A karyotype is a standardized, organized visual profile of an organism’s complete set of chromosomes. It is essentially a laboratory-produced image where chromosomes are isolated from a single cell, photographed, and systematically arranged. The resulting map, often called a karyogram, is a powerful tool used in cytogenetics to assess the number and structure of these genetic packages.
The Foundation of the Karyotype
Chromosomes are the condensed packages of DNA found in the nucleus of a cell. In humans, the typical somatic cell contains 46 chromosomes organized into 23 pairs. These pairs are known as homologous chromosomes, meaning the two members of the pair are similar in length, centromere position, and banding pattern. One chromosome from each homologous pair is inherited from each parent.
The 23 pairs are divided into 22 pairs of autosomes and one pair of sex chromosomes. Autosomes are numbered 1 through 22, with the number 1 pair being the largest and the number 22 pair being the smallest. The sex chromosomes, XX for a female or XY for a male, constitute the 23rd pair and determine biological sex. For the karyotype to be readable, homologous pairs are aligned in descending order of size, with the sex chromosomes placed at the end.
Generating the Chromosomal Map
Creating a karyotype begins with obtaining a sample of actively dividing cells, often from peripheral blood, amniotic fluid, or bone marrow. These cells are cultured in a laboratory medium for several days to ensure a sufficient number of cells are undergoing division.
To capture the chromosomes in their most condensed state, a chemical agent like colchicine is added to arrest the cells during the metaphase stage of mitosis. The arrested cells are then treated with a hypotonic solution, causing them to swell and burst, which spreads the chromosomes out on a microscope slide.
A specialized technique called G-banding uses Giemsa dye to stain the chromosomes. This staining produces a unique pattern of light and dark horizontal bands along the length of each chromosome, allowing each homologous pair to be definitively identified. Finally, the stained chromosomes are viewed under a microscope, photographed, and digitally arranged into the final karyogram, pairing up the homologs and ordering them by size.
How Karyotypes Reveal Genetic Conditions
The karyotype is used to detect large-scale numerical and structural abnormalities in the genome. Numerical abnormalities, known as aneuploidy, involve the gain or loss of entire chromosomes. The most common example is Trisomy 21 (Down syndrome), identified by the presence of three copies of chromosome 21 instead of two. Conversely, the absence of a chromosome, known as monosomy, is seen in conditions like Turner syndrome, where a female has only one X chromosome (45,X).
Karyotyping also identifies major structural changes within chromosomes, including the movement, loss, or duplication of large DNA segments. A translocation occurs when a segment of one chromosome breaks off and attaches to a non-homologous chromosome. This rearrangement is considered balanced if no genetic material is lost or gained, though it can affect reproduction. Other structural defects include deletions, where a part of a chromosome is missing, and inversions, where a segment breaks off, flips around, and reattaches. This visualization provides genetic information necessary for diagnosis and counseling.