Amanda has taught high school science for over 10 years. They have a Master's Degree in Cellular and Molecular Physiology from Tufts Medical School and a Master's of Teaching from Simmons College. They also are certified in secondary special education, biology, and physics in Massachusetts.
What Are Karyotyping Tests?
What controls all the traits in your body? Why do you have green eyes, or even two arms instead of three? The answer is DNA. DNA is arranged into pieces called chromosomes. Humans have 23 pairs of chromosomes, one set from your mom and one set from your dad.
Chromosomes can contain thousands of individual genes and control many traits. So, if there is a change in the structure of number of chromosomes there can be large scale problems with the person's development and health.
One way to test for chromosome abnormalities is called a karyotype. A karyotype is a picture of a person's chromosomes. The doctor can look to see what chromosomes are damaged, or if there are too many or too few.
Today, we're going to look at the procedures to carry out a karyotype and look at different examples of when you would use a karyotype.
To do a karyotype, a doctor will need a sample of cells from the patient. Where the cells come from depends on where you need the examine the chromosomes. Many people use karyotypes to ensure their unborn babies are healthy. In this case, a sample of amniotic fluid surrounding the baby, or a sample from the placenta, the barrier that nourishes the baby in the uterus can be tested.
Other times, there might be a problem with a person's bone marrow, which makes blood cells. A sample of bone marrow can be taken with a large needle. Nearly any part of the body can be used to sample cells for a karyotype.
Once the doctor has a sample of cells they need to be grown in the lab in a culture. This is done to increase the number of cells available for the test. The few cells taken from the initial sample won't be enough to do the karyotype.
The cells are monitored for cell division. When cells are in metaphase, the chromosomes are condensed and lined up in the middle of the cell. This is the optimal time to stain them for the karyotype because they are easy to see.
The cells are arrested in metaphase using a chemical called colcemid. Next the cells are concentrated and a fixative is applied called Carnoy's fixative. This preserves the cells in the state they are in. Next, the cells are stained with a dye called Giemsa, which helps scientists see the chromosomes.
Lastly, the cells are spread on a slide that can be viewed under a microscope. Scientists take a picture of the cells and then figure out which chromosomes are which and organize the picture into a karyotype showing all the chromosomes lined up in order.
Let's look at some examples of patients that may want to, or need to have a karyotyping test done.
Rayana has recently gotten pregnant with her husband. However, they waited until later in life to have a baby. Rayana is 37 and knows the risk of a disease called down syndrome increases as women get older. Down syndrome is a genetic disorder in which a person has an extra copy of chromosome number 21. The chromosome may be simply another copy, giving the person 47 chromosomes total, or it may be an extra chromosome 21 attached to chromosome 14.
A karyotype can be done during pregnancy to assess the baby's risk, called a prenatal karyotype. A sample of the placenta or amniotic fluid is taken and a karyotype can be performed using these cells. Although this type of test usually is very accurate in diagnosing down syndrome, there is about a 1% risk of miscarriage as a result of the procedure.
Rayana decides to go ahead with the procedure. She and her husband want to know if their child has down syndrome so they can prepare to care for a child with special needs. Children with down syndrome have cognitive delays and may have behavioral problems and require increased care. Some parents may elect to make arrangements for adoption or terminate the pregnancy if they are unable to care for their child.
Mark is going in for his yearly physical. He gets his blood work done but expects everything to be normal, since he hasn't been feeling sick at all. However, a few days later he gets a call from his doctor explaining he has a high white blood cell count. Mark goes back to the doctor and receives further tests that indicate he has a high number of immature white blood cells, characteristic of a genetic disorder called chronic myeloid leukemia (CML).
CML is a type of cancer that affects white blood cells. It is caused by an translocation or a part of a chromosome that gets swapped. CML is a translocation between chromosome 22 to 9. The result is a shortened chromosome 22.
A high white blood cell count and immature white blood cells aren't enough to diagnose CML. So doctors will do further genetic testing that may include a karyotype using a blood or bone marrow sample. Bone marrow is the source for all blood cells, so this is the best sample for a karyotype. A karyotype will reveal the size of chromosome 22. Further tests can be used to confirm the specific translocation.
Karyotypes are images of chromosomes used to diagnose genetic disorders. Doctors will take a sample of cells and wait for them to enter metaphase. Then, they arrest cell growth using colcemid and fix cells with Carnoy's fixative. Lastly they stain the chromosomes with Giemsa and create an organized image of the chromosomes.
Karyotypes can be used to diagnose conditions like down syndrome, which is caused by an extra chromosome number 21, or chronic myeloid leukemia (CML) which involves a translocation between chromosome 9 and 22.
Medical Disclaimer: The information on this site is for your information only and is not a substitute for professional medical advice.
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