What is Down syndrome?
Down syndrome is a genetic condition that results when a person is born with three rather than the usual two copies of chromosome 21. Because there are three copies of chromosome 21, Down syndrome is also called “Trisomy 21.”
Are there different types of Down syndrome?
Yes, there are three types: Trisomy 21, occurs in 95% of cases when there are three, rather than two, number 21 chromosomes present in every cell of the body. Instead of the usual 46 chromosomes, a person with Trisomy 21 has 47. Translocation accounts for 4% of all cases. In translocation, part of chromosome 21 breaks off during cell division and attaches to another chromosome, typically chromosome 14. While the total number of chromosomes in the cells remain 46, the presence of an extra part of chromosome 21 causes the characteristics of Down syndrome. Mosaicism is rare and accounts for 1% of all cases. In mosaicism, nondisjunction of chromosome 21 takes place in one – but not all – of the initial cell divisions after fertilization. When this occurs, there is a mixture of two types of cells, some containing the usual 46 chromosomes and others containing 47.
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