Core idea Chromosomal abnormalities are changes in chromosome number or structure that alter gene dosage, gene disruption or genome stability. Their clinical effects range from early miscarriage to recognizable syndromes, infertility, developmental disability, congenital malformations and cancer. The phenotype depends on the chromosome involved, the size and location of the change, mosaicism, uniparental disomy and the genes affected.
Learning outcomes
Explain normal chromosome organisation and notation; distinguish numerical from structural abnormalities; describe nondisjunction, anaphase lag and breakage-rejoining; interpret karyotype, FISH, microarray and sequencing results; recognise major viable aneuploidies; and counsel patients about mosaicism, recurrence risk and diagnostic limits.
1. Normal chromosome organisation
Human somatic cells normally contain 46 chromosomes: 22 pairs of autosomes and one pair of sex chromosomes. Each chromosome contains a short arm (p), a long arm (q), a centromere and telomeres. During the cell cycle, DNA replicates in S phase so each chromosome temporarily consists of two sister chromatids joined at the centromere. A karyotype describes chromosome number and visible structure, not every base-pair sequence.
Genes are distributed along chromosomes and are sensitive to dosage. A deletion may remove one copy of several genes; a duplication may create three copies; a balanced translocation can preserve total DNA but disrupt a gene or alter its regulation. The clinical effect is therefore not predicted by size alone.
2. Numerical abnormalities
Aneuploidy is gain or loss of one or more individual chromosomes, such as trisomy or monosomy. Polyploidy is an extra complete chromosome set, such as triploidy. Aneuploidy usually results from nondisjunction—the failure of homologous chromosomes or sister chromatids to separate—or anaphase lag, in which a chromosome is left behind and excluded from a daughter nucleus.
| Mechanism | Cellular event | Typical consequence |
|---|---|---|
| Meiotic nondisjunction I | Homologous chromosomes fail to separate. | Gametes receive both homologues or none; after fertilisation, trisomy or monosomy may result. |
| Meiotic nondisjunction II | Sister chromatids fail to separate. | Two abnormal and two normal gametes may be produced. |
| Mitotic nondisjunction | Separation fails after fertilisation. | Mosaicism: two or more cell lines in one individual. |
| Anaphase lag | A chromosome fails to migrate into one daughter nucleus. | Loss of a chromosome in a daughter cell, producing mosaic monosomy. |
| Polyploidy | Extra complete haploid set from dispermy or meiotic failure. | Usually severe malformations and early pregnancy loss. |
3. Structural abnormalities
Structural abnormalities follow chromosome breakage with abnormal repair. A deletion removes a segment; a duplication repeats it; an inversion reverses a segment; a translocation moves material between chromosomes; and a ring chromosome forms when both ends break and rejoin. An insertion places a segment into another location. Isochromosomes contain two copies of one arm and loss of the other.
A structural change may be balanced or unbalanced. Balanced carriers may be healthy but produce gametes with partial monosomy or trisomy, causing miscarriage or an affected child. An apparently balanced change can still disrupt a gene or alter a regulatory landscape, so a normal-looking phenotype does not always mean zero risk.
Deletion
Loss of genetic material. Terminal deletions involve an end; interstitial deletions occur between two breakpoints. Severity reflects dosage-sensitive genes and mosaicism.
Duplication
Extra genomic material may increase gene dosage and cause developmental or congenital phenotypes. Duplications can be tandem, inverted or inserted elsewhere.
Inversion
A segment reverses orientation. Pericentric inversions include the centromere; paracentric inversions do not. Carriers may be normal but have reproductive risk from abnormal recombinant chromosomes.
Translocation
Reciprocal exchange may be balanced; Robertsonian translocation joins long arms of acrocentric chromosomes and can predispose to translocation trisomy.
4. Mosaicism and chimerism
Mosaicism means genetically different cell lines derived from one zygote. Post-zygotic nondisjunction or anaphase lag may produce a normal and an abnormal line. The percentage in blood may not represent the percentage in brain, heart, placenta or gonads, so a low blood mosaic result does not automatically predict a mild phenotype.
Chimerism refers to cell lines from two different zygotes, for example after twin fusion or transplantation. Confined placental mosaicism occurs when abnormal cells are present in placenta but not the fetus; it can cause an abnormal screening result while fetal diagnostic testing is normal, or affect placental function and growth.
Mosaicism requires context: report the tissue tested, cell count, method, percentage and detection limit. Never convert “20% mosaic in blood” directly into “20% affected body.”
5. Major viable aneuploidies
Trisomy 21 (Down syndrome)
Trisomy 21 most often results from meiotic nondisjunction, but translocation and mosaic forms occur. Typical findings may include characteristic facial appearance, hypotonia, developmental delay, congenital heart disease, hearing or vision problems, thyroid disease, gastrointestinal anomalies and increased risk of acute leukemia and Alzheimer-type pathology. Features vary widely; a karyotype and clinical assessment are needed rather than diagnosis by appearance alone.
Trisomy 18 (Edwards syndrome)
Trisomy 18 is associated with severe growth restriction, clenched hands with overlapping fingers, characteristic foot posture, congenital heart disease, brain and renal anomalies and high infant mortality. Mosaic or partial forms may be less severe. Care should include clear communication of prognosis, comfort, family goals and specialist review.
