There are three different types of Down Syndrome:
Standard Trisomy 21: This is also known as “nondisjunction.“ The extra chromosome 21 comes from either the egg or sperm cell. This is basically an error in cell division. Between 90% and 95% of all Down Syndrome is Standard Trisomy 21.
Translocation Trisomy 21: This form of Down syndrome is obviously close to my heart due to the fact that Austin has Translocation Trisomy 21. Translocation Trisomy 21 happens when a piece of chromosome 21 is located on another chromosome. The person with Translocation Trisomy 21 will have 46 chromosomes but will have the genetic material of 47 chromosomes. The person with Translocation Trisomy 21 will exhibit all the same characteristics of a person with Standard Trisomy 21 since they have three copies of chromosome 21. Translocation occurs between 3% and 5% of cases of Down Syndrome. This is the ONLY form of Down syndrome that his hereditary…which means it can be passed on to a child from a parent. A balanced translocation occurs when two pieces of chromosomes break off and switch places with each other. If all the chromosomal material is present, just rearranged – that is, switched places (translocated) - this person is "clinically normal" since all the chromosomal material needed is present. There is no way to tell whether or not a person has one of these rearrangements unless you look at his or her blood to examine the chromosomes. In our case, my husband is a balanced carrier.
Mosaic Trisomy 21: This is when a person has a mix of cells, some containing 46 chromosomes and some containing 47 chromosomes. This occurs either because:
The person received 46 chromosomes at fertilization but somewhere during early cell division the chromosome 21 cell pairs failed to split creating a cell with 47 chromosomes and a cell with 45 chromosomes. The cell with 45 chromosomes can not survive but the cell with 47 chromosomes will continue to divide. All cells that come from this cell will contain 47 chromosomes. OR The person received 47 chromosomes at fertilization but later during cell division the extra chromosome is lost. Mosaic Trisomy 21 occurs in 2% to 4% of cases of Down Syndrome. A person with Mosaic Down Syndrome may exhibit all, some, or none of the characteristics of Down Syndrome depending on the percent of cells carrying the extra chromosome and where these cells are located.