Residency · Residency · Medical Genetics Genomics

Chromosome Structure, Banding, and Karyotype Analysis

Chromosome Structure

Basic Architecture

Human somatic cells contain 46 chromosomes organized as 22 pairs of autosomes plus 2 sex chromosomes. Each chromosome consists of three essential structural elements: a centromere, two telomeres, and origins of replication. The position of the centromere defines chromosome morphology, classifying chromosomes as metacentric (centromere near the middle), submetacentric (centromere offset from center), or acrocentric (centromere near one end). The five acrocentric chromosomes (13, 14, 15, 21, and 22) are distinctive because they carry ribosomal RNA genes on short-arm stalks and satellites.

Centromere Biology

Centromeres contain alpha-satellite (alphoid) DNA arrays, typically 0.2 to 5 megabases in length. The histone variant CENP-A replaces histone H3 at centromeric nucleosomes and serves as the epigenetic mark that directs kinetochore assembly. The kinetochore complex built upon this foundation mediates spindle microtubule attachment during cell division. Neocentromeres are a rare but clinically relevant phenomenon in which functional centromeres form at ectopic locations devoid of alpha-satellite DNA. These are particularly relevant in the context of marker chromosomes.

Telomere Structure and Function

Telomeres consist of TTAGGG repeats, typically 5 to 15 kilobases in length. The shelterin complex, comprising TRF1, TRF2, POT1, TIN2, TPP1, and RAP1, protects chromosome ends from degradation and end-to-end fusion. Progressive telomere shortening with each cell division eventually triggers replicative senescence, a phenomenon known as the Hayflick limit. Telomere biology disorders, including dyskeratosis congenita and idiopathic pulmonary fibrosis, result from mutations in telomere maintenance genes such as TERT, TERC, DKC1, and RTEL1.

Chromosome Banding Techniques

G-Banding (Giemsa Banding)

G-banding has served as the gold standard for clinical cytogenetics since the 1970s. The technique involves trypsin digestion of metaphase chromosomes followed by Giemsa staining, producing a characteristic pattern of dark and light bands. At standard resolution, approximately 400 to 550 bands are visible, while high-resolution banding can achieve 550 to 850 bands. G-dark bands correspond to AT-rich, gene-poor, late-replicating regions of the genome, while G-light bands are GC-rich, gene-rich, and early-replicating. The resolution limit for detecting structural abnormalities by G-banding is approximately 5 to 10 megabases.

Other Banding Methods

Q-banding, using quinacrine fluorescence, was historically the first banding method developed by Caspersson in 1968. It is particularly useful for identifying the Y chromosome, which shows bright fluorescence of its heterochromatic long arm. R-banding uses heat denaturation to produce a reverse pattern of G-bands and is useful when G-banding gives technically suboptimal results for terminal regions. C-banding stains constitutive heterochromatin at centromeres and the Y long arm, primarily used to evaluate heterochromatin polymorphisms. NOR staining (silver staining) identifies active nucleolar organizer regions on acrocentric short arms and is useful for characterizing Robertsonian translocations. DAPI/DA staining preferentially stains AT-rich regions and is commonly used as a counterstain in FISH.

Karyotype Nomenclature (ISCN)

International System for Human Cytogenomic Nomenclature

The ISCN provides the standardized nomenclature for describing normal and abnormal karyotypes. The current edition is ISCN 2020, which is updated periodically. The standard format lists the total chromosome number, the sex chromosome complement, and then any abnormalities.

Common Nomenclature Examples

A normal male karyotype is written as 46,XY and a normal female as 46,XX. Trisomy 21 (Down syndrome) is designated 47,XX,+21 or 47,XY,+21. Turner syndrome appears as 45,X, and Klinefelter syndrome as 47,XXY. A balanced reciprocal translocation is written as 46,XX,t(2;5)(p21;q31), while a Robertsonian translocation is designated 45,XY,der(14;21)(q10;q10). Deletions are noted as in 46,XX,del(5)(p15.2) for cri-du-chat syndrome, and inversions as 46,XY,inv(9)(p11q13), which is a common benign variant. An isochromosome is written as 46,X,i(Xq) for the isochromosome Xq seen in a Turner syndrome variant, a ring chromosome as 46,XX,r(18)(p11.2q23), and mosaicism as mos 45,X[15]/46,XX[35].

