Table of ContentsView AllTable of ContentsWhat Is It?Conditions DiagnosedWhen It’s DoneTesting ProcessInterpreting ResultsLimitations
Table of ContentsView All
View All
Table of Contents
What Is It?
Conditions Diagnosed
When It’s Done
Testing Process
Interpreting Results
Limitations
A karyotype test is a type of genetic testing that uses blood or body fluids to analyze your chromosomes. It is commonly used to screen for and confirm chromosomal abnormalities.
This article covers what a karyotype test is, when it’s used, and the steps involved in doing the test. It also explains what the test is used to detect and what it means if the results are abnormal.
PASIEKA/SPL/Getty Images
What Is a Karyotype Test?
A karyotype is a photograph of thechromosomes in a cell. Karyotypes can be taken from:
A karyotype test looks at the size, shape, and number of chromosomes in a sample of cells.
Conditions Diagnosed With a Karyotype Test
Karyotypes can be used to screen for and confirm chromosomal abnormalities such asDown syndromeandcat eye syndrome, and there are several different types of abnormalities which may be detected.
Chromosomal abnormalities:Trisomiesin which there are three copies of one of the chromosomes rather than twoMonosomiesin which only one copy (instead of two) is presentChromosome deletionsin which part of a chromosome is missingChromosome translocationsin which a part of one chromosome is attached to another chromosome (and vice versa in balanced translocations.)
Chromosomal abnormalities:
Examples oftrisomiesinclude:
An example ofmonosomyincludes:
Examples ofchromosomal deletionsinclude:
Translocations: There are many examples of translocations including translocation Down syndrome. Robertsonian translocations are fairly common, occurring in roughly 1 in 1,000 people.
Mosaicismis a condition in which some cells in the body have a chromosomal abnormality while others do not. For example, mosaic Down syndrome or mosaic trisomy 9. Full trisomy 9 is not compatible with life, but mosaic trisomy 9 may result in a live birth.
An Overview of Karyotyping
How a Karyotype Test Is Performed
A karyotype test may sound like a simple blood test, which makes many people wonder why it takes so long to get the results. This test is actually quite complex after collection.
Preparing for Your Appointment
Here’s a look at the steps so you can understand what happens during the time you are waiting for the test.
Sample Collection
The first step in performing a karyotype is to collect a sample.
The following methods may be used to collect a sample from babies, children, and adults:
Transport to the Laboratory
Separating the Cells
In order to analyze chromosomes, the sample must contain cells that are actively dividing. In blood, the white blood cells actively divide. Most fetal cells actively divide as well. Once the sample reaches the cytogenetics lab, the non-dividing cells are separated from the dividing cells using special chemicals.
Growing Cells
In order to have enough cells to analyze, the dividing cells are grown in special media or a cell culture. This media contains chemicals and hormones that enable the cells to divide and multiply. This process of culturing can take three to four days for blood cells, and up to a week for fetal cells.
Synchronizing Cells
Chromosomes are a long string of human DNA. In order to see chromosomes under a microscope, chromosomes have to be in their most compact form in a phase of cell division (mitosis) known as metaphase. In order to get all the cells to this specific stage of cell division, the cells are treated with a chemical that stops cell division at the point where the chromosomes are the most compact.
Releasing the Chromosomes From Their Cells
Staining the Chromosomes
Chromosomes are naturally colorless. In order to tell one chromosome from another, a special dye called Giemsa dye is applied to the slide. Giemsa dye stains regions of chromosomes that are rich in the bases adenine (A) and thymine (T). When stained, the chromosomes look like strings with light and dark bands. Each chromosome has a specific pattern of light and dark bands which enable the cytogeneticist to tell one chromosome from another. Each dark or light band encompasses hundreds of different genes.
Analysis
Counting Chromosomes
The first step of the analysis is counting the chromosomes. Most humans have 46 chromosomes. People with Down syndrome have 47 chromosomes. It is also possible for people to have missing chromosomes, more than one extra chromosome, or a portion of a chromosome that is either missing or duplicated. By looking at just the number of chromosomes, it is possible to diagnose different conditions including Down syndrome.
Sorting Chromosomes
After determining the number of chromosomes, the cytogeneticist will start sorting the chromosomes. To sort the chromosomes, they will compare chromosome length, the placement of centromeres (the areas where the two chromatids are joined), and the location and sizes of G-bands. The chromosome pairs are numbered from largest (number 1) to smallest (number 22). There are 22 pairs of chromosomes, called autosomes, which match up exactly. There are also the sex chromosomes; females have two X chromosomes while males have an X and a Y.
