KARYOTYPE: WHAT IS IT FOR, TYPES, HOW IS IT DONE, ALTERATIONS - BIOLOGY - 2025

The karyotype is a photograph of the complete set of metaphase chromosomes that details aspects of their number and structure. The branch of medical and biological sciences that deals with the study of chromosomes and related diseases is known as cytogenetics.

Chromosomes are the structures in which the genes contained in deoxyribonucleic acid (DNA) molecules are organized. In eukaryotes, they are composed of chromatin, a complex of histone proteins and DNA that is packaged within the nucleus of all cells.

Human karyotype obtained with fluorescent dyes (Source: Plociam ~ commonswiki via Wikimedia Commons

The cells of every living thing on Earth have a particular number of chromosomes. Bacteria, for example, have only one circular, while humans have 46 organized in 23 pairs; and some species of birds have up to 80 chromosomes.

Unlike humans, plant cells generally have more than two homologous (identical) sets of chromosomes. This phenomenon is known as polyploidy.

All the instructions necessary for the growth and development of living beings, unicellular or multicellular, are contained in the DNA molecules that are coiled on the chromosomes. Hence the importance of knowing its structure and characteristics in a species or in any of its individuals.

The term karyotype was used for the first time during the 1920s by Delaunay and Levitsky to designate the sum of the characteristic physical properties of chromosomes: their number, size and structural peculiarities.

Since then, it is used for the same purpose in the context of modern science; and the study of it accompanies many processes of the clinical diagnosis of various diseases in man.

Human karyotype

The set of 46 chromosomes (23 pairs) that make up the human genome is known as a human karyotype and which are arranged graphically according to characteristics such as size and banding pattern, which is evident thanks to the use of special staining techniques.

Schematic representation of the human karyotype (Source: Mikael Häggström via Wikimedia Commons)

Of the 23 pairs of chromosomes, only 1 through 22 are arranged in order of size. In somatic cells, that is, in non-sexual cells, these 22 pairs are found and, depending on the sex of the individual, whether male or female, a pair of X chromosomes (women) or the XY pair (men) is added..

The pairs 1 through 22 are called autosomal chromosomes and are the same in both sexes (male and female), while the sex chromosomes, X and Y, are different from each other.

What is the karyotype for?

The main utility of a karyotype is the detailed knowledge of the chromosomal load of a species and the characteristics of each of its chromosomes.

Although some species are polymorphic and polyploid in relation to their chromosomes, that is, they have variable shapes and numbers of these throughout their life cycle, knowledge of the karyotype usually allows us to infer a lot of important information about them.

Thanks to the karyotype, chromosomal changes on a “large scale” involving large pieces of DNA can be diagnosed. In humans, many mentally disabled diseases or conditions and other physical defects are related to severe chromosomal abnormalities.

Types of karyotypes

Karyotypes are described according to the notation endorsed by the International System of Human Cytogenetic Nomenclature (ISCN).

In this system, the number assigned to each chromosome has to do with its size, and they are usually ordered from largest to smallest. Chromosomes are presented in karyotypes as pairs of sister chromatids with the small arm (p) facing upward.

The types of karyotypes are distinguished by the techniques used to obtain them. Usually the difference lies in the types of staining or "labeling" used to differentiate one chromosome from another.

Here is a brief summary of some of the techniques known to date:

Solid staining

In this, dyes such as Giemsa and orcein are used to stain the chromosomes evenly. It was widely used until the early 1970s, since they were the only known dyes for the time.

G-band or Giemsa stain

It is the most used technique in classical cytogenetics. Chromosomes are previously digested with trypsin and then stained. The pattern of bands obtained after staining is specific for each chromosome and allows detailed studies of its structure.

There are alternative methods to Giemsa staining, but they give very similar results, such as Q banding and reverse R banding (where the dark bands observed are the light bands obtained with G banding).

Constitutive C-band

It specifically stains heterochromatin, especially that found in the centromeres. It also stains some material in the short arms of the acrocentric chromosomes and the distal region of the long arm of the Y chromosome.

Replication banding

It is used to identify the inactive X chromosome and involves the addition of a nucleotide analog (BrdU).

Silver stain

It has been used historically to identify nucleolar organization regions that contain many copies of ribosomal RNA and are found in the centromeric regions.

Distamycin A / DAPI staining

It is a fluorescent staining technique that distinguishes heterochromatin from chromosomes 1, 9, 15, 16 and from the Y chromosome in humans. It is used especially to distinguish the inverted duplication of chromosome 15.

Fluorescent in situ hybridization (FISH)

Recognized as the greatest cytogenetic advance after the 1990s, it is a powerful technique by which submicroscopic deletions can be distinguished. It employs fluorescent probes that specifically bind to chromosomal DNA molecules, and there are multiple variants of the technique.

Comparative Genomic Hybridization (CGH)

It also uses fluorescent probes to differentially label DNA, but uses known comparison standards.

Other techniques

Other more modern techniques do not directly involve the analysis of the chromosomal structure, but rather the direct study of the DNA sequence. These include microarrays, sequencing, and other techniques based on PCR (polymerase chain reaction) amplification.

How is a karyotype performed?

There are various techniques to carry out the study of chromosomes or karyotype. Some are more sophisticated than others, since they allow the detection of small imperceptible alterations by the most commonly used methods.

Cytogenetic analyzes to obtain the karyotype are commonly performed from cells present in the oral mucosa or in the blood (using lymphocytes). In the case of studies carried out in neonates, samples are taken from the amniotic fluid (invasive techniques) or from fetal blood cells (non-invasive techniques).

