What are the different types of Down syndrome?
People with Down syndrome have an extra chromosome 21. They have three copies of chromosome 21 instead of two copies, so Down syndrome is sometimes called “Trisomy 21”. About 95% of the time, the extra copy is due to an error made during the formation of egg or sperm. We have 46 chromosomes that come in 23 pairs. The process of separating the pairs of chromosomes at the time of egg or sperm formation is a delicate process. Sometimes an error occurs and the chromosome pair does not separate the right way and both of the chromosomes of a pair moves to the same cell. This error is called nondisjunction and leads to having either too many or too few chromosomes in the resulting egg or sperm. This kind of error can happen for any chromosome. But most of the time, a fetus that carries that extra chromosome cannot survive. This means that this type of error is the leading cause of pregnancy loss. Sometimes this error occurs right after the formation of the fetus. In this case, some cells have
