Getting a Karyotype for Down Syndrome

A karyotype of your chromosomes can detect any trisomies

Human chromosomes, normal female, human karyotype
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Within every organism, from humans to plants, are tiny building units known as cells. These cells differ per species and within each person, animal, or plant itself. In those cells are chromosomes which contain DNA, the blueprint for how we function. Sometimes, either to get a better idea of how we function or diagnose medical conditions, a picture, known as a karyotype, is taken of our chromosomes.

 

A karyotype is the most common way of analyzing a person's chromosomes. It can be done on any human tissue, but is most often done on blood, skin, amniotic fluid, or chorionic villi. Cells are taken to the lab, grown or cultured, put on slides, photographed, and analyzed.

What Can a Karyotype Tell You?

A karyotype can tell you how many chromosomes a person has as well as the structure of each individual chromosome. Because a karyotype gives you information about each chromosome, it can also tell you the sex of an individual. If two X chromosomes are present, then the individual is female. If an X and a Y chromosome are present, then the individual is a male.

Because a karyotype gives information about the total number of chromosomes present, it can diagnose conditions such as Down syndrome –– the presence of an extra number 21 chromosome (47 chromosomes in total, rather than 46). A karyotype also looks at the structure of each chromosome, so it can tell if a person carries structural chromosome problems such as a translocation or deletion.

(About 2 to 3 percent of Down syndrome cases are due to a translocation involving the number 21 chromosome.)

How Does a Karyotype Differ From FISH Analysis?

A karyotype differs from fluorescent in situ hybridization (FISH) analysis in that it looks very closely at all of the chromosomes that a person has.

 A FISH analysis gives you less information about all of the chromosomes. FISH testing can identify trisomies –– like Down syndrome. However, it can not give you as informative of a picture as a karyotype can. A karyotype will also detect structural differences such as translocations, deletions, ring chromosomes, and others.

Karyotype results take longer to get back than those of a FISH analysis. A karyotype done on blood cells can take anywhere from three to seven days. A karyotype on amniocytes (cells from an amniocentesis) or from chorionic villus sampling can take anywhere from 10 to 14 days, depending on how fast the cells grow. A FISH analysis will come back within three to four days. 

Choosing an Appropriate Testing Type

If you are considering prenatal testing or a karyotype for diagnostic reasons, you should discuss which option is better suited for your specific case. You may also want to discuss and research whether or not insurance will cover the cost of a FISH analysis and karyotype.

If you are pregnant with a high-risk pregnancy, your doctor may recommend FISH testing as well as a karyotype, so that preliminary results are received faster. Quicker results may be desired by those further along in their pregnancies. 

SOURCES:

American College of Obstetricians and Gynecologists (ACOG). Screening for Fetal Chromosomal Abnormalities. ACOG Practice Bulletin, number 77, January 2007.

National Human Genome Research Institute. Fluorescence In Situ Hybridization (FISH) , 2008.

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