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Trisomy 21 or Down syndrome is a chromosomal abnormality that manifests itself in a combination of mental disability and physical malformations of varying degrees and severity. The cause lies in a peculiarity of the genetic makeup of the affected person (genome mutation, chromosomal aberration or aneuploidy). Chromosome 21 (chromosomes are components of cells on which genetic information is stored) or parts of it are present three times instead of twice. This chromosomal disorder is therefore called trisomy 21. Chromosome triplicates can occur when cell division occurs in such a way that two chromosomes of the same number end up in the germ cell instead of one. The fertilized egg then has a total of three chromosomes (since another chromosome has been added): one each from the mother and father and an additional one from either the mother or the father. Support powered by #sp3cialeffects