It is commonly used to screen for and confirm chromosomal abnormalities.
It also explains what the test is used to detect and what it means if the results are abnormal.
What Is a Karyotype Test?
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A karyotype is a photograph of thechromosomes in a cell.
Robertsonian translocations are fairly common, occurring in roughly 1 in 1,000 people.
Mosaicismis a condition in which some cells in the body have a chromosomal abnormality while others do not.
For example, mosaic Down syndrome or mosaic trisomy 9.
Full trisomy 9 is not compatible with life, but mosaic trisomy 9 may result in a live birth.
This test is actually quite complex after collection.
Sample Collection
The first step in performing a karyotype is to collect a sample.
Not all hospitals have cytogenetics labs.
The test sample is analyzed by specially trained cytogenetic technologists, Ph.D. cytogeneticists, or medical geneticists.
Separating the Cells
so that analyze chromosomes, the sample must contain cells that are actively dividing.
In blood, the white blood cells actively divide.
Most fetal cells actively divide as well.
This media contains chemicals and hormones that switch on the cells to divide and multiply.
Synchronizing Cells
Chromosomes are a long string of human DNA.
This is done by treating the cells with a special solution that causes them to burst.
This is done while the cells are on a microscopic slide.
The leftover debris from the cells is washed away, leaving the chromosomes stuck to the slide.
Staining the Chromosomes
Chromosomes are naturally colorless.
When stained, the chromosomes look like strings with light and dark bands.
Each dark or light band encompasses hundreds of different genes.
Analysis
Once chromosomes are stained, the slide is put under the microscope for analysis.
A picture is then taken of the chromosomes.
Counting Chromosomes
The first step of the analysis is counting the chromosomes.
Most humans have 46 chromosomes.
People with Down syndrome have 47 chromosomes.
By looking at just the number of chromosomes, it is possible to diagnose different conditions including Down syndrome.
Sorting Chromosomes
After determining the number of chromosomes, the cytogeneticist will start sorting the chromosomes.
The chromosome pairs are numbered from largest (number 1) to smallest (number 22).
There are 22 pairs of chromosomes, called autosomes, which match up exactly.
A translocation triggers when a part of one chromosome is attached to another chromosome.
A digital picture of the chromosomes is generated with all of the chromosomes arranged by number.
Your genetic counselor can help you understand both what karyotype tests can tell you and what they cannot.
Further studies are needed to evaluate the possible role of gene mutations in disease or miscarriages.
Summary
A karyotype test examines chromosomes in a sample of cells.
The test can be performed on blood, amniotic fluid, tissue from the placenta, and bone marrow.
A normal result from a karyotype test should show exactly 46 chromosomes without any structural changes.
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