Karyotyping

Definitions & Key takeaways

Karyotyping is the process of examining chromosomes to identify structural changes. It is used to detect genetic disorders and can also be used to determine the biological sex of an individual. The results of a karyotype can help to diagnose chromosomal abnormalities such as an abnormal number of chromosomes, called aneuploidy; or a structural abnormality, such as deletions, duplications, inversions, or translocations. Examples of genetic conditions diagnosed through karyotyping include Down syndrome, Turner syndrome, and Klinefelter syndrome.

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Karyotyping is the simple process of seeing what a person’s chromosomes look like. But don’t think of it as a chromosome beauty contest - karyotyping is actually used to detect chromosome number or structure abnormalities, in order to diagnose genetic disorders, like Down syndrome; or even some types of cancer, like leukemia.
Ok, now, chromosomes are found in the nucleus of our cells and they contain our DNA. You can think of DNA like a library, with thousands of books called genes that carry recipes for how to make every single protein found in the cell.
In human somatic cells, so all cells besides the gametes; there are 23 pairs of homologous chromosomes, and in each pair, one chromosome came from each parent.
This adds up to 46 chromosomes in total. Of these 23 pairs, 22 are somatic pairs, which contain genes that code for traits like hair color; and one sexual pair, which determines the biological sex of an individual.
In genetically female individuals, there are two X chromosomes, while in genetically male individuals, there’s an X and a Y chromosome.
However, chromosomes actually look different depending on the phases of the cell cycle - which is the series of events that somatic cells go through from the moment they’re formed until they they divide into two identical daughter cells.
The cell cycle has two phases: interphase, when the cell prepares to divide, and mitosis, when the cell is dividing. In early interphase, each chromosome has a single copy of the genetic information, called a chromatid, so there’s 46 chromosomes and 46 chromatids.
But in later interphase, as the cell prepares for mitosis, each chromosome is copied and pasted, so the amount of DNA, aka the number of chromatids, doubles up.
But two identical chromatids remain joined in a region called the centromere - so they still count as one chromosome. So right before mitosis, there are 46 chromosomes and 92 chromatids.
This way, the two resulting daughter cells have 46 chromosomes and 46 chromatids each, so they have the same DNA as the original cell.
Ok, now karyotyping is actually done by snapping a picture of the chromosomes during mitosis - because that’s when they are at their most condensed, and they’re the most visible.
Mitosis can be broken down into prophase, metaphase, anaphase, and telophase. And metaphase is when the chromosomes, made up of two chromatids each, neatly align on the midline of the cell, like 46 little X shapes - where each side of the X is a chromatid.
So, in order to select chromosomes in metaphase, first you need to choose cells that can easily enter mitosis. And a good option are white blood cells from a blood sample.
Next, white blood cells are incubated in a culture flask with growth factors, which stimulate mitosis. After a while, when a number of cell divisions have taken place, a substance called colchicine is added, which stops mitosis from progressing past metaphase.
Then, the cells are transferred to a conical tube, and a hypotonic solution is added. Hypotonic means it’s more dilute than the intracellular fluid.
The laws of osmosis dictate that fluid goes from the more dilute compartment to the more concentrated compartment - so the cells swell with fluid and eventually burst like a balloon, releasing the chromosomes in the solution.
Then, a drop of the solution is put in a glass slide and preserved with a chemical fixative, like Carnoy’s fluid, which is a mixture of acetic acid and ethanol.
Next, the chromosomes are stained, in order to identify the chromosome sections that are lightly packed and carry mostly genes, called euchromatin; and the parts that are more condensed and don’t have as many genes, called heterochromatin.
These are seen as alternating bright and dark bands that appear along each chromosome, called banding patterns. These are different depending on the technique and stain used.
The most common is G-banding, where the chromosomes are stained with Giemsa. This stain attaches to regions that are rich in adenine and thymine pairs, like heterochromatin, making them darker.
In contrast, Giemsa doesn’t attach as much to euchromatin, which has more guanine and cytosine pairs. So these regions are the bright bands.
