Human Karyotype: Definition, Types, Organization and Chromosomal Abnormalities

It is defined as the grouping of chromosomes within cells, which are organized by:

  • Shape.
  • Size.
  • Features.

The general lines can be said that a human karyotype harbors 46 chromosomes of which are joined in 23 pairs, of which 22 pairs are characterized by being autosomal, while 1 pair corresponds to the sexuality of the individual, that is, XY if it corresponds to a man and XX if it corresponds to a woman.

There may also be other patterns of karyotypes in humans that are called or known as chromosomal abnormalities, failures, or aberrations.

Among the importance of human karyotypes is that diseases, mutations or genetic failures can be discovered through the chromosomal structure.

For example, with a karyotype study, Down syndrome can be discovered and diagnosed by the mutation in chromosomal pair 21, which acquires an extra chromosome, that is, there are three instead of 2, which leads to some physical characteristics already determined a deficiency or mental retardation in the person who suffers from it.

It is common to know karyotypes such as chromosome analysis.

Procedure to perform a karyotype

To perform a karyotype, the chromosomes must be stained. Subsequently, a specific dye is placed that will help to better visualize the chromosomal structure through a microscope.

This procedure will allow the pattern of each chromosome pair to be appreciated more clearly. The cell is also evaluated to investigate the individual’s genetics and thus determine if it has any abnormality or mutation in its DNA.

Classic karyotype

In this type of study, a solution called Giemsa is applied as a dye to distinguish phosphate groups in DNA.

The objective of this solution is to color the chromosome bands and then organize them so that the short arm of the chromosome is located in the upper front part while the long arm is in the lower part.

Types of chromosomes in a karyotype

Within the study of karyotypes, two types of chromosomes can be evaluated:

  • Autosomal.
  • Sexual

The two types of chromosomes differ as follows:

Autosomal chromosomes: they tend to have the same characteristics, either for men or for women, and humans have 22 pairs of this category in their chromosome structure.

Sex chromosomes: these define the sex of the individual, so if they form two XX it will be female but if it forms XY it will be male.

They differ from each other because the X have four separate arms that originate or acquire a similarity to an X, whereas the Y chromosomes only have 2 arms joined at one end that simulates the shape of that letter.

Organization of the human karyotype

When a karyotype study is carried out, the chromosomes are organized from largest to smallest, this is because in the chromosomal structure some small, other medium and large pairs can be found in the 23 pairs.

In turn, the chromosomes are also organized into 7 groups, where the shape of the pairs is evaluated and whether they are:

  • Metacentric.
  • Submetacentric.
  • Acrocentric.

Karyotype for chromosomal abnormalities

Careful karyotype analysis can reveal subtle structural changes, such as chromosomal deletions, duplications, translocations, or inversions.

In fact, as medical genetics becomes increasingly integrated with clinical medicine, karyotypes are becoming a source of diagnostic information for specific birth defects, genetic disorders, and even cancers.

Preparation of karyotypes from mitotic cells

Karyotypes are prepared from mitotic cells that have stopped in the metaphase or prometaphase portion of the cell cycle, when the chromosomes assume their most condensed conformations.

A variety of tissue types can be used as a source for these cells. For cancer diagnosis, typical samples include tumor biopsies or bone marrow samples.

For other diagnoses, karyotypes are often generated from peripheral blood samples or a skin biopsy. For prenatal diagnosis, amniotic fluid or chorionic villus samples are used as a source of cells.

And the banding patterns reveal the structural details of the chromosomes, without any treatment, because the structural details of the chromosomes are difficult to detect with a light microscope.

Therefore, to make the analysis more effective and efficient, cytologists have developed spots that bind with DNA and generate characteristic banding patterns for different chromosomes.

Chromosomal abnormalities

Currently, G-band karyotypes are used routinely to diagnose a wide range of chromosomal abnormalities in individuals.

Although the resolution of karyotyped-detectable chromosomal changes is usually within a few megabases, this may be sufficient to diagnose certain categories of abnormalities.

For example, aneuploidy , which is often caused by the absence or addition of a chromosome, is simple to detect by karyotype analysis.

The cytogenetic can also detect deletions or insertions often much more subtle as deviations from normal patterns of bands. Similarly, translocations are often evident in karyotypes.

When regional changes are observed in the chromosomes studied in karyotypes, researchers are often interested in identifying candidate genes within the critical range whose incorrect expression can cause symptoms in patients.

Consequently, researchers can now apply a variety of molecular cytogenetic techniques to achieve even greater resolution of genomic changes.

Fluorescent in situ hybridization (FISH) and comparative genomic hybridization (CGH) are examples of two approaches that can identify abnormalities at the individual gene level.

Molecular cytogenetics is a dynamic discipline and new diagnostic methods continue to be developed. As these new technologies are implemented in the clinic, we can expect that cytogenetists will be able to make the leap from karyotype to gene with greater efficiency.