What is a Karyotype and Why is It Important?

A karyotype is a form of chromosome testing that provides a snapshot of your complete set of chromosomes.¹ Geneticists may use it to detect irregularities in either structure or in your chromosomes' count (missing or excessive chromosomes).

Karyotyping is the process of isolating your chromosomes from a cell, organizing them in pairs, and then arranging them in numerical order from largest to smallest.² 

Karyotyping can be valuable for diagnosing a suspected genetic disorder or assessing its inheritance risk.

Image from National Human Genome Research Institute

What is a Karyotype?

A karyotype is a lab-produced image of your complete set of chromosomes. Karyotyping is usually done when your chromosomes are in the metaphase stage.

Metaphase is the third phase of the cell division process. During metaphase, your chromosomes are lined up in the middle of the cell.

They are most visible and well-defined this way, so it’s easy to count and identify them.

Karyotyping can be most useful for detecting chromosomal abnormalities. It can help diagnose genetic disorders like Down syndrome, some birth defects, and even certain cancers.

How is a Karyotype Determined?

Now that we have a general idea of what a karyotype is, let's look into how it is produced. The process involves key steps, such as:

Sample Collection

The process of karyotyping starts with obtaining a sample of your cells containing your chromosomes. Geneticists often use blood draws, but amniotic cell sample retrieval is also a possibility when pregnant.

Karyotyping uses chromosomes from mitotic cells or cells capable of dividing. Only cells that can be induced to divide in a lab are used for karyotyping, such as:³

• Peripheral blood lymphocytes— a type of white blood cell
• Skin fibroblasts— main cell found on your skin connective tissue
• Amniocytes— cells found in the amniotic fluid around a developing fetus

However, cells from the bone marrow and chorionic villus can also be used since they divide quickly.³

When determining certain blood diseases or cancers, a bone marrow test may be used to harvest cells for karyotype.

Cell Culture

Geneticists will place the collected cells in the lab to have them cultured. They’ll be put in an environment with sufficient nutrients to stimulate them to divide.

Arresting Cell Division

Geneticists will apply a chemical called colchicine to the cells to interrupt the cell cycle at the metaphase stage. Colchicine is a plant alkaloid extracted from the autumn crocus, a flowering plant.

It prevents the formation of the mitotic spindle fibers necessary for cell division. The cells become stuck in metaphase as a result.

In karyotyping, it’s easier to count and determine abnormalities of metaphase chromosomes. It’s because they are condensed and tightly coiled, which makes them easier to see and count.

Chromosomes are more dispersed and less easily distinguished from one another in later phases of the cell cycle.

Staining

Geneticists will place the cells on a slide and stain them with a special dye. Giemsa stain, a visible light dye, is the most commonly used for karyotyping.

The dye binds with DNA and produces banding patterns for different chromosomes. A banding pattern is made up of the alternating light and dark regions in a chromosome.

It marks the location of genes in a chromosome. 

Karyotype Construction

Geneticists will view the stained cells under a high-powered microscope and capture images of the chromosome. They’ll arrange each chromosome image according to size and shape. 

What Are The Methods Of Collecting Samples For A Karyotype

Here are the different ways to collect samples for karyotyping.

Blood Sample Karyotyping

A blood test is often the simplest way to obtain cells for karyotyping. Blood samples have peripheral blood lymphocytes, which can be used for a karyotype.

A specialist will draw your blood sample from a vein in your arms. They will then send your specimen to a lab to be cultured and stimulated to divide.

Once the chromosomes enter metaphase, the geneticists will stain and photograph them for chromosome analysis.

Amniotic Fluid Karyotyping

Amniotic fluid karyotyping is performed with the steady guidance of ultrasound images when you’re pregnant. The medical specialist inserts a long thin needle into your abdomen and draws out amniotic fluid. 

The amniotic fluid contains fetal skin cells shed by the fetus. Geneticists can retrieve the skin cells by extracting the fluid using a syringe.⁴

The needle goes through the amniotic sac and collects a small fluid sample for analysis. 

Amniotic fluid karyotyping is generally a safe procedure. Still, to exercise caution, geneticists typically perform it between 15 and 20 weeks of pregnancy to minimize the risk of complications, such as miscarriage.⁵

Chorionic Villus Sampling

Chorionic Villus Sampling (CVS) is a procedure for obtaining cells from the chorionic villi tissues. Because the chorionic villi tissues are part of the placenta, they usually share the same chromosomes as the unborn fetus.

Geneticists can do a CVS 10 to 13 weeks into the pregnancy. They perform it while being steadily guided by an ultrasound scan. This helps prevent foreign materials from entering the sac.⁶

Why is Karyotyping Important?

Karyotyping can help detect genetic disorders and abnormalities related to your chromosomes. It is an important way to determine if you (or your fetus, if you’re pregnant) have a genetic disease.

Karyotyping does this by looking for problems with chromosome structure or number, which often cause these illnesses.

A karyotype can check if an unborn child has extra or missing chromosomes in prenatal testing. It can also assess if any of the chromosomes have structural abnormalities.

There are 46 human chromosomes in each cell, which come in pairs. The first 22 pairs are called autosomes.

The 23rd pair is the sex chromosomes. Males and females have different sex chromosomes. Men have one X and one Y chromosome, while women have two X chromosomes.

Abnormal chromosome count and anatomy can lead to health, growth, and development problems.

What are Some Common Chromosomal Abnormalities?

It’s easy to see how significant karyotyping is in finding chromosomal problems, given how crucial it is to have the right number and shape of chromosomes.

Some of the most common chromosomal abnormalities a karyotype can detect include:

• Down syndrome (Trisomy 21) — Occurs when there’s an excess or a third copy of chromosome 21. It affects a person’s physical features, cognitive functions, and behavior.⁷ For instance, people with Down Syndrome are typically shorter than average.
• Edwards’ syndrome (Trisomy 18) — Happens when there’s an extra or a third copy of chromosome 18. Most babies born with Edwards’ syndrome have low chances of surviving after birth since they are usually born with many health problems like heart defects, low birth weight, and respiratory problems.⁸ 
• Patau syndrome (Trisomy 18) — Results from an extra copy of chromosome 13. Most babies born with Patau syndrome don’t live more than a year because of heart problems and severe mental impairment.
• Klinefelter syndrome — Happens when a boy baby has an extra X chromosome (XXY), which may slow puberty or affect their ability to reproduce.
• Turner syndrome — Occurs when a baby girl has a missing or impaired X chromosome, resulting in problems with heart, neck, and height.

What are the Limitations of Karyotyping?

Despite being a valuable tool for detecting genetic disorders, karyotyping has some limitations. Karyotyping is limited by its inability to accurately detect:⁹

• Small-scale chromosomal changes
• Certain genetic conditions like cystic fibrosis, Tay-Sachs disease, sickle cell disease, and dwarfism¹⁰
• Complex aberrations or chromosomal changes that involve at least two chromosomes
• Marker chromosomes or extra chromosomes that don’t belong in any of the 23 pairs 

Frequently Asked Questions About Karyotypes