Types of Down syndrome
• Trisomy 21: Occurring in about 95% of people with Down syndrome, it is caused due to an error in cell division before or at the time of conception. In this type a pair of the 21st chromosome does not separate properly in the egg or sperm and this extra chromosome is then found in every cell of the body.
• Mosaicism: The error in separation of the 21st chromosome occurs for a few cell divisions after fertilization due to which some cells have 47 chromosomes and others have 46.The extent of physical issues depends on the ratio of the presence of these two kinds of cells. It occurs in 1-2% of all people with Down syndrome.
• Translocation: In this type, a part of chromosome 21 breaks off and attaches itself to another chromosome (often chromosome 14) and all cells in the body have this extra piece of chromosome 21.It occurs in 3-4% of babies born with Down’s syndrome.
Karyotyping is the test used to identify which type of Down’s syndrome is present in the individual.