Band Designation

Each chromosome arm (p for short, q for long) is divided into regions, bands, and sub-bands. Numbering increases away from the centromere, so 1p36 refers to the most distal band on the short arm of chromosome 1. Landmark bands define regional boundaries, and sub-bands provide higher resolution, as in 15q11.2.

Numerical Chromosome Abnormalities

Aneuploidy

The most common viable trisomies are trisomy 21 (occurring in approximately 1 in 700 births), trisomy 18 (1 in 5,000), and trisomy 13 (1 in 16,000). Autosomal monosomy is uniformly lethal, and Turner syndrome (45,X) is the only viable monosomy. Aneuploidy most commonly results from meiotic nondisjunction, with the maternal age effect being strongest for trisomy 21. Rescue mechanisms, including trisomy rescue (which can lead to uniparental disomy) and monosomy rescue, can modify the consequences of the initial nondisjunction event.

Polyploidy

Triploidy (69 chromosomes) occurs in 1 to 3% of pregnancies and is usually lethal. The parental origin of the extra haploid set matters: diandric triploidy produces a partial hydatidiform mole phenotype, while digynic triploidy results in a growth-restricted fetus with a small placenta. Tetraploidy (92 chromosomes) is very rare and almost always lethal.

Sex Chromosome Aneuploidy

Klinefelter syndrome (47,XXY) occurs in about 1 in 600 males and is characterized by tall stature, infertility, and learning difficulties. The 47,XYY karyotype occurs in about 1 in 1,000 males and usually produces a mild phenotype with tall stature. Triple X syndrome (47,XXX) occurs in about 1 in 1,000 females, typically with a mild phenotype and possible learning difficulties. Turner syndrome (45,X) occurs in about 1 in 2,500 females and presents with short stature, ovarian insufficiency, and cardiac and renal anomalies.

Structural Chromosome Abnormalities

Translocations

Reciprocal translocations involve the exchange of segments between non-homologous chromosomes. Balanced carriers are phenotypically normal but face a risk of producing unbalanced offspring. Robertsonian translocations involve the fusion of two acrocentric chromosomes at or near their centromeres. The most common are rob(13;14) and rob(14;21). Carriers of rob(14;21) face a risk of having offspring with trisomy 21, estimated at approximately 1% for paternal carriers and 10 to 15% for maternal carriers.

Deletions and Duplications

Terminal deletions involve loss of a chromosomal segment including the telomere and are stabilized by telomere capture or healing. Interstitial deletions involve loss of material within a chromosome arm. Duplications, representing an extra copy of a chromosomal segment, generally produce milder phenotypes than the corresponding deletion.

Inversions

Paracentric inversions have both breakpoints on the same side of the centromere. Recombinant chromosomes from paracentric inversions are typically inviable. Pericentric inversions, with breakpoints on both sides of the centromere, carry a higher risk of producing viable unbalanced offspring. The inversion inv(9)(p11q13) is a common population variant that is generally not clinically significant.

Other Structural Abnormalities

Isochromosomes consist of a chromosome with two identical arms; i(Xq) is commonly seen in Turner syndrome. Ring chromosomes form when both telomeric regions are deleted and the broken ends fuse into a circle. They are associated with "ring syndrome" (growth failure) due to dynamic mosaicism. Marker chromosomes are small supernumerary chromosomes whose clinical significance depends on their euchromatic content, which can be characterized by FISH or microarray. Insertions, in which a segment from one chromosome is inserted into another, carry a particularly high recurrence risk for unbalanced offspring, approximately 50%.

