Karyotyping is a laboratory technique that visualizes an individual’s complete set of chromosomes, creating a highly organized image called a karyotype. Its primary purpose is to identify numerical or large structural abnormalities within these chromosomes, helping to understand various health conditions.
What Karyotyping Reveals
A karyotype displays an individual’s chromosomes, arranged in pairs. These pairs are ordered by size, from largest to smallest, with the sex chromosomes (X and Y) at the end. In humans, a normal karyotype consists of 23 pairs, totaling 46 chromosomes: 22 pairs of autosomes and one pair of sex chromosomes.
Karyotyping reveals numerical abnormalities, such as an incorrect number of chromosomes. For instance, Down syndrome (Trisomy 21) is identified by an extra copy of chromosome 21. Turner syndrome involves only one X chromosome (XO), while Klinefelter syndrome is characterized by an extra X chromosome in males (XXY). The technique also detects large structural abnormalities, such as large deletions or duplications of chromosome segments. Translocations, where a piece of one chromosome breaks off and attaches to another, and inversions, where a segment is reversed, can also be observed.
Why Karyotyping is Performed
Karyotyping is frequently used in prenatal diagnosis to assess a developing baby for chromosomal conditions, particularly with advanced maternal age or a family history of genetic disorders. This test can be conducted using samples obtained through procedures like amniocentesis or chorionic villus sampling.
The technique is also employed to investigate unexplained developmental delays or intellectual disabilities in infants and children, helping to identify underlying chromosomal causes. For couples experiencing infertility or recurrent miscarriages, karyotyping can determine if chromosomal abnormalities in either partner are contributing factors. Karyotyping plays a role in the diagnosis and monitoring of certain cancers, such as leukemia and lymphoma, where specific chromosomal rearrangements like the Philadelphia chromosome in chronic myelogenous leukemia can indicate the disease and guide treatment.
How Karyotyping is Done
Karyotyping begins by collecting a cell sample from the individual. Common sources include blood, bone marrow, amniotic fluid, or chorionic villus samples. For blood samples, white blood cells are typically used because they have a nucleus containing chromosomes and are capable of active division.
Once collected, the cells are cultured in a laboratory dish, allowing them to grow and divide over several days. To visualize the chromosomes clearly, cell division is then arrested in metaphase, a stage where chromosomes are most condensed and visible. The cells are treated to spread the chromosomes apart, making them easier to analyze.
The chromosomes are stained, often using Giemsa dye, which produces unique light and dark banding patterns along their length. These distinctive patterns allow for the identification of individual chromosomes and the detection of structural changes. The stained chromosomes are then photographed through a microscope, and the images are digitally arranged into a karyogram, ordered by size and centromere position for analysis.
Limitations of Karyotyping
Karyotyping has limitations in what it can detect. The resolution of conventional karyotyping is limited to detecting chromosomal abnormalities larger than 5 to 10 megabases (Mb). This means that very small deletions or duplications, often referred to as microdeletions or microduplications, may not be visible under a microscope.
Karyotyping is also unable to identify single-gene mutations, which are changes within a single gene that can cause conditions like cystic fibrosis or sickle cell anemia. It focuses on the overall structure and number of chromosomes, not on the sequence of DNA within individual genes. Therefore, a normal karyotype result does not exclude the presence of all genetic conditions, as many disorders are caused by changes too subtle for this method to detect.