The human body consists of a billion cells equipped with 23 pairs of chromosomes and the genes found in them. A full set of chromosomes creates a karyotype. Unfortunately, it happens that the chromosome system is disturbed – it comes to the so-called translocation.
The human body consists of a billion cells equipped with 23 pairs of chromosomes and the genes found in them. A full set of chromosomes creates a karyotype. Unfortunately, it happens that the chromosome system is disturbed – it comes to the so-called translocation, which, although invisible at first glance, can stand in the way of desired parenthood. Read this article and find out what chromosomal translocations are and how they can affect human fertility.
No visible symptoms in the karyotype
Such translocation may not change the external appearance (phenotype) or affect the life expectancy of the carrier. Patient usually learns about its existence only when, despite attempts, cannot conceive a child or is struggling with the problem of habitual miscarriages. Where exactly do chromosomal translocations come from?
So far this has not been fully explained. Chromosomes can crack. Then their fragment breaks and moves, and then attach to another chromosome. Translocation can occur at the stage of egg and sperm formation or during the first division of the embryo cells in the first days of pregnancy. It is worth remembering that such irregularities occur regardless of our will. Their presence is a coincidence. Therefore, none of the partners should blame themselves for this condition. There are several translocation divisions. They are shared among others for balanced and unbalanced translocations.
Balanced and unbalanced translocation – what are they?
A balanced translocation occurs when two fragments of two different chromosomes break off and swap places. No genes are lost during this exchange. The amount of genetic material remains normal, only its distribution changes.
Unbalanced translocation is a different type of translocation. It is one in which the amount of genetic material changes during such exchange. A fragment of one chromosome is missing or is added to another chromosome. These types of changes are most often visible in the external appearance of the host and may cause disease symptoms.
Robertsonian translocation
A unique example of unbalanced translocation is the Robertsonian translocation that occurs between chromosomes 13, 14, 15, 21 and 22. It consists in the fact that the chromosome from one pair is almost completely attached to the chromosome from another pair, a small number of genes are lost, copies of which occur also on other chromosomes. This is the most common type of translocation in humans and, as in the case of balanced translocations, it does not change the external appearance and does not cause disease symptoms.
Interestingly, a couple in which one parent has a balanced translocation may have a child with an unbalanced translocation. In addition, such a translocation may occur in a child despite the normal karyotypes of both parents. We say about such translocation that it was created “de novo”, and thus as a completely new mutation.
Balanced translocation as a cause of habitual miscarriages?
Scientific studies show that among couples struggling with habitual miscarriages, between 3 and 6% of partners are carriers of chromosomal translocations. Most often they are Robertsonian translocations, but also balanced translocations, inversions and mosaicism. Couples affected by these abnormalities are more likely to have early miscarriage, stillbirth, and birth of a child with congenital malformation syndrome.
Karyotype test allows to detect chromosomal translocation
Couples who are unable to become pregnant or who experience habitual miscarriages are recommended to perform cytogenetic diagnostics, so-called karyotype test. This is the basic research used in genetic diagnostics. It evaluates the number and structure of individual chromosomes in the karyotype. Using cytogenetic testing, we can detect both chromosome number abnormalities, e.g., monosomes or trisomies, and their structures – e.g., deletions, inversions or just translocations. From the patients’ side, this is a fairly simple test – a small amount of peripheral blood is taken into it.
Expecting a baby? You can check the chromosomes now!
Parents who carry balanced chromosomal translocation are often afraid that they will pass it on to their child. These types of irregularities are indeed hereditary and can be passed on to future generations. However, this is not the rule. The child of such parents may have completely normal chromosomes, inherit a balanced translocation from the parent, or be born with an unbalanced translocation, which may, unfortunately, negatively affect its development.
Fortunately, parents can learn about the presence of a translocation in a child before his or her birth. A test that allows this is a trophoblast biopsy (chorionic villus sampling). Normally, this test is performed between 11 and 13 weeks of pregnancy and involves collecting a fragment of the chorion with a thin biopsy needle. The whole procedure takes place in the presence of a specialist and under the control of an ultrasound machine. The acquired material is subjected to genetic analysis in the laboratory.
Another test that allows assessing a child’s chromosomes at the prenatal stage is amniocentesis. This test is usually performed around the 15th to 20th week of pregnancy. It involves puncturing the fetal bladder with a thin needle and collecting a small amount of fluid surrounding the fetus (amniotic fluid). This fluid contains fetal cells from which its genetic material can be obtained, and then the chromosome system can be assessed.
Preimplantation diagnosis, cell adoption – alternative methods of family planning
Couples whose genetic abnormalities cannot get pregnant naturally can help themselves by using the in vitro procedure. The treatment can be preceded by Preimplantation Genetic Diagnosis (PGD). Thanks to it, laboratory diagnostics are able to select embryos without genetic disorders, including chromosome rearrangements such as translocations. Thanks to preimplantation diagnosis, parents have the opportunity to eliminate the risk of passing on the genetic change to the child.
Couples who, due to their health condition, cannot use their own reproductive cells for the in vitro procedure can use the cells of an anonymous male or female donor (depending on whether the fertility disorders are caused by a male or female factor).