Looking at the Structure
In addition to looking at the total number of chromosomes and the sex chromosomes, the cytogeneticist will also look at the structure of the specific chromosomes to make sure that there is no missing or additional material as well as structural abnormalities like translocations. A translocation occurs when a part of one chromosome is attached to another chromosome. In some cases, two pieces of chromosomes are interchanged (a balanced translocation) and other times an extra piece is added or missing from one chromosome alone.
The Final Result
What Karyotype Results Mean
Normal resultslook like the following:
Abnormal resultsmay involve:
Such abnormalities may be linked to certain genetic syndromes or conditions.
Limits of Karyotype Testing
Note that while karyotype testing can give a lot of information on chromosomes, this test cannot tell you whether specific gene mutations, such as those which causecystic fibrosis, are present. Your genetic counselor can help you understand both what karyotype tests can tell you and what they cannot. Further studies are needed to evaluate the possible role of gene mutations in disease or miscarriages.
It’s also important to note that at times karyotype testing may not be able to detect some chromosomal abnormalities, such as when placental mosaicism is present.
At the current time, karyotype testing in the prenatal setting is quite invasive, requiring amniocentesis or chorionic villus sampling. However,evaluating cell-free DNAin a pregnant person’s blood sample is now common as a much less invasive alternative for the prenatal diagnosis of genetic abnormalities in a fetus.
Summary
A karyotype test examines chromosomes in a sample of cells. It is often performed to help identify chromosomal conditions such as down syndrome, Klinefelter syndrome, and trisomy 18. The test can be performed on blood, amniotic fluid, tissue from the placenta, and bone marrow. A normal result from a karyotype test should show exactly 46 chromosomes without any structural changes.
14 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
U.S. National Library of Medicine. Medline Plus.Karyotype genetic test.
Levy PA, Marion R.Trisomies.Pediatrics In Review. 2018;39(2):104-106. doi:10.1542/pir.2016-0198
Witters G, Van Robays J, Willekes C, et al. Trisomy 13, 18, 21,Triploidy and Turner syndrome: the 5T’s. Look at the hands.Facts Views Vis Obgyn. 2011;3(1):15–21.
Learn Genetics, Genetic Science Learning.Examples of unbalanced chromosomal arrangements.
Song J, Li X, Sun L, et al.A family with Robertsonian translocation: a potential mechanism of speciation in humans.Mol Cytogenet. 2016;9:48. doi:10.1186/s13039-016-0255-7
Copel, JA, D’Alton, ME.Obstetric Imaging: Fetal Diagnosis and Care. New York, NY: Elsevier.
MedlinePlus.Karyotype genetic test.
UVA Health Medical Center.Cytogenetics laboratory.
Howe B, Umrigar A, Tsien F.Chromosome preparation from cultured cells.JoVE. 2014;(83):50203. doi:10.3791/50203
O’Connor, C.Karyotyping for chromosomal abnormalities.Nature Education2008:1(1):27.
National Center for Advancing Translational Sciences.FAQs about chromosome disorders.
Grace MR, Hardisty E, Dotters-katz SK, Vora NL, Kuller JA.Cell-free DNA screening: complexities and challenges of clinical implementation.Obstet Gynecol Surv. 2016;71(8):477-87. doi:10.1097/OGX.0000000000000342
Kumar, Vinay, Abul K. Abbas, and Jon C. Aster. Robbins and Cotran Pathologic Basis of Disease. Philadelphia: Elsevier-Saunders. Print.Norton, M., and B. Rink.Changing Indications for Invasive Testing in an Era of Improved Screening.Seminars in Perinatology. 40(1):56-66.Shah, M., Cinnioglu, C., Maisenbacher, M., Comstock, I., Kort, J., and R. Lathi.Comparison of Cytogenetics and Molecular Karyotyping for Chromosome Testing of Miscarriage Specimens.Fertility and Sterility. 107(4):1028-1033.
Kumar, Vinay, Abul K. Abbas, and Jon C. Aster. Robbins and Cotran Pathologic Basis of Disease. Philadelphia: Elsevier-Saunders. Print.
Norton, M., and B. Rink.Changing Indications for Invasive Testing in an Era of Improved Screening.Seminars in Perinatology. 40(1):56-66.
Shah, M., Cinnioglu, C., Maisenbacher, M., Comstock, I., Kort, J., and R. Lathi.Comparison of Cytogenetics and Molecular Karyotyping for Chromosome Testing of Miscarriage Specimens.Fertility and Sterility. 107(4):1028-1033.
Meet Our Medical Expert Board
Share Feedback
Was this page helpful?Thanks for your feedback!What is your feedback?OtherHelpfulReport an ErrorSubmit
Was this page helpful?
Thanks for your feedback!
What is your feedback?OtherHelpfulReport an ErrorSubmit
What is your feedback?
