At a glance
Also known as
Cytogenetics; Cytogenetic Analysis; Karyoptyping; Chromosome Karyotype; Chromosome analysis
Why get tested?
To detect abnormalities, in order to help diagnose genetic diseases, some birth defects and certain haematologic and lymphoid disorders
When to get tested?
When pregnancy screening tests are abnormal; when signs of a chromosomal abnormality-associated disorder are present; when a specific abnormality has been detected in a family member; sometimes when a person has leukaemia, lymphoma, myeloma, myelodysplasia or another cancer and an acquired chromosome abnormality is suspected
A blood sample drawn from a vein in your arm; a sample of or chorionic villus from a pregnant woman; a or tissue sample
Confused about genetics?
See our Genetics Information page
Test preparation needed?
What is being tested?
Chromosome studies or karyotyping is a test that evaluates the number and structure of a person's in order to detect abnormalities. Chromosomes are thread-like structures within each cell and contain the body's genetic blueprint. Each chromosome contains thousands of in specific locations. These genes are responsible for a person’s inherited physical characteristics and they have a profound impact on growth, development, and function.
Humans have 46 chromosomes, present as 23 pairs. Twenty-two pairs are found in both genders (autosomes), and one pair (sex chromosomes) is present as either XY (in males) or XX (in females). Normally, all cells in the body that have a nucleus will contain a complete set of the same 46 chromosomes, except for the reproductive cells (eggs and sperm), which contain a half set of 23. This half set is the genetic contribution that will be passed on to a child. At conception, half sets from each parent combine to form a new set of 46 chromosomes in the developing fetus.
Chromosomal abnormalities include both numerical and structural changes. For numerical changes, anything other than a complete set of 46 chromosomes represents a change in the amount of genetic material present and can cause health and development problems. For structural changes, the significance of the problems and their severity depends upon the chromosome that is altered. The type and degree of the problem may vary from person to person, even when the same chromosome abnormality is present.
A karyotype examines a person's chromosomes to determine if the right number is present and to determine if each chromosome appears normal. It requires experience and expertise to perform properly and to interpret the results. While theoretically almost any cells could be used to perform testing, in practice it is usually performed on a chorionic villus sample or to evaluate a fetus and on (a white blood cell) from a blood sample to test at other ages. Alternatively, white blood cells may be obtained from bone marrow aspirations to look for changes in individuals suspected of having haematologic or lymphoid diseases (e.g., leukaemia, lymphoma, myeloma, refractory anaemia).
The test is performed by:
- Taking a sample of a person's cells and culturing them in to promote cell division . This is done in order to select a specific time during the cells' growth phase when the chromosomes are easiest to distinguish.
- Isolating the chromosomes from the nucleus of the cells, placing them on a slide, and treating them with a special stain.
- Taking microphotographs of the chromosomes.
- In jigsaw puzzle fashion, rearranging the pictures of the chromosomes to match up pairs and arrange them by size and structure, from numbers 1 to 22, followed by the sex chromosomes as the 23rd pair.
- The pictures also allow the chromosomes to be vertically oriented. Each chromosome looks like a striped straw. It has two arms that differ in length (a short arm (p) and a long arm (q)), a pinched-in area between the arms called a centromere, and a series of light and dark horizontal bands. The length of the arms and the location of the bands help determine top from bottom.
- Once the chromosome photo arrangement is completed, a laboratory specialist evaluates the chromosome pairs and identifies any abnormalities that may be present.
Normal male karyotype
Example of a peripheral blood karyotype, normal male
Example of an abnormal bone marrow karyotype showing chromosome 9 and chromosome 22 (9;22) rearrangement, indicative of chronic myelogenous leukemia (CML) or a subtype of acute lymphoblastic leukemia (ALL)
Example of an abnormal karyotype showing an extra chromosome 21 (Trisomy 21) indicative of Down syndrome.
Images courtesy of: Mary Lowery Nordberg, PhD ©AACC
Some chromosomal disorders that may be detected include:
- Down syndrome (Trisomy 21), caused by an extra chromosome 21; this may occur in all or most cells of the body.
- Edwards syndrome (Trisomy 18), caused by an extra chromosome 18.
- Patau syndrome (Trisomy 13), caused by an extra chromosome 13.
- Klinefelter syndrome, caused by an extra X chromosome, the most common sex chromosome abnormality in males
- Turner syndrome, caused by missing one X chromosome in females.
- Chronic myelogenous leukaemia, a classic 9:22 translocation that is diagnostic of the disease.
How is the sample collected for testing?
- A blood sample is obtained by inserting a needle into a vein in the arm.
- Amniotic fluid and chorionic villi are collected from a pregnant woman by a doctor using amniocentesis or chorionic villus sampling procedures.
- Bone marrow or tissue sample collections require a procedure to be performed.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the articles under: Coping with discomfort and anxiety. See also our video on the front page, "Having a blood test"
Another article, Follow a sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.