Robertsonian translocations: description, features and characteristics

The entire volume of genetic material lies only in 46 pairs of chromosomes. And the chromosomes, as is known from biology, are in the nucleus of the cell. A healthy person has a karyotype of 23 pairs of diploid chromosomes. That is, 46 XX is a chromosome set of a woman, and 46 XY is a male chromosome set. When a chromosome breaks, the main "bearer" of the genetic code, various kinds of violations occur.

Mutations are inherent not only to man. Small changes in gene material contribute to the diversity of manifestations of nature. With the so-called balanced translocation, the change in chromosomes occurs without loss of information and without unnecessary duplication. Most often this happens with meiosis (division of the chromosome), in addition, sometimes parts of the chromosomes are duplicated (duplication occurs), and then the consequences are unpredictable. But we will consider only the Robertsonian translocations, their features and consequences.

Robertsonian translocations - what is it? The genetic problems of mankind

Due to the rupture of the chromosome near the centromere, structural changes occur in the human genetic code. The gap can be single, but can be repeated. One shoulder of the chromosome after the rupture (more often a short shoulder) is lost. But there are cases when the gap occurs simultaneously in 2 chromosomes, whose short shoulders change places. It happens that only certain parts of the shoulder are subjected to translocation. But such short shoulders in acrocentric-type chromosomes (in which the centromere divides the chromosome into a longer and shorter shoulders) never carry vital information. In addition, the loss of such elements is not so important, since this hereditary material is copied in other acrocentric chromosomes.

But when the detached short shoulders fuse with the short shoulders of another gene, and the remaining long ones also weld together, this translocation is no longer balanced. Such "permutations" of the genetic material are the Robertson translocation.

He investigated and described this kind of translocation of W. Robertson in 1916. And his name was also called an anomaly. Robertsonovskaya translocation can lead to the development of cancer, but it can and does not affect the appearance and health of the carrier. However, in most cases, if one of the parents has such a translocation, the child is born with deviations.

How common is a mutation?

Thanks to the improvement of technology and the development of genetics as a science, today one can know in advance whether there are anomalies in the karyotype of a future child. Now it is possible to make statistics: how often do gene anomalies appear? According to modern data, Robertson's translocations are found in one newborn out of a thousand. Most often, the translocation of 21 chromosomes is diagnosed.

Small chromosomal translocations absolutely do not threaten the wearer himself. But when important elements of the code are touched, the child can be born dead or die in a few months, as, for example, happens with the Patau syndrome. But the Patau syndrome is very rare. Somewhere 1 case for 15 thousand births.

Factors contributing to the appearance of translocation in chromosomes

In nature, there are spontaneous mutations, that is, nothing caused. But the environment makes its own adjustments to the development of the genome. Some factors contribute to the increase in mutational changes. These factors are usually called mutagenic. The following factors are known:

• Exposure to nitrogenous bases;
• Foreign DNA biopolymers;
• Taking alcohol by the mother during pregnancy;
• Influence of viruses during pregnancy.

The most common is translocation due to the harmful effects of irradiation on the body. Influences ultraviolet radiation, proton and X-ray radiation, as well as gamma rays.

Which chromosomes undergo changes?

The translocations of chromosome 13, 14, 15 and 21 are undergoing. The most popular and dangerous translocation is the Robertsonian translocation between 14 and 21 chromosomes.

If, as a result of meiosis, an additional chromosome (trisomy) is formed in the fetus with such a translocation, the child will be born with Down's syndrome. A similar precedent is possible if a Robertsonian translocation occurred between 15 and 21 chromosomes.

Translocation of chromosomes of group D

Robertson's translocation of chromosomes of group D affects only acrocentric chromosomes. Chromosomes 13 and 14 participate in translocations in 74% of cases and they are called unbalanced translocations, which often have no dangerous consequences for life.

However, there is one circumstance that can accompany such anomalies. Robertson's translocation 13, 14 in men can lead to a violation of the fertility of such a male carrier (chromosome set 45 XY). Because of the loss of both pairs of chromosomes rather than two pairs of chromosomes, because of the loss of both pairs of chromosomes, there is often only one, having 2 long ones, gametes of such a man can not give a viable offspring.

The same Robertsonian translocation 13, 14 in a woman also reduces her ability to have a baby. Months are present in such women, and yet there have been cases when they gave birth to healthy children. But statistics still shows that these are rare cases. Basically, their children are not viable.