Trisomy 13 (Patau syndrome)
Trisomy 13 may cause holoprosencephaly, cleft lip or palate, microphthalmia, polydactyly, congenital heart and renal anomalies and severe neurodevelopmental impairment. The diagnosis should be confirmed and management individualised; counselling must avoid assuming that every child has the same course.
45,X and sex-chromosome aneuploidies
Turner syndrome (45,X or mosaic variants) may present with short stature, gonadal dysgenesis, infertility, webbed neck, coarctation of the aorta, renal anomalies, hearing problems and specific learning difficulties. Klinefelter syndrome (47,XXY) commonly causes small firm testes, infertility, reduced androgenisation, tall stature and variable language or executive difficulties. 47,XXX and 47,XYY have variable, often subtle phenotypes. Explain that a sex-chromosome result does not predict personality or worth.
6. Microdeletions, duplications and copy-number change
Some clinically important abnormalities are too small for routine karyotyping. Chromosomal microarray detects genome-wide gains and losses at higher resolution and can identify pathogenic copy-number variants in developmental delay, congenital anomalies, autism or multiple malformations. It may also reveal variants of uncertain significance or consanguinity-related regions of homozygosity, so pre-test counselling is essential.
7. Laboratory diagnosis
| Method | Strength | Important limitation |
|---|---|---|
| Karyotype | Visualises chromosome number and large structural changes; detects balanced rearrangements. | Requires dividing cells and has limited resolution; small copy-number changes may be missed. |
| FISH | Rapid targeted detection in selected loci or cell types; useful for mosaicism and interphase cells. | Only answers the probes chosen; a normal FISH does not exclude abnormalities elsewhere. |
| Chromosomal microarray | Genome-wide copy-number analysis at higher resolution than karyotype. | Usually does not detect balanced translocations or low-level mosaicism reliably; may find VUS. |
| QF-PCR or rapid aneuploidy testing | Fast assessment of selected common aneuploidies. | Limited chromosome coverage and not a complete fetal karyotype. |
| Sequencing | Detects sequence variants and selected structural changes. | Does not replace copy-number or chromosome testing unless the platform and analysis are validated. |
8. Prenatal screening versus diagnosis
Ultrasound, serum screening and cell-free DNA estimate the probability of selected aneuploidies. A screen-positive result is not a diagnosis. Diagnostic testing uses chorionic villus sampling or amniocentesis for karyotype, microarray or a targeted familial test. Explain the procedure, timing, risks, turnaround time and what each test cannot detect.
If ultrasound shows multiple anomalies, a normal rapid aneuploidy test does not end the evaluation. Consider microarray, targeted testing, fetal MRI or exome sequencing according to the phenotype and local expertise. When a parent carries a balanced rearrangement, offer parental karyotyping and explain the possible unbalanced outcomes.
9. Reading chromosome notation
In 47,XX,+21, 47 is the total chromosome count, XX indicates female sex chromosomes and +21 indicates an extra chromosome 21. 46,XY,t(14;21)(q10;q10) describes a male with a Robertsonian translocation involving chromosomes 14 and 21. mos 46,XX/47,XX,+21 indicates mosaic normal and trisomy-21 cell lines. Students should always ask which specimen and how many cells were analysed.
10. Causes and risk factors
Most aneuploid conceptions arise sporadically through meiotic error, and parental age changes the probability of some trisomies. Structural abnormalities may be inherited from a balanced carrier or arise de novo. Ionising radiation, chemicals and infection should not be blamed without evidence; the cause is often not identifiable. A careful family history and laboratory confirmation are more useful than assigning guilt.
11. Clinical assessment after a chromosome diagnosis
Assessment is syndrome-specific but commonly includes cardiac examination and echocardiography, hearing and vision review, growth and development, thyroid testing, renal imaging, blood counts, feeding and airway assessment, and screening for neurologic, gastrointestinal or orthopedic complications. Link the chromosome result to a surveillance plan rather than leaving it as a label.
12. Counselling and recurrence risk
Explain whether the abnormality is full, mosaic, partial, inherited or de novo. For a de novo trisomy, recurrence risk is usually close to age-related risk but not zero; for a parental balanced translocation, risk depends on the exact rearrangement and reproductive history. Offer parental studies when they will clarify risk. Use absolute numbers, explain uncertainty and discuss prenatal or preimplantation options only within available law and services.
Counselling checklist
- Use the exact laboratory wording and a diagram of the chromosomes.
- Explain phenotype variability and what is not predicted by the result.
- Ask what the family already knows and what decision the result affects.
- Discuss current treatment, surveillance, educational and social support.
- Offer testing of relatives only with consent and appropriate counselling.
- Provide a written summary, referral contact and follow-up date.
13. Examination summary
Chromosome disorders arise from abnormal number, abnormal structure or both. Nondisjunction and anaphase lag cause numerical change; breakage and abnormal repair cause deletions, duplications, inversions, translocations and rings. Mosaicism results from a post-zygotic event and varies by tissue. Karyotype detects large and balanced changes, FISH answers targeted questions, microarray detects copy-number change and sequencing detects nucleotide-level variants. A screen is not a diagnosis, and a chromosome result must be translated into phenotype-specific care and respectful counselling.
