Waiting for a baby is the best time for parents, but also the worst. Many worry that the baby may be born with any disabilities, physical or mental disabilities.
Science does not stand still, it is possible to check the baby for small developmental abnormalities. Almost all of these tests can show if everything is normal with the child.
Why does it happen that the same parents can have completely different children - a healthy child and a child with disabilities? This is determined by genes. In the birth of an underdeveloped toddler or a child with physical disabilities, gene mutations associated with a change in the structure of DNA influence. Let's talk about this in more detail. Consider how this happens, what gene mutations are, and their causes.
What are mutations?
Mutations are a physiological and biological change in cells in the structure of DNA. The reason may be radiation (during pregnancy, you can not take x-ray photographs, for injuries and fractures), ultraviolet rays (long exposure to the sun during pregnancy or being in a room with UV lamps on). Also, such mutations can be inherited from their ancestors. All of them are divided into types.
Gene mutations with a change in the structure of chromosomes or their number
Chromosomal mutations are mutations in which the structure and number of chromosomes are changed. Chromosomal regions can fall out or double, move into a non-homologous zone, and turn one hundred eighty degrees from the norm.
The reasons for the appearance of such a mutation is a violation during cross -over.
Gene mutations are associated with changes in the structure of chromosomes or their number, are the cause of serious disorders and diseases in the baby. Such diseases are incurable.
Types of chromosomal mutations
In total, two types of basic chromosomal mutations are distinguished: numerical and structural. Aneuploidies are species by the number of chromosomes, that is, when gene mutations are associated with a change in the number of chromosomes. This is the occurrence of an additional or several of the latter, the loss of any of them.
Gene mutations are associated with structural changes when chromosomes break apart, and then reunite, disrupting the normal configuration.
Types of Numerical Chromosomes
According to the number of chromosomes, mutations are divided into aneuploidy, that is, species. Consider the main, find out the difference.
Trisomy is the occurrence of an extra chromosome in the karyotype. The most common occurrence is the appearance of the twenty-first chromosome. It becomes the cause of Down syndrome, or, as they also call this disease, the trisomy of the twenty-first chromosome.
Patau syndrome is detected on the thirteenth, and on the eighteenth chromosome , Edwards syndrome is diagnosed . These are all autosomal trisomies. Other trisomies are not viable, they die in the womb and are lost in spontaneous abortions. Those individuals who have additional sex chromosomes (X, Y) are viable. The clinical manifestation of such mutations is very small.
Gene mutations associated with a change in the number arise for certain reasons. Trisomies can most often occur when chromosomes are homologous in anaphase (meiosis 1). The result of this discrepancy is that both chromosomes enter only one of the two daughter cells, the second remains empty.
Less often, nondisjunction of chromosomes may occur. This phenomenon is called a violation in the divergence of sister chromatids. It occurs in meiosis 2. This is precisely the case when two completely identical chromosomes settle in the same gamete, causing a trisome zygote. Nondisjunction occurs in the early stages of the egg crushing process, which has been fertilized. Thus, a clone of mutant cells arises that can span a larger or smaller portion of tissue. Sometimes clinically manifested.
Many associate the twenty-first chromosome with the age of a pregnant woman, but this factor to this day has not been unequivocally confirmed. The reasons why the chromosomes do not diverge remain unknown.
Monosomy is the absence of any of the autosomes. If this happens, then in most cases the fetus cannot be taken out, premature birth occurs in the early stages. The exception is monosomy due to the twenty-first chromosome. The reason why monosomy occurs may be non-divergence of chromosomes, and the loss of the chromosome during its path in anaphase to the cell.
By sex chromosomes, monosomy leads to the formation of a fetus with a karyotype XO. The clinical manifestation of such a karyotype is Turner syndrome. In eighty percent of cases out of a hundred, the appearance of monosomy on the X chromosome occurs due to a violation of the father’s meiosis. This is due to the nondisjunction of the X and Y chromosomes. Basically, a fetus with a karyotype of XO dies in the womb.
According to sex chromosomes, trisomy is divided into three types: 47 XXY, 47 XXX, 47 XYY. Klinefelter's syndrome is trisomy 47 XXY. With such a karyotype, the odds of having a baby are divided into fifty to fifty. The cause of this syndrome may be non-divergence of chromosomes X or non-divergence of X and Y spermatogenesis. The second and third karyotypes can occur only in one out of a thousand pregnant women, they practically do not appear and in most cases are discovered by experts quite by accident.
These are gene mutations associated with a change in the haploid set of chromosomes. These sets can be tripled and quadrupled. Triploidy is most often diagnosed only when spontaneous abortion has occurred. There were several cases when the mother managed to bear such a baby, but they all died before reaching the age of one month. The mechanisms of gene mutations in the case of triploid cause a complete discrepancy and nondisjunction of all chromosome sets of either female or male germ cells. Also, double fertilization of one egg can serve as a mechanism. In this case, degeneration of the placenta occurs. This rebirth is called cystic drift. As a rule, such changes lead to the development of mental and physiological disorders in the baby, and termination of pregnancy.
