WHAT ARE THE TYPES OF DOWN SYNDROME?
Down syndrome is a genetic condition in which a person has three rather than the usual two copies of chromosome 21. Because there are three copies of chromosome 21, some people call Down syndrome Trisomy 21.
THREE CATEGORIES OF DOWN SYNDROME
Categories of Down Syndrome
- Trisomy 21 – This is the most common form of Down syndrome. It occurs in 95% of cases. Trisomy 21 occurs when there are three, rather than two, number 21 chromosomes in every cell in the body. This happens prior to or at conception through a cell division process called nondisjunction. If a pair of 21st chromosomes in either the sperm or the egg fails to separate, the extra chromosome is replicated in every cell of the embryo’s body. This additional genetic material that alters the course of development and causes the traits associated with the condition.
- Translocation – This occurrence accounts for around 3%-4% of Down syndrome cases. In translocation, either an extra part or the whole extra chromosome 21 becomes attached or “trans-located” to a different chromosome. The translocation typically occurs on chromosome 14. Though the total number of chromosomes in the cell stays at 46, the extra piece of chromosome 21 alters development and causes the traits associated with the condition.
- Mosaic – Mosaicism accounts for around 1% of all Down syndrome cases. In this instance, nondisjunction of chromosome 21 happens on one — but not all — of the initial cell divisions after fertilization. As a result, there is a mixture of two types of cells: some containing the usual 46 chromosomes and others containing 47. Cells with 47 chromosomes contain the extra chromosome 21.
Down syndrome is the most frequently occurring chromosomal abnormality and shows up in approximately one in every 792 live births. The condition occurs equally among people of all ethnic, racial, religious, and socioeconomic backgrounds. Children diagnosed with Down syndrome can face other related health challenges. For example, it is not uncommon for a child with Down syndrome to experience low muscle tone that requires extra help with gross motor developmental milestones like crawling and walking.
If you need help understanding a diagnosis of Down syndrome, know that you are not alone. The Warren Center is here for you. We offer host of support services to improve the outlook of children diagnosed with the condition. For more information, contact us.