Karyotyping is a laboratory technique used to create a visual representation of an organism’s chromosomes. The process provides a standardized picture of the entire set of chromosomes, allowing scientists to analyze their number and structure. Chromosomes are composed of deoxyribonucleic acid (DNA) tightly packed with proteins, and this method offers a comprehensive snapshot of an individual’s genome and is a foundational tool in the field of cytogenetics.
Creating the Chromosome Map
The generation of a karyotype begins with obtaining a sample of actively dividing cells, often sourced from peripheral blood, amniotic fluid, or bone marrow. Since red blood cells lack a nucleus, the analysis focuses on white blood cells, which are capable of division and contain the full set of chromosomes. These cells are then placed in a specialized culture medium and incubated for several days.
Once the cells are proliferating, a chemical agent like colchicine is introduced to the culture. Colchicine works by arresting cell division at the metaphase stage, when the chromosomes are at their most condensed state under a microscope. The arrested cells are then treated with a hypotonic saline solution, causing them to swell and burst, which spreads the chromosomes out onto a slide.
To make the chromosomes visible, they are treated with a specialized stain, most commonly Giemsa stain in a process known as G-banding. This staining technique produces a pattern of light and dark stripes along the length of each chromosome. These unique banding patterns function like molecular barcodes, allowing each specific chromosome to be identified. The final steps involve photographing the spread chromosomes and then using specialized software to arrange the individual images into a standardized chart.
Understanding Karyotype Organization
The resulting chart, also called a karyogram, organizes the data according to a specific set of criteria. Chromosomes are sorted and aligned into homologous pairs, meaning each pair consists of two chromosomes that are approximately the same size and contain the same genes in the same locations. Humans normally possess 23 pairs of chromosomes, totaling 46 individual structures.
The first 22 pairs are known as autosomes, and they are arranged in descending order of size, beginning with chromosome 1 as the largest and ending with chromosome 22 as the smallest. The final pair is comprised of the sex chromosomes, which determine the biological sex of the individual. A female typically has two X chromosomes (XX), while a male possesses one X and one Y chromosome (XY).
The light and dark banding patterns created by the G-banding stain are used for the interpretation of the karyotype. These stripes correspond to differences in the DNA composition and structure along the chromosome, and they are consistent for every chromosome of a particular type. Analyzing these patterns allows cytogeneticists to detect subtle structural changes that might not be apparent from size alone. The centromere position, which is the constricted point on the chromosome, is also analyzed and used in the organization process.
Detecting Genetic Conditions
Karyotyping is used to identify two primary types of chromosomal irregularities: numerical and structural abnormalities. Numerical abnormalities involve having an incorrect number of chromosomes, a condition known as aneuploidy. This results from nondisjunction, which is the failure of chromosomes to separate correctly during cell division.
A common example of a numerical abnormality is Trisomy 21, also known as Down syndrome, where an individual has three copies of chromosome 21 instead of the usual two copies. Other conditions caused by extra or missing sex chromosomes include Turner syndrome, characterized by a single X chromosome (45,X), and Klinefelter syndrome, where a male has an extra X chromosome (47,XXY). These numerical changes affect development and are often associated with distinct clinical presentations.
Structural abnormalities involve alterations to one or more chromosomes, which can include the loss, gain, or rearrangement of genetic material. Examples include translocations, where segments of two non-homologous chromosomes are exchanged, or deletions, where a portion of a chromosome is missing. Inversions, which occur when a segment of a chromosome breaks off, flips around, and reattaches, can also be detected. Karyotyping is used in prenatal diagnosis and in cases of unexplained developmental delay or infertility.