Chromosomal examination (karyotype)
What are chromosomes?
- chromosomes are blocks of genes of typical shape and number which are located in nuclei of all cells
- humans have 23 pairs of chromosomes = 46 chromosomes
- one member of the pair comes from the mother, the other one from the father
- mature sex cells - eggs and sperm - have half the number of chromosomes, which are paired at conception
Depending on the size and characteristic lines, we can divide chromosomes into a set - karyotype. Women have two sex chromosomes X, whereas men have one sex chromosome X and one sex chromosome Y. Normal female karyotype is written down as 46, XX and a normal male karyotype as 46, XY.
Chromosomal defects (aberrations)
Chromosomal aberrations are deviations in the number or shape of chromosomes. They usually present a risk of serious changes in genetic structure of the carriers or their children. These changes in genetic structure affect the function of multiple organs. Currently there is no specific prevention or targeted treatment of chromosomal defects and life expectancy of the concerned patients is often unsatisfactory.
Numerical chromosomal abnormality (aneuploidy)
- occurs when a chromosomal “trio” emerges at conception in each cell instead of a pair of chromosomes
- means that the person in question has in total 47 chromosomes in each cell instead of 46
- trisomy mainly emerges randomly at conception
- trisomy is very frequent in the following chromosomes:
- trisomy of chromosome 21 is the cause of Down syndrome
- trisomy of chromosome 18 is the cause of Edwards syndrome
- trisomy of chromosome 13 is the cause of Patau syndrome
- the risk of trisomy rises with maternal age
Structural chromosomal aberrations
- are present only in parts of chromosomes
- with loss (deletion) of genetic information
- with excess (duplication) of genetic information
Chromosomal aberrations can be inherited from the mother (maternal), from the father (paternal), or newly emerging (de novo). Aneuploidy mainly emerges incidentally at conception and its risk increases with maternal age. While some aneuploid embryos can develop until the birth of the child (e.g. Down syndrome), the majority of aneuploid pregnancies end in spontaneous miscarriage.
Some chromosomal aberrations do not always present changes in genetic makeup of cells - the aberration is balanced and its carrier is mostly healthy. However, there is a risk that carriers have an unbalanced part of their sex cells (eggs for women, sperm for men), and it can then manifest itself as infertility or recurrent miscarriages. For this reason, karyotype examination of both partners is a basic preconception genetic testing.
What are the precautions when chromosomal aberration is found in future parents?
In cases of significant risk of chromosomal defects for the future baby and/or risk of pregnancy failure we recommend assisted conception (IVF) and selection of chromosomally balanced embryos that are suitable for transfer into the uterus (preimplantation diagnosis).