Trisomy 8 is a genetic disorder where three copies of chromosome 8 exist in every cell of an individual. You can review my earlier general trisomy post to refresh your memory on how that can occur. Recall that the chromosomes are numbered from largest (chromosome 1) to smallest. The larger a chromosome is, the more genetic information - instructions for cell development and function - it contains. Chromosome 8 is not the largest, but still contains a significant amount of DNA. Generally with three copies of chromosome 8 in each cell, too much damage is present for viability beyond early fetal development. Statistically Trisomy 8 is responsible for about 0.7% of miscarriages.
Some individuals have a different form of Trisomy 8 called mosaic syndrome, or T8mS. In T8mS only some of the person's cells have three copies of chromosome 8. The person may have cells in one type of tissue (e.g. blood or muscle) that have two copies of chromosome 8 and cells in another tissue (e.g. skin or eye) that have three copies. T8mS results from non-disjunction occurring during the process of mitosis (cell-division) rather than during meiosis (fertilization). Depending on when the mitosis goes awry there may be only a few effects, or several. However, the percentage of cells affected does not apparently correlate with the severity or which symptoms will appear.
Several distinct physical features are potential indicators of T8mS, including: low-set or abnormal ear shape, bulbed nose, strabismus (eye turns in), structural heart problems, palate abnormalities, and deep hand and feet creases. T8mS can also cause cognitive impairment and delays that may be mild or moderate. Treatment depends greatly on the symptoms of each individual. Heart problems can usually be corrected with surgery as can cleft palate. Strabismus can be treated with an eye-patch therapy or surgery.
More information on Trisomy 8 can be found here. During my research for this post, I also ran across the website for a support group called Unique. They distribute detailed information and provide support for individuals with T8mS and other genetic disorders.
