Chromosomes are thread-like structures found inside the nucleus of every cell, carrying an organism’s genetic information as organized packages of DNA and associated proteins. Karyotyping is a technique that allows scientists and clinicians to visualize this entire chromosomal arrangement. This process provides a comprehensive map of an individual’s genome, systematically arranging the chromosomes to check for completeness and structural integrity. This visualization method is a foundational tool in cytogenetics, the study of chromosomes.
Defining Karyotyping and the Chromosomal Blueprint
Karyotyping formally refers to the laboratory-produced image of a person’s complete set of chromosomes, isolated from a single cell and arranged in a standardized format. The human chromosomal blueprint normally consists of 46 chromosomes, organized into 23 homologous pairs. Twenty-two pairs are autosomes, numbered 1 to 22, while the remaining pair consists of the sex chromosomes (XX for females, XY for males).
Cytogeneticists arrange these homologous pairs based on their size, the position of the centromere, and the unique light and dark banding pattern created by specialized staining. Analyzing this pattern allows for the precise identification of each chromosome pair.
The Steps of Creating a Karyotype
The process begins with obtaining a sample of actively dividing cells. These cells can be sourced from:
- Peripheral blood
- Amniotic fluid
- Chorionic villi
- A tissue biopsy
Cells must be cultured in a laboratory environment to stimulate growth and division. Once the cells are rapidly dividing, a chemical agent like colchicine is introduced to halt the cell cycle specifically during metaphase.
Metaphase is the ideal stage for visualization because the chromosomes are maximally condensed and most visible. The cells are then treated with a hypotonic solution to swell them, causing the chromosomes to spread out when fixed onto a glass slide. The standard staining method used is G-banding, which involves treating the chromosomes with an enzyme, trypsin, and then staining them with Giemsa dye.
This G-banding technique produces a characteristic pattern of light and dark bands along the length of each chromosome. After imaging the spread chromosomes, the final step involves digitally cutting out each individual chromosome and arranging them in homologous pairs according to the standardized format, creating the final karyogram.
Interpreting Results and Identifying Chromosomal Changes
The completed karyogram is analyzed for any deviations from the normal count or structure. Karyotyping is particularly effective at detecting two main categories of chromosomal changes: numerical and structural abnormalities. Numerical abnormalities, known as aneuploidies, involve having too many or too few copies of an entire chromosome.
A common example is Trisomy 21 (Down syndrome), where an individual possesses three copies of chromosome 21 instead of the usual two. Structural abnormalities involve rearrangements within the chromosome, such as translocations, deletions, or inversions. A translocation occurs when a segment of one chromosome breaks off and attaches to a different, non-homologous chromosome.
Translocations are classified as either balanced, meaning no genetic material is lost or gained, or unbalanced, which results in a net gain or loss of material. Results are reported using a standardized nomenclature, which includes the total chromosome number, the sex chromosomes, and any detected abnormality. For example, a normal male is denoted as 46, XY, while a female with Trisomy 21 is written as 47, XX, +21.
Key Clinical Applications
Karyotyping is widely used across several medical fields to diagnose and assess genetic risks. In prenatal diagnosis, the test screens for chromosomal conditions in a developing fetus, especially when risk is increased due to maternal age or family history. Samples for this purpose are typically obtained through amniocentesis or chorionic villus sampling.
The technique is also an important tool in reproductive medicine for couples experiencing infertility or recurrent pregnancy loss. A parent may carry a balanced translocation that does not affect their own health but increases the risk of producing offspring with an unbalanced, often nonviable, chromosome complement. Karyotyping identifies these hidden structural rearrangements, guiding reproductive planning.
In oncology, karyotyping helps detect acquired chromosomal changes characteristic of certain cancers, such as leukemias. These acquired abnormalities, like specific translocations, aid in confirming a cancer diagnosis and predicting disease aggressiveness. Identifying these genomic signatures can also inform treatment decisions, pointing to targeted therapies.
Karyotyping Compared to Other Genetic Tests
Despite its utility, conventional karyotyping is limited by its resolution, detecting changes larger than 5 to 10 megabases (Mb) of DNA. This means that very small deletions, duplications, or single-gene mutations—often responsible for genetic disorders—are missed by this method. The technique also requires living cells that can be successfully cultured and arrested at metaphase, which is not always possible.
Newer genomic technologies, such as array comparative genomic hybridization (aCGH), offer a much higher resolution, capable of detecting changes as small as 50 to 200 kilobases. While aCGH is superior for detecting small copy number variations, it cannot detect balanced structural rearrangements (where no net DNA is lost or gained). Karyotyping remains relevant because it is the only test that can visualize the entire chromosome set, including balanced translocations and mosaicism (the existence of two or more genetically distinct cell lines).