The Value of Karyotyping

What This Simple Test Can Tell You About Your Health

Male Karyotype
Example of a Karyotype. Credit: Media for Medical / Contributor / Getty Images

A karyotype is, quite literally, a picture of the chromosomes that exist within a single cell. Chromosomes are the thread-like structures inside the nucleus of cells that are made of protein and a single molecule of deoxyribonucleic acid (DNA). DNA contains the specific instructions that make each type of living creature unique, according to the National Human Genome Research Institute.

A doctor will often order a karyotype in order to figure out if a condition is caused by some sort of underlying genetic problem.

It's a painless test, involving little more than a needle prick to obtain a tissue sample, but it can tell a lot about a person's health. Here's an overview of how karyotyping is done and what it can reveal.

How Karyotypes Are Performed

A karyotype can be created from different types of tissue. Usually, a blood sample is all that's needed, but if there's a concern that a developing fetus has a chromosomal abnormality, amniotic fluid may need to be drawn. This involves inserting a needle into a pregnant woman's abdomen—a procedure that's relatively safe and painless.

After a sample is collected, it will be analyzed in a laboratory by a specialist called a cytogeneticist who will separate the cells, grow them so that there are enough to analyze, and then release the chromosomes from the cells. At this point, the chromosomes will be stained, analyzed, counted, and then sorted.

The cytogeneticist will also look at the structure of the chromosome to make sure there is no missing or added genetic material.

Once this process is complete, the findings are written up and the test results passed along to the physician who ordered the karyotype, typically within three to seven days. It takes a little longer to create a karyotype of a prenatal sample, however—anywhere from 10 to 14 days.

A Picture of Health

The first thing a karyotype reveals is the number of chromosomes a person has: In a normal sample, there are 23 pairs of 46 chromosomes.

A karyotype also confirms biological gender: Two X chromosomes signify female; an X and a Y chromosome signify male.

Beyond this basic information, a cytogeneticist can use a karyotype to diagnose any syndrome or condition that's caused by having extra or missing chromosomes. For example, Down syndrome results when there is a full or partial extra copy of chromosome 21, according to the National Down Syndrome Society. For this reason, Down syndrome is sometimes referred to as trisomy 21.

Other conditions that can be diagnosed using karyotyping include:

  • Edward syndrome (trisomy 18)
  • Patau syndrome (trisomy 13)
  • Turner syndrome (one X)
  • Klinefelter syndrome (XXY)
  • XYY syndrome
  • Triple X syndrome (XXX)

A karyotype also can reveal deletions in chromosomes that are the underlying cause of conditions such as Cri-du-Chat syndrome (chromosome 5) or Williams syndrome (chromosome 7), as well as translocations in which one part of a chromosome is found on or switched with another part of a different chromosome. From 2 percent to 3 percent of Down syndrome cases are caused by a translocation on chromosome 21, for example.

The possibility that a karyotype will reveal a serious genetic problem in you or an unborn child may make the idea of having one scary, but keep in mind that the more knowledge you have about a health concern, the you'll be able to deal with it.

Sources:

Medline Plus. "Karyotyping." 

National Down Syndrome Society.  "What Is Down Syndrome?"

National Human Genome Research Institute. "Chromosomes." June 16, 2015.

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