Consequences of translocations

We have already found out that some structural changes are quite normal and do not carry a threat. The single Robertson translocation is determined only by analysis. But repeated translocation in a set of chromosomes of the next generation is already dangerous.

Robertson's translocation 15 and 21, in combination with other structural changes, may even be deplorable. All the consequences of individual structural changes in the karyotype will be described in more detail. Recall that a karyotype is an individual set of chromosomes in the nucleus.

Trisomy and translocation

In addition to translocations, genetics distinguish such an anomaly as trisomy in the chromosome. Trisomy means that the fetal karyotype has a triploid set of one of the chromosomes, instead of the put two copies, mosaic trisomy sometimes occurs. That is, the triploid set is not observed in all cells of the body.

Trisomy in combination with Robertson's translocation leads to very serious consequences: such as the Patau syndrome, Edwards syndrome and the more common Down syndrome. In some cases, the recruitment of such anomalies leads to miscarriage in the early stages.

Down Syndrome. Manifestations

It should be noted that translocations involving 21 and 22 chromosomes are more stable. Such anomalies do not lead to fatal outcomes, are not semi-lethal, but simply lead to deviation in development. Thus, trisomy 21 in combination with Robertson's translocation in a karyotype in the analysis of the fetal karyotype is an obvious "sign" of Down's syndrome, a genetic disease.

Down's syndrome is characterized by both physical and mental abnormalities. The prognosis of life in such people is favorable. Despite heart defects and some physiological changes in the skeleton, their body functions normally.

Characteristic features of the syndrome:

• Flat face;
• Increased language;
• A lot of skin on the neck, gathering in creases;
• Clinodactyly (curvature of fingers);
• Epicanthus;
• Heart failure is possible in 40% of cases.

People with such a syndrome begin to walk slowly, say words. And also it is more difficult to study them than other children of the same age.

Nevertheless, they are capable of fruitful work in society and with a certain support and proper work with such children in the future they are well socialized.

Syndrome Patau

The syndrome is less common than Down's syndrome, but there are a lot of vices of this kind in such a child. Almost 80% of children with this diagnosis die within 1 year of life.

In 1960, he studied this anomaly and found out the causes of genetic malfunction Klaus Patau, although before him in 1657 he described the syndrome of T. Bartolini. The risk of such violations increases in those women who give birth after 31 years.

In such children, numerous physical defects are combined with severe disruption of the development of psychomotor. Characteristic for the syndrome:

• microcephaly;
• Abnormal hands, often superfluous fingers are formed;
• Low set ears of irregular shape;
• Cleft lip;
• Short neck;
• narrow eyes;
• Obviously "sunken" nose bridge;
• Kidney and heart defects;
• Cleft lip or palate;
• In pregnancy there is only one umbilical artery.

A small number of surviving infants are provided with medical care. And they can live for a long time. But congenital anomalies still affect the nature of life and its short duration.

Edwards Syndrome

Trisomy of chromosome 18 on the background of translocation leads to Edwards syndrome. This syndrome is less well known. With this diagnosis, the child barely survives until six months. The law of natural selection will not allow the development of a being with numerous deviations.

In general, the number of different vices in Edwards syndrome is about 150. There are malformations of the blood vessels, heart, internal organs. It is always present in such newborn hypoplasia of the cerebellum. Abnormalities in the structure of the fingers are possible. Very often there is such a distinctive anomaly as the deformation of the foot.

What analyzes determine the anomalies in the period of intrauterine development?

For the analysis of fetal karyotype, it is necessary to obtain the material - the fetal cells.

There are several analyzes. Let's see how it all happens.

1. Chorionic villus biopsy. The analysis is carried out at week 10. These villi are the immediate part of the placenta. This particle of biological material will tell all about the future fruit.

2. Amniocentesis. Using a needle, several fetal cells and amniotic fluid are taken. They are taken most often at week 16 of pregnancy, and in a few weeks the couple can get detailed information about the well-being of the child.

This analysis is sent to mothers who have a higher risk of having a child with abnormalities. Usually the genetic analysis is sent to those couples who have:

1) there were causeless miscarriages;

2) the couple could not conceive a child for a long time;

3) ties of a closely related nature were present in the family.

Such young people may have Robertsonian translocations of a chromosome. And therefore they must do an analysis in advance on their karyotype in order to know what are the chances to endure and give birth to a healthy child.