The entire volume of genetic material lies in only 46 pairs of chromosomes. And chromosomes, as is known from biology, are located in the nucleus of a cell. A healthy person has a karyotype of 23 pairs of diploid chromosomes. That is, 46 is the chromosome set of a woman, and 46 is the male set of chromosomes. 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 gene material contribute to the diversity of nature. With the so-called balanced translocation, a change in the 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 consider only Robertson translocations, their features and consequences.
Robertson translocations - what is it? Gene problems of mankind
Due to chromosome rupture near the centromere, structural changes occur in the human genetic code. The gap can be single, and sometimes repeated. One shoulder of the chromosome after a break (usually a short shoulder) is lost. But there are cases when the gap occurs simultaneously in 2 chromosomes, the short shoulders of which change places. It happens that only certain parts of the shoulder are translocated. But such short shoulders in acrocentric chromosomes (in which the centromere divides the chromosome into longer and shorter shoulders) never carry vital information. Moreover, the loss of such elements is not so important, since this hereditary material is copied in other acrocentric chromosomes.
But when the separated short shoulders fuse with the short shoulders of a different gene, and the remaining long shoulders also fuse together, then such a translocation is no longer balanced. Such “rearrangements” of genetic material are Robertson translocations.
Investigated and described this type of translocation W. Robertson in 1916. And an anomaly was named after him. Robertson translocation can lead to the development of cancer, but may not affect the appearance and health of the carrier. However, the child in most cases, if one of the parents has such a translocation, is born with abnormalities.
How common is a mutation?
Thanks to the improvement of technology and the development of genetics as a science, today you can find out in advance whether there are anomalies in the karyotype of an unborn child. Now there is an opportunity to conduct statistics: how often do gene abnormalities appear? According to modern data, Robertson translocations are found in one out of a thousand newborns. Most often, translocation of 21 chromosomes is diagnosed.
Small chromosomal translocations absolutely do not threaten the host itself. But when important elements of the code are touched, a child can be born dead or die in a few months, as, for example, happens with Patau syndrome. But 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, not caused by anything. But the environment is making adjustments to the development of the genome. Some factors contribute to the increase in mutational changes. These factors are commonly called mutagenic. The following factors are known:
- exposure to nitrogen bases;
- alien DNA biopolymers;
- drinking alcohol by the mother during pregnancy;
- the effect of viruses during pregnancy.
Most often, translocation occurs due to the harmful effects of radiation on the body. It affects ultraviolet radiation, proton and x-rays, as well as gamma rays.
What chromosomes undergo changes?
Chromosomes 13, 14, 15 and 21 are translocated. The most popular and dangerous translocation is Robertson translocation between chromosomes 14 and 21.
If as a result of meiosis an additional chromosome (trisomy) is formed in the fetus with such a translocation, the baby will be born with Down syndrome. The same precedent is possible if Robertson translocation occurred between 15 and 21 chromosomes.
Group D chromosome translocation
Robertson translocation of group D chromosomes affects only acrocentric chromosomes. Chromosomes 13 and 14 participate in translocations in 74% of cases and are called unbalanced translocations, which often have no dangerous consequences for life.
However, there is one circumstance that may accompany such anomalies. Robertson translocation 13, 14 in men can lead to impaired fertility of such a male carrier (chromosome set 45 CU). Due to the fact that, due to the loss of both short arms, instead of 2 pairs of chromosomes, more often there is only one that has 2 long chromosomes; the gametes of such a man cannot produce viable offspring.
The same Robertson translocation 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 healthy children. But statistics still show that these are rare cases. Mostly their children are not viable.
The consequences of translocations
We have already found out that some structural changes are quite normal and do not pose a threat. A single Robertson translocation is determined only by analysis. But re-translocation in the next-generation chromosome set is already dangerous.
Robertson 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 an inherent 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 laid 2 copies, mosaic trisomy sometimes takes place. That is, the triploid set is not observed in all cells of the body.
Trisomy in combination with Robertson translocation leads to very serious consequences: such as Patau syndrome, Edwards and the more common Down syndrome. In some cases, a set of such abnormalities 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 deaths, are not semi-lethal, but simply lead to a deviation in development. So, trisomy 21 in combination with Robertson translocation in the karyotype in the analysis of the fetal karyotype is a clear “sign” of Down syndrome, a genetic disease.
Down Syndrome is characterized by both physical and mental disabilities. The prognosis of life in such people is favorable. Despite heart defects and some physiological changes in the skeleton, their body functions normally.
Characteristic signs of the syndrome:
- flat face;
- enlarged language;
- a lot of skin on the neck, gathering in folds;
- clinodactyly (curvature of the fingers);
- epicanthus;
- heart disease is possible in 40% of cases.
People with this syndrome begin to walk more slowly, pronounce words. And it’s also harder to learn than other children of the same age.
Nevertheless, they are capable of fruitful work in society and with some support and proper work with such children in the future they are well socialized.
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, he studied this anomaly and found out the causes of a genetic failure Klaus Patau, although before him in 1657 he described T. Bartolini's syndrome. The risk of such disorders is increased in those women who give birth after 31 years.
In such children, numerous physical defects are combined with a severe violation of the development of psychomotor. Characteristic for the syndrome:
- microcephaly;
- abnormal hands, extra fingers often form;
- low set ears of irregular shape;
- cleft lip;
- short neck;
- narrow eyes;
- clearly "sunken" nose bridge;
- malformations of the kidneys and heart;
- cleft lip or palate;
- during pregnancy, there is only one umbilical cord artery.
A small number of surviving infants receive medical care. And they are able to live a long time. But congenital anomalies still affect the nature of life and its short duration.
Edwards Syndrome
Trisomy of chromosome 18 against the background of translocation leads to Edwards syndrome. This syndrome is less known. With this diagnosis, the child barely survives to six months. The law of natural selection will not allow a creature with numerous deviations to develop.
In general, the number of various malformations in Edwards syndrome is about 150. Malformations of the blood vessels, heart, and internal organs are present. Cerebellar hypoplasia is always present in such newborns . Possible anomalies in the structure of the fingers. Very often, such a distinctive anomaly is manifested as deformation of the foot.
What tests determine abnormalities during fetal development?
To conduct an analysis on the fetal karyotype, it is necessary to obtain material - fetal cells.
There are several analyzes. We highlight how this all happens.
1. Biopsy of chorionic villi. The analysis is carried out at 10 weeks. These villi are a direct particle of the placenta. This particle of biological material will tell everything about the future fetus.
2. Amniocentesis. Using a needle, several fetal cells and amniotic fluid are taken. They are most often taken at 16 weeks of gestation, and after a few weeks the couple can receive detailed information about the well-being of the baby.
Mothers who have an increased risk of having a baby with an abnormality are sent for such an analysis. Typically, pairs in which:
1) there were unreasonable miscarriages;
2) the couple could not conceive a child for a long time;
3) there were closely related ties in the family.
Such young people may have Robertson translocations of some kind of chromosome. And therefore, they must make an analysis in advance on their karyotype in order to know what are the chances to endure and give birth to a healthy child.