Chromosomal Abnormalities and Fluorescent In Situ Hybridization
Chromosomal abnormalities occur when there are deviations in the structure or number of chromosomes. These deviations are broadly categorized into numerical and structural anomalies.
Numerical Abnormalities
Numerical abnormalities involve a change in the total number of chromosomes from the standard 46 in human somatic cells.
- Aneuploidy: The presence of an abnormal number of chromosomes.
- Monosomy: The loss of a single chromosome, such as Turner syndrome (45,X).
- Trisomy: The gain of an extra chromosome, such as Down syndrome (Trisomy 21), Patau syndrome (Trisomy 13), or Edwards syndrome (Trisomy 18).
- Polyploidy: The presence of extra sets of chromosomes, which is usually incompatible with human life.
Structural Abnormalities
Structural abnormalities arise when a chromosome undergoes breakage and incorrect rejoining.
- Deletion: A segment of a chromosome is lost. An example is Cri-du-chat syndrome (deletion of the short arm of chromosome 5).
- Duplication: A segment of a chromosome is repeated, leading to extra genetic material.
- Inversion: A segment of a chromosome breaks, reverses its orientation, and reattaches.
- Translocation: A segment of one chromosome breaks off and attaches to a different chromosome.
- Ring Chromosome: The ends of a chromosome break off and join together to form a circle.
Fluorescent In Situ Hybridization (FISH)
Fluorescent In Situ Hybridization is a molecular cytogenetic technique used to detect and locate specific DNA sequences on chromosomes. It acts as a bridge between traditional cytogenetics and molecular genetics.
Working Principle
The technique relies on the ability of single-stranded DNA probes to bind (hybridize) to their complementary sequences on the target chromosome.
- Probe Preparation: DNA probes are synthesized and labeled with fluorescent dyes.
- Denaturation: The target chromosomal DNA and the fluorescent probe are denatured (made single-stranded) using heat or chemicals.
- Hybridization: The probe binds to the specific complementary sequence on the target chromosome.
- Detection: The hybridized probe is visualized using a fluorescence microscope.
Applications of FISH
- Identification of Microdeletions: Detects small chromosomal deletions that are too minute for standard G-banding karyotyping to identify.
- Prenatal Diagnosis: Used to rapidly screen for common aneuploidies in fetal cells.
- Cancer Genetics: Identifies specific gene amplifications (such as HER2 in breast cancer) or chromosomal translocations that drive cancer progression.
- Gene Mapping: Pinpoints the physical location of specific genes on chromosomes.
Comparison of Diagnostic Methods
| Method | Resolution | Primary Use Case |
| Karyotyping | Low (5-10 Megabases) | Whole genome overview, large aneuploidy |
| FISH | Moderate (50-100 Kilobases) | Targeted diagnosis of specific loci |
| Chromosomal Microarray | High (10-50 Kilobases) | Genome-wide detection of copy number changes |
Key Concepts in Cytogenetics
- Centromeres are the primary constricted regions of chromosomes that facilitate proper segregation during cell division. Telomeres are protective caps at the ends of chromosomes that prevent degradation and fusion.
- The term mosaicism describes a condition where an individual possesses two or more populations of cells with different genotypes, often resulting from non-disjunction during early embryonic development.
- Robertsonian translocation is a specific type of structural rearrangement involving acrocentric chromosomes, such as 13, 14, 15, 21, and 22. In this process, the long arms of two acrocentric chromosomes fuse, and the short arms are lost. This type of translocation is a common cause of familial Down syndrome.
- Non-disjunction is the failure of homologous chromosomes or sister chromatids to separate during meiosis or mitosis. This event is the primary cause of aneuploidy in humans.
- Heterochromatin consists of tightly packed, repetitive DNA sequences that are generally transcriptionally inactive. It stains darkly in banding techniques and is concentrated around centromeres. Euchromatin is less condensed, transcriptionally active, and contains the majority of the expressed genes.
Karyotype nomenclature follows international standards. For example, 46,XX denotes a normal female, while 46,XY denotes a normal male. An individual with Down syndrome is represented as 47,XX,+21. A deletion on the short arm of chromosome 5 is written as 46,XX,del(5p).
