Karyotype is the number and appearance of chromosomes in the nucleus of a eukaryotic cell. The term is also used for the complete set of chromosomes in a species, or an individual.
A karyotype is a photograph of the chromosomes in a cell. Karyotypes can be taken from blood cells, fetal skin cells (from amniotic fluid or the placenta), or bone marrow cells.
Karyotype analysis involves visualization of chromosomes under a microscope. Cells are collected from an individual, induced to divide, and then arrested at metaphase. A karyotype test examines these dividing cells. The chromosomes are stained with certain dyes that show a pattern of light and dark bands (called the banding pattern). The banding pattern for each chromosome is specific and consistent allowing identification of each of the 24 chromosomes.
Chromosome spreads can be photographed, cut out, and assigned into the appropriate chromosome number or they can be digitally imaged using a computer. The pairs of chromosomes are arranged by their size and appearance. The chromosomes can be divided into seven groups (A-G) based on descending order of size and position of the centeromere. The standard nomenclature for describing a karyotype is based on the International System for Human Cytogenetic Nomenclature (ISCN).
1. Sample Collection
The first step in karyotyping is to take a sample of cells. The sample cells can come from a number of different tissues. This can include:
- bone marrow
- amniotic fluid
A karyotype will be done on the white blood cells which are actively dividing (a state known as mitosis). During pregnancy, the sample can either be amniotic fluid collected during an amniocentesis or a piece of the placenta collected during a chorionic villi sampling test (CVS). The amniotic fluid contains fetal skin cells which are used to generate a karyotype.
The preparation required for karyotyping depends on the method to take a sample of blood cells for testing. Samples can be taken in various ways, including:
- a blood draw
- a bone marrow biopsy, which involves taking a sample of the spongy tissue inside certain bones
- an amniocentesis, which involves taking a sample of amniotic fluid from the uterus
Sampling can be done using various methods, depending on area of body being tested. For example, amniocentesis to collect the sample if amniotic fluid to be tested.
2. Transport to the Laboratory
Karyotypes are performed in a specific laboratory called a cytogenetics lab––a lab which studies chromosomes. Not all hospitals have cytogenetics labs.
3. Separating the Cells
In order to 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. The non-dividing cells are separated from the dividing cells using special chemicals.
4. Growing Cells
In order to have enough cells to analyze, the dividing cells are grown in special media or a cell culture. This media contains chemicals and hormones that enable the cells to divide and multiply.
5. Synchronizing Cells
Chromosomes are a long string of human DNA. In order to see chromosomes under a microscope, chromosomes have to be in their most compact form in a phase of cell division (mitosis) known as metaphase. In order to get all the cells to this specific stage of cell division, the cells are treated with a chemical which stops cell division at the point where the chromosomes are the most compact.
6. Releasing the Chromosomes From Their Cells
In order to see these compact chromosomes under a microscope, the chromosomes have to be out of the white blood cells. This is done by treating the white blood 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 white blood cells is washed away, leaving the chromosomes stuck to the slide.
7. Staining the Chromosomes
Chromosomes are naturally colorless. In order to tell one chromosome from another, a special dye called Giemsa dye is applied to the slide. Giemsa dye stains regions of chromosomes that are rich in the bases adenine (A) and thymine (T). When stained, the chromosomes look like strings with light and dark bands. Each chromosome has a specific pattern of light and dark bands which enable the cytogeneticist to tell one chromosome from another. Each dark or light band encompasses hundreds of different genes.
Once chromosomes are stained, the slide is put under the microscope for analysis. A picture is then taken of the chromosomes. By the end of the analysis, the total number of chromosomes will be determined and the chromosomes arranged by size.
These stained cells are examined under a microscope for potential abnormalities. Abnormalities can include:
- extra chromosomes
- missing chromosomes
- missing portions of a chromosome
- extra portions of a chromosome
- portions that have broken off of one chromosome and reattached to another
9. Counting Chromosomes
The first step of the analysis is counting the chromosomes. Most humans have 46 chromosomes.
