B. Types of Down Syndrome
- Down syndrome can typically occur in 3 ways:
- Trisomy 21: In this type, the person has three copies of chromosome 21, instead of two copies. This happens due to the abnormal cell division during the development of the sperm cell or the egg cell. Trisomy 21 is the cause of Down syndrome 95% of the time.
- Mosaic Down syndrome: This is a rare form of Down syndrome, where the individual has an extra copy of chromosome 21 only in some cells. Abnormal cell division after fertilization causes this mosaic of normal and abnormal cells.
- Translocation Down syndrome: In this type, Down syndrome occurs when a portion of chromosome 21 becomes attached (translocated) onto another chromosome. These individuals have the two copies of chromosome 21 as usual, but they also have some extra genetic material from chromosome 21 attached to a different chromosome.