Now, there’s other techniques that are used for different purposes. R-banding, for example, also uses Giemsa, but with a twist, so it shows a Reverse pattern of G-banding.
So in this case, euchromatin is dark, while heterochromatin is bright. C-banding, is a technique that uses Giemsa to stain and study mostly the Centromeres.
Finally, Q-banding uses a fluorescent stain called Quinacrine, which creates a similar pattern as G-banding but can only be seen under UV light.
Now, once the chromosomes are stained, they’re easy to sort based on their size and the position of their centromere. Size-wise, chromosomes can be big, medium-sized, and small.
Centromere-wise, remember that the centromere divides the chromosome into two “arms”. The shorter, or “petit” arm, is called “p arm”.
And the longer, or “queue” of the chromosome, is called the “q arm”. So, when the centromere is just in the middle, both arms are about the same length, and that’s characteristic of metacentric chromosomes.
When the centromere is right below the middle, the p arm is shorter, and that’s called a submetacentric chromosome. Finally, when the centromere is at the very end of the chromosome, the p arm is really tiny, and those are called acrocentric chromosomes.
Based on that, chromosomes are placed next to each other from bigger to smaller, with the centromeres aligned, and their short arms upside.
The resulting collage is called a karyogram. Finally, chromosomes are classified into seven groups, from A to G.
The A group has the big metacentric chromosomes, or pairs 1,2, and 3. Group B has big, but submetacentric chromosomes, or pairs 4 and 5.
Group C has medium-sized, submetacentric chromosomes, which are the pairs 6 through 12, and also the X chromosome. Group D has medium-sized, acrocentric chromosomes, or pairs 13, 14, and 15.
Group E has small chromosomes, which can be metacentric, like pair 16, or submetacentric, like pairs 16, 17, and 18. Group F also has small metacentric chromosomes, like pairs 19 and 20, which are even smaller than those in group E.
Finally, group G is made up of small acrocentric chromosomes, like pairs 21, 22, and the Y chromosome. On a side note, chromosomes X and Y are usually placed together at the end of the karyogram to represent the sex chromosomes and differentiate them from the 22 somatic pairs.
Ok, now, for any two given individuals that don’t have a genetic disorder, the karyogram looks about the same. So, for example, Will.I.Am’s karyogram would look just like Adam Levine’s, even though they look quite different from one another.
So the karyogram doesn’t give any information about what hair color a person might have. However, it can identify chromosomal abnormalities that indicate a genetic disorder.
These include having an abnormal number of chromosomes, called aneuploidy; or structural abnormalities, like deletion, duplication, inversion, or translocation of a chromosome segment.
So first up, Aneuploidy means having additional or missing chromosomes. For example, in Down syndrome also known as Trisomy 21, there is an extra chromosome 21, whereas in Turner syndrome, there’s only one sex chromosome - a single X.
Next let’s look at structural abnormalities. With deletions, a chunk of the chromosome goes missing.
An example is Cri du chat, or 5p- syndrome, when the tip of chromosome 5’s short arm is missing. Duplication means a chromosome has a duplicate segment that attaches to another segment that codes for the same genes, resulting in duplicate genes.
For example chromosome 1q21.1 duplication syndrome, where the karyogram shows two of the same bands one after another in the long arm of chromosome 1.
With inversions, a part of the chromosome breaks off and reattaches to the same chromosome, but it gets flipped around in the process.
Finally, there’s translocations, where a part of one chromosome breaks off and is exchanged for a part of another, non-homologous chromosome, which may result in a fusion between genes that were initially separated.
A classic example is the Philadelphia chromosome, which is a chromosome 22 with a bit of chromosome 9 on it - so on a karyogram, chromosome 9 looks shorter, whereas chromosome 22 looks longer than normal.
Right where they come together, there’s a fusion between the BCR gene - initially in chromosome 22 - and the ABL gene - originally in chromosome 9.
And the presence of a fusion gene ABL- BCR leads to uncontrolled cell division, which results in a type of leukemia. Alright, as a quick recap… Karyotyping is the process of retrieving, staining, taking a picture, and organizing the chromosomes of an individual by size and position of the centromere, in order to analyze them and determine whether there’s any chromosomal abnormality.
This can be an abnormal number of chromosomes, called aneuploidy; or a structural abnormality, such as deletions, duplications, inversions, or translocations.