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Karyotype and Down Syndrome

How Does a Karyotype Diagnose Down Syndrome?


Updated May 08, 2014

What Is a Karyotype?

A karyotype is the traditional way that a person’s chromosomes are analyzed. 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. It gives information about 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.

How Does A Karyotype Diagnose Down Syndrome?

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 for a total of 47 chromosomes, 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% of Down syndrome cases are due to a translocation involving the number 21 chromosome.)

I've Heard About FISH Analysis. How Does a Karyotype Differ?

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. In addition to telling you the number of chromosomes a person has, a karyotype gives you information about the structure of the chromosomes. It will detect structural differences such as translocations, deletions, ring chromosomes and others. A FISH analysis gives you less information about all of the chromosomes.

What Are the Advantages of a Karyotype?

The major advantage of a karyotype is that it gives a very thorough picture of an individual's chromosomes.

What Are the Disadvantages of Karyotype?

The big disadvantage of a karyotype is that results can take longer than those of a FISH analysis. A karyotype done on blood cells can take 3 to 7 days; a karyotype on amniocytes (cells from an amniocentesis) or from chorionic villi can take anywhere from 10 to 14 days, depending on how fast the cells grow.

Questions to Ask Your Healthcare Provider about Testing

  • How long will it take to get karyotype results?
  • Does the lab you use also do a FISH testing, and do you recommend that I get that, too?
  • How long does it take to get the results of FISH testing?
  • Does my insurance cover the cost of a FISH analysis and karyotype?

Bottom Line

A karyotype will give you complete information about the structure and number of a persons chromosomes. It will diagnose Down syndrome due to trisomy 21, translocation Down syndrome, other trisomies and other structural chromosome defects. While it may take longer than FISH analysis, it offers a much more complete picture of someone’s chromosomes.


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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