What are chromosomes?
Every human being has 46 chromosomes in each body cell, on which all hereditary information is stored. Two of them, X and Y, are the sex chromosomes. These 46 chromosomes consist of 23 pairs of chromosomes. The sex chromosome in girls is designated 46XX because it has two X chromosomes. Boys have one X and one Y chromosome each, so the hereditary units are abbreviated as 46XY.
Cell division and fertilization
As the only cells in the body, the gametes (eggs, sperm) have half the chromosome set (23 chromosomes): Oocytes harbor the 23X set of chromosomes in their nucleus, and the nucleus of sperm cells has either the 23X or 23Y set of chromosomes.
A healthy child can develop if there are no abnormalities in the chromosomes in the mother and father and the fusion and subsequent cell divisions occur normally.
Chromosome defects
There are a number of ways in which deviations from the normal chromosome order (called chromosome aberrations) can occur. Such errors can occur during any cell division, including the formation of the egg and sperm cells, as well as during the fusion of the egg and sperm cells during fertilization.
For example, during cell division, chromosomes may be incorrectly distributed among daughter cells. In the context of reproduction, this means that the number of chromosomes in the child is reduced or increased. The shape of individual chromosomes can also change, for example, a part can break off (structural chromosome defects).
Children with defective hereditary disposition
- The best known chromosomal abnormality is trisomy 21 (Down syndrome). Trisomy 21 means that chromosome number 21 is present three times instead of only twice. Affected children are inhibited in their development and show malformations (such as heart defects) more often than other children.
- Children with other trisomies (for example, three chromosomes number 13 or 18) are either not viable at all or have a significantly shorter life expectancy than healthy children and suffer from severe disabilities.
- If a fetus has the hereditary disposition 45X0, this means that it is missing one sex chromosome. A girl then develops with the so-called Turner syndrome (Ullrich-Turner syndrome). Women with this hereditary disease are short in stature, have a widened neck (pterygium colli) and cannot have children. However, they are usually normally gifted.
Examinations and diagnostics of chromosomal anomalies
- Amniocentesis. The amniotic fluid taken with a fine hollow needle contains cells of the child that can be examined. Chemical analysis also provides evidence of chromosomal disease.
- Placental examination (chorionic villus sampling) from the placenta (CVS)
- Umbilical cord puncture: ultrasound-controlled blood sampling from the umbilical cord and examination of the chromosome set of the fetal blood cells
- Direct fetal puncture: direct sampling of fetal somatic cells or puncture of the urinary bladder under ultrasound control
- Blood tests for chromosomal abnormalities: They can detect chromosomal abnormalities in the unborn child with a high, but not one hundred percent certainty. However, false-positive and false-negative findings are possible, which must then be further clarified by the above-mentioned examinations.
