This type of counselor is trained to help you understand what karyotype test results mean for you, such as your risk for having a child with an inherited genetic condition like Down syndrome.
A genetic counselor can help you make well-informed decisions. Ask to have genetic counseling before making a decision about a karyotype test. Karyotype testing can be done using almost any cell or tissue from the body. A karyotype test usually is done on a blood sample taken from a vein. For testing during pregnancy, it may also be done on a sample of amniotic fluid or the placenta. For this type of test, cells are collected from the fetus using amniocentesis or chorionic villus sampling.
Bone marrow aspiration may be used for a karyotype test. Karyotype testing is usually done using a blood sample from a vein. You may feel nothing at all from the needle, or you may feel a quick sting or pinch. There is very little chance of having a problem from this test. A small bruise may form at the site. Author: Healthwise Staff. This information does not replace the advice of a doctor. Healthwise, Incorporated, disclaims any warranty or liability for your use of this information.
Your use of this information means that you agree to the Terms of Use. Learn how we develop our content. To learn more about Healthwise, visit Healthwise. This makes it possible for your doctor to view the chromosomes under a microscope.
These stained cells are examined under a microscope for potential abnormalities. Abnormalities can include:. This information is important in determining if there are any genetic abnormalities.
A normal test result will show 46 chromosomes. Two of these 46 chromosomes are sex chromosomes, which determine the sex of the person being tested, and 44 of them are autosomes. The autosomes are unrelated to determining the sex of the person being tested.
Females have two X chromosomes, while males have one X chromosome and one Y chromosome. Abnormalities that appear in a test sample could be the result of any number of genetic syndromes or conditions.
Mosaic Down syndrome is a rare, less severe form of Down syndrome. We explain the condition, its cause, symptoms, and how to diagnose it. Curly hair is determined by factors you inherit from your biological parents. Here's how it works. Health Conditions Discover Plan Connect. Why the Test Is Useful. Preparation and Risks.
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. 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. Once chromosomes are stained, the slide is put under the microscope for analysis. A picture is then taken of the chromosomes. By the end of the analysis, the total number of chromosomes will be determined and the chromosomes arranged by size. 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. After determining the number of chromosomes, the cytogeneticist will start sorting the chromosomes. To sort the chromosomes, a cytogeneticist 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 chromosomes pairs are numbered from largest number 1 to smallest number 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. 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. In the end, the final karyotype shows the total number of chromosomes, the sex, and any structural abnormalities with individual chromosomes. A digital picture of the chromosomes is generated with all of the chromosomes arranged by number.
It's important to 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 cause cystic fibrosis , are present.
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