Best A karyotype is an actual photograph of the chromosomes from one cell. Karyotypes are usually done using blood cells, fetal skin cells (from amniotic fluid or the placenta) and occasionally bone marrow cells. While a karyotype is used to confirm that a person has Down syndrome due to an extra chromosome number 21, it actually gives much more information.
From a patient’s perspective, a karyotype is a simple blood test. But what happens to the blood after it is collected is actually quite complex. This step by step guide will help you understand why it takes so long (up to a week) to get karyotype results.
GENETIC DISORDERS
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
Cystic fibrosis
Tay-Sachs disease
Sickle cell disease
Dwarfism
Breast cancer
Cleft lip/palate
Heart defects
These are only a few examples. The complete list would contain thousands of genetic conditions that cannot be detected by chromosome analysis.