The reasons why a karyotype is performed are diverse, but many times they are done for the purposes of diagnosing diseases, fertility studies, or to find out the causes of recurrent miscarriages or fetal deaths and cancers, among other reasons.

The steps to perform a karyotype test are as follows:

1-Obtaining the sample (whatever its source).

2-Separation of cells, a vitally important step, especially in blood samples. In many cases it is necessary to separate dividing cells from dividing cells using special chemical reagents.

3-Cell growth. Sometimes it is necessary to grow the cells in a suitable culture medium to obtain a greater quantity of them. This may take more than a couple of days, depending on the type of sample.

4-Synchronization of cells. To observe the condensed chromosomes in all cultured cells at the same time, it is necessary to “synchronize” them by means of chemical treatments that stop cell division when the chromosomes are more compact and, therefore, visible.

5-Obtaining chromosomes from cells. To see them under the microscope, the chromosomes must be "pulled" out of the cells. This is usually achieved by treating these with solutions that cause them to burst and disintegrate, freeing the chromosomes.

6-Staining. As highlighted above, chromosomes must be stained by one of many available techniques in order to be able to observe them under the microscope and perform the corresponding study.

7-Analysis and counting. Chromosomes are observed in detail to determine their identity (in the case of knowing it in advance), their morphological characteristics such as size, position of the centromere and banding pattern, the number of chromosomes in the sample, etc.

8-Classification. One of the most arduous tasks of cytogeneticists is that of the classification of chromosomes by comparing their characteristics, since it is necessary to determine which chromosome is which. This is because since there is more than one cell in the sample, there will be more than one pair of the same chromosome.

Chromosomal abnormalities

Before describing the different chromosomal alterations that may exist and their consequences for human health, it is necessary to become familiar with the general morphology of chromosomes.

Chromosome morphology

Chromosomes are structures that appear linear and have two “arms”, a small one (p) and a larger one (q) that are separated from each other by a region known as the centromere, a specialized site of DNA that participates in spindle anchoring. mitotic during mitotic cell division.

The centromere can be located in the center of the two arms p and q, far from the center or close to one of their ends (metacentric, submetacentric or acrocentric).

At the ends of the short and long arms, the chromosomes have “caps” known as telomeres, which are particular DNA sequences rich in TTAGGG repeats and which are responsible for protecting DNA and preventing fusion between chromosomes.

At the beginning of the cell cycle, chromosomes are seen as individual chromatids, but as the cell replicates, two sister chromatids form that share the same genetic material. It is these chromosomal pairs that are seen in the karyotype photographs.

Chromosomes have different degrees of "packing" or "condensation": heterochromatin is the most condensed form and is transcriptionally inactive, whereas euchromatin corresponds to the looser regions and is transcriptionally active.

In a karyotype, each chromosome is distinguished, as previously highlighted, by its size, the position of its centromere, and the banding pattern when stained with different techniques.

Chromosomal abnormalities

From the pathological point of view, specific chromosomal alterations can be specified that are regularly observed in human populations, although other animals, plants, and insects are not exempt from these.

Abnormalities often have to do with deletions and duplications of regions of a chromosome or entire chromosomes.

These defects are known as aneuploidies, which are chromosomal alterations that involve the loss or gain of a complete chromosome or parts of it. The losses are known as monosomies and the gains are known as trisomies, and many of these are lethal to developing fetuses.

There can also be cases of chromosomal inversions, where the order of the gene sequence changes due to simultaneous breaks and erroneous repairs of some region of the chromosome.

Translocations are also chromosomal alterations that involve changes in large portions of chromosomes that are exchanged between non-homologous chromosomes and may or may not be reciprocal.

There are also alterations that are related to direct damage to the sequence of genes contained in chromosomal DNA; and there are even some related to the effects of genomic "marks" that material inherited from one of the two parents may bring with it.

Human diseases detected with karyotypes

Cytogenetic analysis of chromosomal alterations before and after birth is essential for the comprehensive clinical care of infants, regardless of the technique used for this purpose.

Down syndrome is one of the most commonly detected pathologies from the karyotype study, and it has to do with the nondisjunction of chromosome 21, which is why it is also known as trisomy 21.

Karyotype of a human with trisomy on chromosome 21 (Source: US Department of Energy Human Genome Program. Via Wikimedia Commons)

Some types of cancer are detected by studying the karyotype, since they are related to chromosomal changes, especially the deletion or duplication of genes directly involved with oncogenic processes.

Certain types of autism are diagnosed from karyotype analysis, and duplication of chromosome 15 has been shown to be involved in some of these conditions in humans.

Among other pathologies associated with deletions on chromosome 15 is Prader-Willi syndrome, which causes symptoms such as lack of muscle tone and respiratory deficiencies in infants.

The “crying cat” syndrome (from the French cri-du-chat) implies the loss of the short arm of chromosome 5 and one of the most direct methods for its diagnosis is through the cytogenetic study of the karyotype.

The translocation of parts between chromosomes 9 and 11 characterizes patients suffering from bipolar disorder, specifically related to the disruption of a gene on chromosome 11. Other defects on this chromosome have also been observed in various birth defects.

According to a study carried out by Weh et al. In 1993, more than 30% of patients suffering from multiple myeloma and plasma cell leukemia have karyotypes with chromosomes whose structures are aberrant or abnormal, especially on chromosomes 1, 11 and 14.

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