10. Sorting Chromosomes
After determining the number of chromosomes, the cytogeneticist will start sorting the chromosomes. To sort the chromosomes, a cytogeneticist will compare chromosome length, the placement of centromeres (the areas where the two chromatids are joined), and the location and sizes of G-bands. The chromosomes pairs are numbered from largest (number 1) to smallest (number 22). There are 22 pairs of chromosomes, called autosomes, which match up exactly. There are also the sex chromosomes, females have two X chromosomes while males have an X and a Y.
11. Looking at the Structure
In addition to looking at the total number of chromosomes and the sex chromosomes, the cytogeneticist will also look at the structure of the specific chromosomes to make sure that there is no missing or additional material as well as structural abnormalities like translocations. A translocation occurs when a part of one chromosome is attached to another chromosome. In some cases, two pieces of chromosomes are interchanged (a balanced translocation) and other times an extra piece is added or missing from one chromosome alone.
12. The Final Result
The final karyotype shows the total number of chromosomes, the sex, and any structural abnormalities with individual chromosomes. A digital picture of the chromosomes is generated with all of the chromosomes arranged by number.
Karyotypes can be used to screen for and confirm chromosomal abnormalities such as Down’s syndrome and Cat Eye Syndrome, and there are several different types of abnormalities which may be detected.
Genetic counselors rely on karyotypes to diagnose abnormal pregnancies. Amniocentesis is a routine procedure used in prenatal screening that involves removing amniotic fluid for karyotype analysis. It also can be helpful in certaincases to obtain karyotypes from parents to determine carrier status, which can be relevant to recurrence risks in future pregnancies. Karyotype also may help determine the cause of infertility in patients having reproductive difficulties.
An unusual number of chromosomes, incorrectly arranged chromosomes, or malformed chromosomes can all be signs of a genetic condition. Genetic conditions vary greatly, but two examples are Down syndrome and Turner syndrome. Karyotyping can be used to detect a variety of genetic disorders. For example, a woman who has premature ovarian failure may have a chromosomal defect that karyotyping can pinpoint. The test is also useful for identifying the Philadelphia chromosome. Having this chromosome can signal chronic myelogenous leukemia (CML). Genetic abnormalities that indicate serious birth defects, such as Klinefelter syndrome can be detected.
- Trisomies in which there are three copies of one of the chromosomes rather than two
- Monosomies in which only one copy (instead of two) is present
- Chromosome deletions in which part of a chromosome is missing
- Chromosome translocations in which a part of one chromosome is attached to another chromosome (and vice versa in balanced translocations.)
Examples of trisomies include:
Example of monosomy includes:
Examples of chromosomal deletions include
Translocations – There are many examples of translocations including translocation Down syndrome,, Robertsonian translocations.
Mosaicism is 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.
- Infants or children who have medical conditions which suggest a chromosomal abnormality that has not yet been diagnosed.
- Adults who have symptoms suggestive of a chromosomal abnormality (for example, men with Klinefelter’s disease may go undiagnosed until puberty or adulthood.) Some of the mosaic trisomy disorders may also go undiagnosed.
- Infertility: A genetic karyotype may be done for infertility. As noted above, some chromosomal abnormalities may go undiagnosed until adulthood. A woman with Turner syndrome or a man with one of the variants of Klinefelter’s may not be aware of the condition until they are coping with infertility.
- Prenatal testing: In some cases, such as translocation Down syndrome, the condition may be hereditary and parents may be tested if a child has been born with a Down syndrome. (It’s important to note that most of the time Down syndrome is not a hereditary disorder but rather a chance mutation.)
- Stillbirth: A karyotype is often done as part of the testing following a stillbirth.
- Recurrent miscarriages: A parental karyotype of recurrent miscarriages may give clues as to the reasons for these devastating recurring losses. It’s thought that chromosomal abnormalities, such as trisomy 16, are the cause of at least 50% of miscarriages.
- Leukemia: Karyotype testing may also be done to help diagnose leukemias, for example, by looking for the Philadelphia chromosome found in some people with chronic myelogenous leukemia or acute lymphocytic leukemia.
Limits of Karyotype Testing
- Specific gene mutations can not be found , such as those which cause cystic fibrosis
May not be able to detect some chromosomal abnormalities, such as when placental mosaicism is present.
- Karyotype testing in the prenatal setting is quite invasive, requiring amniocentesis or chorionic villus sampling.