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Table of Contents View All. Table of Contents. Genetics Basics. What a Karyotype Can Show. How They're Performed. Was this page helpful? Thanks for your feedback! Sign Up. What are your concerns? Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Updated visitor guidelines. You are here Home » Karyotype Test. Top of the page. Test Overview Karyotype is a test to identify and evaluate the size, shape, and number of chromosomes in a sample of body cells. Why It Is Done Karyotyping is done to: Find out whether the chromosomes of an adult have a change that can be passed on to a child.
Find out whether a chromosome defect is preventing a woman from becoming pregnant or is causing miscarriages. Mosaicism is 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.
There are many situations in which a karyotype may be recommended by your healthcare provider. These might include:. 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. Let's take a look at these steps so you can understand what is happening during the time you are waiting for the test. The first step in performing a karyotype is to collect a sample. In newborns, a blood sample containing red blood cells, white blood cells, serum, and other fluids is collected.
A karyotype will be done on the white blood cells which are actively dividing a state known as mitosis. During pregnancy, the sample can either be amniotic fluid collected during an amniocentesis or a piece of the placenta collected during a chorionic villi sampling test CVS. The amniotic fluid contains fetal skin cells which are used to generate a karyotype. Karyotypes are performed in a specific laboratory called a cytogenetics lab——a lab which studies chromosomes. Not all hospitals have cytogenetics labs.
The test sample is analyzed by specially trained cytogenetic technologists, Ph. 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. 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. 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 which stops cell division at the point where the chromosomes are the most compact.
In order to see these compact chromosomes under a microscope, the chromosomes have to be out of the white blood cells. This is done by treating the white blood cells with a special solution that causes them to burst. This is done while the cells are on a microscopic slide. The leftover debris from the white blood cells is washed away, leaving the chromosomes stuck to the slide.
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. According to international conventions, human autosomes, or non-sex chromosomes, are numbered from 1 to 22, in descending order by size, with the exceptions of chromosomes 21 and 22, the former actually being the smallest autosome.
The sex chromosomes are generally placed at the end of a karyogram. Within a karyogram, chromosomes are aligned along a horizontal axis shared by their centromeres. Individual chromosomes are always depicted with their short p arms—p for "petite," the French word for "small"—at the top, and their long q arms—q for "queue"—at the bottom. Centromere placement can also be used to identify the gross morphology, or shape, of chromosomes. For example, metacentric chromosomes, such as chromosomes 1, 3, and 16, have p and q arms of nearly equal lengths.
Submetacentric chromosomes, such as chromosomes 2, 6, and 10, have centromeres slightly displaced from the center. Acrocentric chromosomes, such as chromosomes 14, 15, and 21, have centromeres located near their ends.
Arranging chromosomes into a karyogram can simplify the identification of any abnormalities. Note that the banding patterns between the two chromosome copies, or homologues, of any autosome are nearly identical.
Some subtle differences between the homologues of a given chromosome can be attributed to natural structural variability among individuals. Occasionally, technical artifacts associated with the processing of chromosomes will also generate apparent differences between the two homologues, but these artifacts can be identified by analyzing 15—20 metaphase spreads from one individual. It is highly unlikely that the same technical artifact would occur repeatedly in a given specimen.
Today, G-banded karyograms are routinely used to diagnose a wide range of chromosomal abnormalities in individuals. Although the resolution of chromosomal changes detectable by karyotyping is typically a few megabases, this can be sufficient to diagnose certain categories of abnormalities.
For example, aneuploidy , which is often caused by the absence or addition of a chromosome, is simple to detect by karyotype analysis.
Cytogeneticists can also frequently detect much more subtle deletions or insertions as deviations from normal banding patterns. Likewise, translocations are often readily apparent on karyotypes. When regional changes in chromosomes are observed on karyotypes, researchers often are interested in identifying candidate genes within the critical interval whose misexpression may cause symptoms in patients.
This search process has been greatly facilitated by the completion of the Human Genome Project , which has correlated cytogenetic bands with DNA sequence information. Consequently, investigators are now able to apply a range of molecular cytogenetic techniques to achieve even higher resolution of genomic changes.
Fluorescence in situ hybridization FISH and comparative genomic hybridization CGH are examples of two approaches that can potentially identify abnormalities at the level of individual genes. Molecular cytogenetics is a dynamic discipline, and new diagnostic methods continue to be developed.
As these new technologies are implemented in the clinic, we can expect that cytogeneticists will be able to make the leap from karyotype to gene with increasing efficiency. Caspersson, T. Differential banding of alkylating fluorochromes in human chromosomes. Experimental Cell Research 60 , — doi Gartler, S. The chromosome number in humans: A brief history. Nature Reviews Genetics 7 , — doi Speicher, M. Karyotyping human chromosomes by combinatorial multi-fluor FISH.
Nature Genetics 12 , — link to article. Strachan, T. Human Molecular Genetics , 2nd ed. Wiley, New York, Tjio, J. The chromosome number of man. Hereditas 42 , 1—6 Trask, B. Human cytogenetics: 46 chromosomes, 46 years and counting. Nature Reviews Genetics 3 , — doi Chromosome Mapping: Idiograms. Human Chromosome Translocations and Cancer.
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