The entire amount of genetic material is laid down in just 46 pairs of chromosomes. And chromosomes, as we know from biology, are located in the nucleus of the cell. A he althy person has a karyotype of 23 pairs of diploid chromosomes. That is, 46 XX is the female chromosome set, and 46 XY is the male chromosome set. When a chromosome breaks, the main "carrier" of the genetic code, various kinds of violations occur.
Mutations are not unique to humans. Small changes in the genetic material contribute to the diversity of the manifestation 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 during meiosis (chromosome division), in addition, sometimes parts of the chromosomes are duplicated (duplication occurs), and then the consequences are unpredictable. But we will only consider Robertsonian translocations, their features and consequences.
Robertsonian translocations - what is it? Gene problems of humanity
Due to the gapchromosomes near the centromere, structural changes occur in the human genetic code. The gap can be single, and sometimes repeated. One arm of the chromosome after the break (usually the short arm) is lost. But there are cases when the break occurs simultaneously in 2 chromosomes, the short arms of which are interchanged. It happens that only certain parts of the shoulder undergo translocation. But such short arms in chromosomes of the acrocentric type (in which the centromere divides the chromosome into longer and shorter arms) 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 separated short arms fuse with the short arms of another gene, and the remaining long arms are also soldered together, then such a translocation is no longer balanced. Such "rearrangements" of genetic material are Robertsonian translocations.
W. Robertson studied and described this type of translocation in 1916. And the anomaly was named after him. Robertsonian translocation can lead to the development of cancer, but may not affect the appearance and he alth of the carrier. However, a child in most cases, if one of the parents has such a translocation, is born with deviations.
How common is a mutation?
Thanks to the improvement of technology and the development of genetics as a science, today it is possible to know in advance whether there are anomalies inkaryotype of the unborn child. Now it is possible to conduct statistics: how often do gene anomalies appear? According to current data, Robertsonian translocations occur in one newborn out of a thousand. Most often, a translocation of chromosome 21 is diagnosed.
Small chromosomal translocations absolutely do not threaten the carrier itself. But when important elements of the code are affected, the child may be stillborn or die after a few months, as, for example, happens with Patau syndrome. But Patau syndrome is very rare. About 1 in 15,000 births.
Factors contributing to the appearance of translocation in chromosomes
In nature, there are spontaneous mutations, that is, not caused by anything. But the environment makes its own adjustments to the development of the genome. Several factors contribute to the increase in mutational changes. These factors are called mutagenic. The following factors are known:
- effect of nitrogenous bases;
- foreign DNA biopolymers;
- mother drinking during pregnancy;
- impact of viruses during pregnancy.
Most often, translocation occurs due to the harmful effects of radiation on the body. Affects ultraviolet radiation, proton and X-ray radiation, as well as gamma rays.
Which chromosomes undergo changes?
Chromosome 13, 14, 15 and 21 undergo translocation. The most popular and dangerous translocation is the Robertsonian translocation between 14 and 21chromosomes.
If meiosis produces an extra chromosome (trisomy) in a fetus with this translocation, the baby will be born with Down syndrome. The same precedent is possible if a Robertsonian translocation occurs between chromosomes 15 and 21.
Translocation of group D chromosomes
Robertsonian translocation of group D chromosomes affects only acrocentric chromosomes. Chromosomes 13 and 14 are involved in translocations in 74% of cases and they are called unbalanced translocations, which often do not have life-threatening consequences.
However, there is one circumstance that may accompany such anomalies. Robertsonian translocation 13, 14 in men can lead to impaired fertility of such a male carrier (chromosome set 45 XY). Due to the fact that, due to the loss of both short arms, instead of 2 pairs of chromosomes, only one more often remains, having 2 long ones, the gametes of such a man cannot give viable offspring.
The same Robertsonian translocation of 13, 14 in a woman also reduces her ability to have a baby. Menstruation is present in such women, and yet there have been cases when they gave birth to he althy children. But statistics still show that these are rare cases. Most of their children are not viable.
