Question: What is (are) Trisomy 17 mosaicism ?
Answer: Trisomy 17 mosaicism is a chromosomal abnormality in which there are three copies of chromosome 17 in some cells of the body, rather than the usual two copies. Trisomy 17 mosaicism is one of the rarest trisomies in humans. It is often incorrectly called trisomy 17 (also referred to as full trisomy 17), which is when three copies of chromosome 17 are present in all cells of the body. Full trisomy 17 has never been reported in a living individual in the medical literature. Few cases of trisomy 17 mosaicism have been described, most having been detected during pregnancy through a test called amniocentesis. Only a few individuals have had a confirmed diagnosis of trisomy 17 mosaicism after birth. Because the proportion and location of cells with trisomy 17 differs from case to case, the presence and severity of signs and symptoms may vary significantly from person to person.
Source: GARD

Question: What causes Trisomy 17 mosaicism ?
Answer: What causes trisomy 17 mosaicism? Trisomy 17 mosaicism can arise due to errors in cell division that occur after conception. For example, at the time of conception, the fetus may actually have trisomy 17 in all of its cells; however, during cell division, some of the cells lose the extra chromosome 17. Alternatively, the fetus may initially have had only two copies of chromosome 17, but due to errors in cell division some of the cells end up with an extra copy of chromosome 17. Either of these two scenarios result in trisomy 17 mosaicism. To read more about trisomy mosaicism, visit the following links from the Medical Genetics Department at the University of British Columbia in Canada. What is mosaicism? How does trisomy mosaicism occur?
Source: GARD