Karyogram: Definition, Classification, Score, Cytogenetic Card and Procedure for the Study of Chromosomes

Also called a cytogenetic map, it is a genetic term that refers to the visual appearance of a chromosome once it is stained and examined under a microscope.

Particularly important are the visually distinct regions, called light and dark bands, that give each chromosome a unique appearance.

This characteristic allows a person’s chromosomes to be studied in a clinical test known as a karyotype , which allows scientists to look for chromosomal abnormalities .

The size, shape, and pattern of the karyotype bands are schematically depicted in a karyotype.

In a karyogram, the graphic representation must be ordered and in pairs of the homologous chromosomes.

As the chromosomes are the same in all cells, it can be done in any, although blood cells are usually used.

Cytogenetics

Cytogenetics is the field of genetics that studies chromosomes, their structure, their composition, and their role in the evolution and development of disease.

The history of cytogenetics begins in 1903, when the American Walter Sutton and Boveri the German Teodor established the chromosomal theory of inheritance, proposing that genetic material is found in chromosomes.

Sutton and Boveri initiated a scientific revolution that allowed the characterization of numerous previously unexplained malformative syndromes and the emergence of new specialties in Human Genetics with applications in prevention, diagnosis and medical therapy.

Also in the field of cytogenetics are research on biodiversity, studies of genetic improvement of plants and animals, and even studies of fertility, both human and animal.

Chromosome classification

In 1956 the researchers Tjio and Levan showed that 46 chromosomes make up the human karyotype, 23 pairs.

Chromosomes can be differentiated by size and shape.

In 1960 a group of researchers ordered the human chromosomes from largest to smallest by size, and within the same size, by the position of the centromere .

This set of characteristics such as shape, size, position of the centromere, the bands that show when staining, among others, allows the chromosomes of the various species to be distinguished, it is called a karyotype.

The karyotype and the number of chromosomes are particular to each species.

Human beings have a total of 46 chromosomes, or 23 pairs, in the nucleus of each diploid cell, made up of 22 pairs of autosomal chromosomes and 1 pair of sex chromosomes (XY and XX).

Somatic chromosomes were later classified into seven groups distinguished by the letters from A to G, classified into seven groups:

  • Group A: is made up of pairs 1, 2 and 3, which are very large chromosomes, where 1 and 3 are metacentric chromosomes and 2 is a submetacentric chromosome.
  • Group B: consists of pairs 4 and 5, which are large and submetacentric chromosomes.
  • Group C: is made up of pairs 6, 7, 8, 9, 10, 11 and 12, all medium and submetacentric chromosomes.
  • Group D: is formed by pairs 13, 14 and 15, acrocentric medium chromosomes with satellite.
  • Group E: is made up of pairs 16, 17 and 18, they are small chromosomes, 16 is a metacentric and submetacentric chromosome, 17 and 18.
  • Group F: is formed by pairs 19 and 20, they are small and metacentric chromosomes.
  • Group G: is formed by pairs 21 and 22, they are small and acrocentric chromosomes.

Pair 23 is made up of the sex chromosomes, an X chromosome, of medium metacentric size and similar to group C chromosomes, and a Y chromosome, small and acrocentric, similar to group G chromosomes.

Score and cytogenetic card

Each human chromosome has a short arm (“p” for petit in French meaning small) and a long arm (“q” queue in French for tail or tail), separated by a centromere.

The ends of the chromosome are called telomeres.

Each arm of the chromosome is divided into regions or cytogenetic bands visible under the microscope using certain stains.

The bands are called p1, p2, p3, q1, q2, q3, etc., from the centromere to the telomere.

At higher resolution, subbands appear within bands.

The subbands are numbered in the same order.

For example, the location of the CFTR gene on the cytogenetic map is 7q31.2, that is:

  • Chromosome 7, q arm, band 3, subband 1 and sub-subband 2.

The ends of the chromosome are called ptel and qtel. The notation 7qtel indicates the end of the long arm of chromosome 7.

Procedure for the study of chromosomes

The procedure for studying human chromosomes is to culture lymphocytes and at the time of division treat them with colchicine.

Colchicine disrupts mitosis at metaphase.

From the shape and number of chromosomes of a species, especially mitotic chromosomes, in metaphase phase, the karyotype of the species can be described.

This is the most propitious moment for the observation of chromosomes. They are then dipped in a hypotonic solution to swell and disperse. They are eventually stained, which is known as chromosome paint.

Chromosome painting is a technique used to mark or “paint” the chromosome sample to be analyzed.

Fluorescent-labeled samples are called DNA “probes,” in analogy to space-traveling probes.

As the chromosomes are arranged in pairs, the probe will form a hybrid with its corresponding pair, which will be easily identified by color.

This procedure is known as fluorescent in situ hybridization.

With this technique it is possible to map the genome of various organisms, for evolutionary studies, biological classification studies and for studies of genetic alterations in diseases.

The so-called conventional stains such as Giemsa, Acetic Orcein, Schiff’s reagent, hematoxylin or eosin, among others, can be used in the methodologies that identify the “bands” of chromosomes.

More recently, with the advent of various techniques in Molecular Biology, cytogenetics has accumulated refinements, especially those related to the location of genes or repetitive DNA sequences.

These studies are reflected in the karyogram or cytogenetic map in drawings or photographs.

Chromosome study uses

From the study of the karyogram, genetic investigations of that individual can be carried out, which allow detecting possible syndromes.

As the chromosomes are the same in all cells, it can be done in any, although blood cells are usually used.

To carry out these types of studies in the fetus, the cells are obtained by amniocentesis, taking the cells from the umbilical cord blood or from the amniotic fluid.

The karyogram is used to detect chromosomal abnormalities in the following cases:

  • Genetic disorders.
  • Congenital malformations.
  • Mental retardation.
  • A delay in puberty.
  • Gonadal disorders.
  • A study of couples with a history of recurrent reproductive loss.
  • A study of couples with a history of malformed or stillborn children.

The alterations presented in the chromosomes as a consequence, can cause fertility or health problems.

The alterations in the chromosomes can be due to the absence of a chromosome, a duplication, the loss of some “piece”.

In those cases when a chromosome is missing, we are in the case of aneuploidy .

The most common aneuploidies are trisomy on chromosome 21 or Down syndrome, trisomy on chromosome 18 or Edwards syndrome, and trisomy on chromosome 13 or Patau syndrome.

Alterations in the chromosomal structure can influence, in the cases of recurrent abortions, ovarian failures or implantation failures in the case of women, and in men, alterations in seminal quality or sterility due to azoospermia.