Consequences of translocations
We have already found out that some structural changes are quite normal and do not pose a threat. A single Robertsonian translocation is determined only by analysis. But repeated translocation tothe next generation of chromosomes is already dangerous.
Robertsonian translocation 15 and 21 in combination with other structural changes can even be deplorable. We will describe all the consequences of individual structural changes in the karyotype in more detail. Recall that a karyotype is a set of chromosomes inherent in an individual in the nucleus.
Trisomies and translocations
In addition to translocations, geneticists 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 prescribed 2 copies, mosaic trisomy sometimes occurs. That is, the triploid set is not observed in all cells of the body.
Trisomy in combination with Robertsonian translocation leads to very serious consequences: such as Patau syndrome, Edwards syndrome and the more common Down syndrome. In some cases, a set of such anomalies leads to early miscarriage.
Down Syndrome. Manifestations
It should be noted that translocations involving 21 and 22 chromosomes are more stable. Such anomalies do not lead to death, are not semi-lethal, but simply lead to a deviation in development. Thus, trisomy 21 in combination with a Robertsonian translocation in the karyotype in the analysis of the karyotype of the fetus is a clear "sign" of Down syndrome, a genetic disease.
Down 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, theirthe body is functioning normally.
Characteristic signs of the syndrome:
- flat face;
- increased tongue;
- a lot of skin on the neck, going into folds;
- clinodactyly (curvature of fingers);
- epicanthus;
- heart disease is possible in 40% of cases.
People with this syndrome begin to walk more slowly, to pronounce words. And it is also more difficult for them to learn than other children of the same age.
Still, they are capable of fruitful work in society, and with some support and proper work with such children, they will socialize well in the future.
Patau Syndrome
The syndrome is less common than Down syndrome, but such a child has a lot of defects of various kinds. Almost 80% of children with this diagnosis die within 1 year of life.
In 1960, Klaus Patau studied this anomaly and found out the causes of the genetic failure, although before him in 1657 he described T. Bartolini's syndrome. The risk of such disorders increases in those women who give birth to a child after 31.
In such children, numerous physical defects are combined with severe psychomotor developmental disorders. Characteristic of the syndrome:
- microcephaly;
- abnormal hands, often with extra fingers;
- low-set irregular ears;
- hare lip;
- short neck;
- narrow eyes;
- obviously "sunken" bridge of the nose;
- defects of the kidneys and heart;
- cleft lip or palate;
- pregnancy has only one umbilical artery.
A small number of surviving babies receive medical attention. And they can live for a long time. But congenital anomalies still affect the nature of life and its shortness.
Edwards Syndrome
Trisomy of chromosome 18 due to translocation leads to Edwards syndrome. This syndrome is less well known. With this diagnosis, the child barely lives up to six months. The law of natural selection will not allow a creature with many deviations to develop.
In general, the number of different malformations in Edwards syndrome is about 150. There are malformations of the blood vessels, heart, and internal organs. Cerebellar hypoplasia is always present in such newborns. Anomalies in the structure of the fingers are possible. Very often, such a distinctive anomaly as deformity of the foot appears.
What tests detect abnormalities during fetal development?
To analyze the fetal karyotype, it is necessary to obtain material - fetal cells.
There are several tests. Let's see how it all happens.
1. Biopsy of chorionic villi. The analysis is carried out at week 10. These villi are a direct part of the placenta. This particle of biological material will tell everything about the future fetus.
2. Amniocentesis. With the help of a needle, several fetal cells and amniotic fluid are taken. They are taken most often at the 16th week of pregnancy, and after a few weeks, the couple can receive detailedchild welfare information.
Mothers who have an increased risk of giving birth to a child with abnormalities are sent for such an analysis. Usually, those couples who have:
1) had unexplained miscarriages;
2) the couple could not conceive for a long time;
3) there were closely related ties in the genus.
Such young people may have Robertsonian translocations of some chromosome. And therefore, they must do an analysis on their karyotype in advance in order to know what are the chances of enduring and giving birth to a he althy child.
