CJ Almacen
5/14/2014
Biology 101

Trisomy 21
According to genetics, the cause of Trisomy 21 is when three copies are made, rather than the usual two copies of genes on chromosome 21. Typically, parents of an individual affected with this genetic disorder are genetically normal. The chances of parents with one child affected with Down syndrome and having a second child with the syndrome as well are at 1% risk, if both parents are determined to have normal karyotypes. In Trisomy 21, there are three genetic variations of Trisomy 21 mechanisms that can occur. The first and most common (about 90% of Down syndrome cases), is known as non-disjunction, in which an extra chromosome (chromosome 21) originates in the development of either the sperm or the egg. When the sperm and the egg combine to create the fertilized egg, three (instead of two) chromosomes 21 are present. As the cell divides, the chromosome is repeated in every cell. The second genetic mechanism is known as mosaic Trisomy 21. This variation is a rare form and less than 2% of Down syndrome cases. Though there are similarities with non-disjunction, individuals with mosaic Trisomy 21 differ due to the extra chromosome 21 being present in some of the cells, not all of them. This type of mechanism occurs during abnormal cell division after fertilization. The name “mosaic” originates from a randomized order of normal and abnormal cells. During cellular mosaicism, the random combination of can be visible in different cells of the same type; while with tissue