Down syndrome, a congenital birth defect, occurs when an error in cell division causes the fetus to have three copies of chromosome 21 instead of two. Down syndrome, also called trisomy 21, causes mental retardation and specific physical characteristics. In 95 percent of people with Down syndrome, all the cells in the body have three copies of chromosome 21. In 1 to 2 percent of people, only some cells have three copies of the chromosome while others have the normal number, The Children's Hospital of Philadelphia states.
Causes
A child normally receives half his chromosomes, 23, from each parent, for a total of 46. Children with Down syndrome have an extra copy of chromosome 21 because of an error in cell replication and division at fertilization. Mosaicism occurs when some of the early cells have 47 chromosomes but other have the normal number of 46. As the cells continue to divide, all the cell lines that develop from the cell with 46 chromosomes will have the normal number of chromosomes, while all the cell lines that develop from the abnormal chromosome will be abnormal.
Diagnosis
Mosaicism is diagnosed by analyzing between 20 and 25 blood cells from a baby with Down syndrome, pediatrician Len Leshin, M.D., of Corpus Christi Texas explains. If some of the cells have 46 chromosomes while others have 47, mosaicism is diagnosed. The degree of mosaicism is usually expressed as a percentage; if 10 out of 20 cells are abnormal, the person has 50 percent mosaicism. A blood test examines only blood cells. Mosaicism may occur at a different percentage in other parts of the body, and may affect some cell lines and not others.
Symptoms
Children with mosaicism might not have the typical facial features and physical characteristics of Down syndrome, depending on which cell lines are affected and the percentage of mosaicism. Children with mosaicism had slightly higher IQs than other Down syndrome children in some studies, according to Down Syndrome Online, but because there was an overlap of scores in both groups, researchers did not make broad generalizations about IQ and developmental abilities. Some children with non-mosaicism will score higher than some children with mosaicism. Children with non-mosaicism and mosaicism had similar speech delays, lead author Sue Buckley reported in "Down Syndrome News and Update" in 2002.
Treatment
There's no treatment for mosaicism.
Considerations
Some children with mosaicism don't physically appear to have Down syndrome. Parents must carefully consider whether to tell other people, including schools, that their child has Down syndrome. Children with mosaicism still have developmental delays common in Down syndrome, albeit to a lesser degree in some cases. They may also have the same risk for cardiac and gastrointestinal problems that often affect Down syndrome children.