What gene mutations are associated with a change in the structure of chromosomes
Structural changes in chromosomes are the result of rupture (destruction) of the chromosome. As a result, these chromosomes join, disrupting their former appearance. These modifications may be unbalanced and balanced. Balanced do not have an excess or lack of material, therefore, do not appear. They can appear only in those cases if there was a gene at the site of chromosome destruction that is functionally important. A balanced set may have unbalanced gametes. As a result, fertilization of an egg with such a gamete can cause the appearance of a fetus with an unbalanced chromosome set. With this set, the fetus has a number of malformations, severe types of pathology appear.
Types of structural modifications
Gene mutations occur at the level of gamete formation. This process cannot be prevented, nor can it be known that such mutations can occur. There are several types of structural modifications.
This change is due to the loss of part of the chromosome. After such a rupture, the chromosome becomes shorter, and its separated part is lost during further cell division. Interstitial deletions - this is the case when one chromosome breaks at once in several places. Such chromosomes usually create an unviable fetus. But there are cases when the kids survived, but because of this set of chromosomes they had Wolf-Hirschhorn syndrome, "cat scream."
These gene mutations occur at the level of organization of double sections of DNA. Basically, duplication can not cause such pathologies that cause deletions.
Translocation occurs due to the transfer of genetic material from one chromosome to another. If a gap occurs simultaneously in several chromosomes and they exchange segments, this becomes the reason for the occurrence of reciprocal translocation. The karyotype of such a translocation has a total of forty-six chromosomes. The translocation itself is detected only by a detailed analysis and study of the chromosome.
Changing the nucleotide sequence
Gene mutations are associated with a change in the sequence of nucleotides when expressed in a modification of the structures of certain DNA regions. According to the consequences, such mutations are divided into two types - without a shift in the reading frame and with a shift. In order to know exactly the causes of changes in DNA, you need to consider each type separately.
Frameless mutation
These gene mutations are associated with a change and replacement of nucleotide pairs in the DNA structure. With such substitutions, the length of the DNA is not lost, but the loss and replacement of amino acids is possible. It is likely that the protein structure will be preserved, this will be the degeneracy of the genetic code. Let us consider in detail both development options: with and without amino acid substitution.
Amino Acid Substitution Mutation
Replacement of the amino acid residue in the composition of the polypeptides is called missense mutations. There are four chains in the human hemoglobin molecule - two "a" (it is located on the sixteenth chromosome) and two "b" (encoding on the eleventh chromosome). If "b" is a normal chain, and it contains one hundred forty-six amino acid residues, and the sixth is glutamine, then hemoglobin will be normal. In this case, glutamic acid should be encoded with a GAA triplet. If, due to the GAA mutation, it was replaced by GTA, then instead of glutamic acid, valine forms in the hemoglobin molecule. Thus, instead of normal HbA hemoglobin, another HbS hemoglobin will appear. Thus, the replacement of one amino acid and one nucleotide will cause a serious serious illness - sickle cell anemia.
This disease manifests itself in the fact that red blood cells become in shape like a sickle. In this form, they are not able to deliver oxygen normally. If at the cellular level, homozygotes have the HbS / HbS formula, then this leads to the death of the child in early childhood. If the formula is HbA / HbS, then the red blood cells have a weak form of change. Such a slight change has a beneficial quality - the body's resistance to malaria. In those countries where there is a danger of contracting malaria the same as the common cold in Siberia, this change has a useful quality.
Amino Acid Mutation
Nucleotide substitutions without amino acid exchange are called seismic mutations. If in the region of DNA encoding the “b” chain the GAA is replaced by the GAG, then due to the fact that the genetic code is in excess, glutamic acid cannot be replaced. The structure of the chain will not be changed, there will be no modifications in red blood cells.
Frame Shift Mutations
Such gene mutations are associated with changes in DNA length. The length may become shorter or longer, depending on the loss or addition of nucleotide pairs. Thus, the entire structure of the protein will be completely changed.
Intragenic suppression may occur. This phenomenon occurs when there is a place for two mutations that cancel each other out. This is the moment the nucleotide pair joins after one has been lost, and vice versa.
Nonsense mutations
This is a special group of mutations. It rarely occurs; in her case, stop codons appear. This can happen both with the loss of nucleotide pairs and with their addition. When stop codons appear, the synthesis of polypeptides is completely stopped. So null alleles can form. None of the proteins will correspond to this.
There is such a thing as intergenic suppression. This is a phenomenon when a mutation in some genes suppresses mutations in others.
Are changes detected during pregnancy?
Gene mutations associated with changes in the number of chromosomes can be determined in most cases. To find out if the fetus has defects in development and pathology, a screening is prescribed in the first weeks of pregnancy (from ten to thirteen weeks). This is a series of simple examinations: sampling for blood tests from a finger and vein, ultrasound. On ultrasound, the fetus is examined in accordance with the parameters of all limbs, nose and head. These parameters, in case of strong non-compliance with the norms, indicate that the baby has developmental defects. This diagnosis is confirmed or refuted based on the results of a blood test.
Also, under the close supervision of doctors are future mothers, whose babies may experience mutations at the gene level, which are inherited. That is, these are the women in whose relatives there were cases of the birth of a child with mental or physical disabilities identified by Down syndrome, Patau and other genetic diseases.