Karyotyping is a test that allows doctors to examine chromosomes in a sample of cells and pinpoint specific genetic causes of a disease. We conduct the test to evaluate a couple with a history of miscarriages, or to examine a child or baby with unusual physical features or developmental delays, which can indicate an underlying genetic abnormality.
- Doctors can count the number of chromosomes and examine the structure of chromosomes.
- Abnormalities in both the number and structures of chromosomes can cause a genetic disorder.
A sample of tissue is taken and placed in a special dish and allowed to grow in a laboratory. Cells are taken from the growing tissue and stained so that the chromosomes can be identified under a microscope. A picture is taken of all 46 chromosomes, in their pairs, from one cell. This picture is called a "karyotype." A normal female karyotype is written as 46, XX, and a normal male karyotype is written as 46, XY, indicating the normal number of chromosomes and the male and female chromosome pairs. Karyotyping is over 99.9 percent accurate.
Extended banding chromosome studies
Extended banding is a test that allows doctors to examine chromosomes at a higher resolution than karyotyping. For this reason, it's also known as a high resolution chromosome study. It differs from karyotyping in that doctors examine smaller pieces of the chromosome in order to identify smaller structural chromosomal abnormalities that may not be visible through a karyotype analysis.
Fluorescent In Situ Hybridization (FISH)
This chromosomal study lets doctors determine how many copies of a specific DNA segment are present in a cell, and to identify chromosomes with a structural problem. First, a tissue sample is taken from which doctors can examine DNA. A segment of DNA is modified and labeled so that it will look fluorescent under a special microscope. This chemically modified NDA is called a probe. Probes can find and attach to other segments of DNA with a similar sequence.
One example of when the test is used is when doctors suspect that a pregnant woman is having a baby with trisomy 21, or Down syndrome. If an amniocentesis is performed, a FISH study can be done on the cells in the amniotic fluid. First, a probe would be made for chromosome #21 to determine how many copies of the #21 chromosome the baby has. Doctors then examine cells under a special microscope. A baby with trisomy 21 would have cells with three "signals," or three brightly colored areas where the probe matched with three #21 chromosomes.
Fish studies can detect structural chromosomal abnormalities that cannot be found using extended banding.
Chromosomal microarray analysis
This laboratory test can view chromosomes at an even higher resolution than FISH techniques. The test looks for identification of a change in DNA copy number. This change can reflect changes in the general population that don't cause genetic disorders. However, some changes in copy number indicate a chromosomal abnormality.
- Types of chromosomal abnormalities include small chromosomal rearrangements, small duplicates of chromosomal material (trisomy) or small deletion of chromosomal material (monosomy).
Doctors perform this test by comparing a sample DNA with a control sample of DNA. Both samples are arranged in a particular order (array) on a glass slide. Fluorescent dyes are attached to the DNA samples and placed in a special scanner that measures the brightness of each fluorescent area.