How does trisomy 21 occur?
Trisomy 21 is a disease caused by the triple presence of the 21st chromosome. In healthy cells the chromosomes are duplicated, so that the human being has a total of 46 chromosomes. A patient with trisomy 21 has 47 chromosomes and suffers from Down syndrome.
The triple presence of the 21st chromosome is in most cases caused by a defect in meiosis. Meiosis turns a cell with 4 chromatids into four germ cells with one chromatid each by two divisions. However, errors can occur in both maturation divisions.
As a result, the chromatids are not distributed correctly among the germ cells and germ cells with two chromatids follow. These germ cells have duplicated individual chromosomes, in Down syndrome the 21st chromosome. If such an egg fuses with a healthy sperm, the embryo has the 21st chromosome in three copies.
The symptoms of this numerical chromosomal aberration are many and varied and can be of varying degrees of severity. Such children are often mentally retarded, undergrown and have congenital heart defects. The risk of disease increases with the age of the mother, so that many older pregnant women prefer to have an amniocentesis.