Practical Report Interpretation

Indications for Karyotype

Karyotyping is indicated when aneuploidy or a sex chromosome abnormality is suspected, in individuals with multiple congenital anomalies or intellectual disability (though microarray has increasingly replaced karyotype as the first-tier test for this indication), for parents experiencing recurrent pregnancy loss, as part of infertility evaluation (especially for azoospermia), for hematologic malignancies (bone marrow karyotype), and to confirm mosaic or structural abnormalities not fully resolved by microarray.

Limitations

The resolution of conventional karyotyping is limited to approximately 5 to 10 megabases. It requires actively dividing cells, whether PHA-stimulated lymphocytes, amniocytes, or fibroblasts. Turnaround time is 7 to 14 days for constitutional studies and 3 to 5 days for oncology. Karyotyping cannot detect submicroscopic deletions or duplications, point mutations, or methylation abnormalities. Cryptic rearrangements, such as subtelomeric rearrangements, may be missed.

<image>A high-resolution ideogram of the complete human karyotype (46,XY) showing G-banded chromosomes arranged in standard Denver classification order (groups A through G plus sex chromosomes). Each chromosome is depicted with numbered bands at the 550-band level. Centromere positions are clearly indicated, and acrocentric chromosomes (13, 14, 15, 21, 22) are labeled with their satellite stalks and NOR regions on the short arms. A legend distinguishes G-dark (AT-rich) from G-light (GC-rich) bands.</image>

<image>A diagram illustrating the meiotic segregation outcomes for a Robertsonian translocation carrier rob(14;21). The center shows the trivalent formed during meiosis I with the der(14;21), normal 14, and normal 21. Six possible segregation outcomes are shown radiating outward: alternate segregation producing a balanced carrier and a normal karyotype (highlighted in green as viable), and adjacent segregation producing trisomy 21, monosomy 21, trisomy 14, and monosomy 14 (with monosomy outcomes crossed out as lethal and trisomy 14 marked as lethal). The expected liveborn ratios are annotated.</image>

<image>A step-by-step visual protocol for G-banding: (1) peripheral blood sample in heparinized tube, (2) PHA-stimulated lymphocyte culture in an incubator at 37 degrees for 72 hours, (3) colcemid arrest at metaphase showing a mitotic spindle being disrupted, (4) hypotonic KCl treatment showing cell swelling, (5) fixation with methanol-acetic acid (Carnoy fixative), (6) slide preparation with chromosome spreading, (7) trypsin digestion and Giemsa staining, (8) microscopic analysis showing banded metaphase spread, and (9) digital karyotype arrangement with matched homologs. Each step is illustrated as a simple laboratory diagram with brief labels.</image>

Clinical Pearls

G-banding resolution is limited to approximately 5 to 10 megabases, meaning that submicroscopic copy number variants require chromosomal microarray or FISH for detection. Robertsonian translocation carriers involving chromosome 21 have a significant risk for offspring with Down syndrome, and genetic counseling with prenatal testing should be offered. Inversions of chromosome 9, specifically inv(9)(p11q13), and heterochromatin length polymorphisms are common benign variants that should not be over-interpreted clinically. It is essential to correlate karyotype findings with the clinical phenotype, as apparently balanced rearrangements may cause disease through gene disruption at breakpoints or through cryptic imbalance detectable only by microarray. Ring chromosomes produce dynamic mosaicism due to mitotic instability, which can lead to variable phenotypes and progressive features over time. Insertions carry up to 50% risk for unbalanced offspring, making them one of the highest recurrence risk structural rearrangements.

References

• ISCN 2020: An International System for Human Cytogenomic Nomenclature. McGowan-Jordan J, Hastings RJ, Moore S (eds). Karger, 2020.
• Gardner RJM, Amor DJ. Gardner and Sutherland's Chromosome Abnormalities and Genetic Counseling. 5th ed. Oxford University Press, 2018.
• Shaffer LG, Bejjani BA. A cytogeneticist's perspective on genomic microarrays. Hum Reprod Update. 2004;10(3):221-226.
• ACMG Standards and Guidelines for constitutional cytogenomic microarray analysis. Genet Med. 2013;15(11):901-909.

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