Down Syndrome is a genetic variation of the chromosome set. Instead of the usual 23 pairs of chromosomes in each human cell, people with Down Syndrome harbor an extra chromosome.
Chromosome 21 is present in triplicate (Trisomy 21).
There are 3 different forms of Down Syndrome.
The most common is the free trisomy (described above). This is present in 95 % of all cases.
In fewer cases Down Syndrome occurs as a translocation trisomy or mosaic trisomy.
The translocation trisomy results either spontaneously during the division of the egg or sperm cell, or is inherited via mother or father. In those cases the chromosome 21 is also present three times, but one of them is attached to another chromosome.
If chromosome 21 is not present in triplicates in all body cells, which means there are some cells with normal chromosome sets, we are speaking from a mosaic trisomy.
Statistics about whether a child is born with Down Syndrome or not say that the risk increases with maternal age.
A chromosome analysis is used to diagnose Down Syndrome.
Characteristics before birth:
cardiac defect, growth disorders, microcephaly, long eye relief, short femur, sandals gap, intestinal obstruction, large amount of amniotic fluid.
Doctors can analyze the chromosomes through amniocentesis or chorionic villi biopsy. These are invasive procedures which are associated with risks.
Characteristics after birth:
brushfield spots (discoloration of the iris), sickle-shaped fold of the skin at the corners of the eyes, third fontanelle, hypotonia, sandals gap, simian crease.
The life expectancy of people with Down Syndrome is almost as high as that of other people.