions, amplifications and aneuploidy can activate oncogenes or inactivate tumour suppressors. They are acquired in tumour cells and are not automatically inherited by children. Results may guide targeted therapy and prognosis.

11. Emergency medicine applications

• Newborn with respiratory distress or multiple anomalies: stabilise airway, breathing and circulation; assess glucose, temperature and cardiac disease while arranging genetics and imaging.
• Unexplained neonatal hypocalcaemic seizure: treat the seizure and calcium emergency, then investigate congenital syndromes, including chromosomal copy-number disorders when indicated.
• Recurrent miscarriage or stillbirth: consider parental balanced rearrangement, aneuploidy or other causes; refer for counselling rather than offering unsupervised testing.
• Adult with sudden arrhythmia or cardiomyopathy: a syndromic or familial chromosome disorder may be relevant, but urgent resuscitation and ECG-based management come first.
• Known chromosome disorder with fever or shock: do not attribute deterioration to the syndrome; evaluate sepsis, bleeding, airway, cardiac and metabolic causes normally.

12. Genetic counselling and communication

• Explain what test is being ordered, what it can detect and what it cannot.
• Discuss possible results: pathogenic, benign, uncertain, incidental or non-diagnostic.
• Protect privacy and obtain consent for testing and sharing results.
• Offer family testing only when clinically justified and with counselling.
• Use respectful, person-centred language; a chromosome finding does not define the whole person.

References and further reading

• NCBI Bookshelf: Chromosomes and human genetics.
• MedlinePlus Genetics: What is a chromosome?.
• National Human Genome Research Institute genetics glossary.
• Use local laboratory, genetics, newborn-screening and consent policies when ordering or interpreting chromosome tests.