Karyotyping, or blood chromosome analysis, is a fairly standard test in the infertility world. However, most people who undergo the test don't have a good understanding of why it is done or what the results reveal. The prefix "karyo" refers to the fact that the nucleus of the cell is studied, and the base word "type" refers to the fact that the test is a characterization, or analysis of the character. Put together, the word karyotype means a characterization of the cell nucleus.
A look inside of the cell
Our bodies are made up of billions of cells. There are many different kinds of cells, including blood cells, skin cells, and bone cells. Our genetic material is contained in a structure within the cell called the nucleus. Every cell in the body, with the exception of the red blood cell, has its own nucleus containing a complete set of genetic material.
A look inside of the nucleus
Inside of the nucleus of the cell there are 46 rod-shaped structures called chromosomes. The chromosomes come in pairs, 23 coming from one's mother and 23 coming from one's father. Scientists label the chromosomes with numbers, calling them chromosome number 1 through chromosome number 22. The 23rd pair of chromosomes are the sex chromosomes, usually two X's in the case of a female or an X and a Y chromosome in the case of a male.
The chromosomes vary in size according to the number, with chromosome number 1 being the largest and chromosome number 22 being labeled the smallest of the numbered chromosomes. The X chromosome is about the same size as the number 7 chromosome, and the Y chromosome is much smaller, about the size of a number 21 or 22 chromosome.
Each chromosome has two "arms," or sections. The top section is called the "p" arm, because it is the small (petite) arm, and the bottom section is called the "q" arm, because q follows p in the alphabet and the scientists who chose these names were apparently not very creative. The structure that separates the two arms is called the centromere.
A look inside the chromosome
Each chromosome contains hundreds or thousands of genes each, depending on the size of the chromosome. Genes are units of heredity and are constructed of DNA. For the most part, individual genes are too small to be seen even through a powerful microscope. Chromosomes, however, can be seen under a microscope.
Back to the karyotype
So, what is the karyotype? The karyotype is a photograph of all of the chromosomes of an individual cell. The chromosomes in the karyotype are usually lined up into rows in pairs, so that they are in order from the largest to the smallest in size. The photograph is taken through a microscope, so that the longest chromosomes usually appear to be about an inch long. In reality, of course, they are so small that they are not visible to the naked eye.
Karyotype analysis
Karyotypes are usually constructed by laboratory technologists and analyzed by cytogeneticists, Ph.D. or M.D. doctors who specialize in the study of chromosomes. The cells must be grown and advanced to a specific cell stage that is optimal for analysis. The process of growing the cells, dropping them onto slides, arranging the chromosomes into a karyotype, and analysis by a cytogeneticist usually takes between one and three weeks.
What a karyotype tells us
Karyotype analysis determines the number of chromosomes in the cells and whether there are any pieces of chromosomal material that are missing, extra, or rearranged. Any variation from the normal chromosome number and arrangement can have implications for a person's fertility and the risk for having a baby with birth defects. The following chromosomal conditions can be detected by karyotype analysis:
Down syndrome (an extra 21 chromosome)
Turner syndrome (females with only one X chromosome)
Klinefelter syndrome (males with two X chromosomes and a Y chromosome)
Other sex chromosome conditions, such as XXX or XYY
Any other aneuploidy, which is an extra or missing single chromosome
Triploidy, which is an extra set of chromosomes, so there are three of each and 69 total
Mosaic chromosome conditions, in which a portion of the cells show an abnormal number or arrangement of chromosomes and a portion show a normal number and arrangement
Translocations, which are rearrangements of the chromosome material
Large missing or extra pieces of chromosome material that are visible under a microscope
What a karyotype cannot tell us
There are many genetic conditions that are the result of single gene mutations. Additionally, there are many genetic conditions that are caused by multiple genes interacting. These conditions cannot be detected by chromosome analysis. Examples of conditions that cannot be detected by karyotyping are:
- Very small deletions of the chromosomal material
- Very small duplications of the chromosomal material
- Some very subtle chromosome rearrangements
These are only a few examples. The complete list would contain thousands of genetic conditions that cannot be detected by chromosome analysis.
Need more help?
If you have questions about your karyotype analysis or an analysis performed on a family member, you should contact a genetic counselor. You can find a genetic counselor in your area by looking on the website of the National Society of Genetic Counselors at www.nsgc.org.
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About the author: Melissa K. Gabriel, M.S., is a Board Certified Genetic Counselor with Genetic Resources Medical Group in Los Angeles, California. She can be contacted by telephone at (877) 655-3755 or by email at mgabriel@dnaMD.com. Genetic Resources Medical Group provides a full spectrum of genetic services, and they focus primarily on genetic assessment for couples or individuals with fertility issues. Their website address is www.dnaMD.com.
