Down syndrome is caused by an additional chromosome in an individual’s genetic makeup. Chromosomes contain genes that determine how a baby’s body develops and works during and after pregnancy. Normally, an infant is born with 46 chromosomes. However, individuals with down syndrome possess an extra copy of chromosome 21. This article focuses on the types of down syndrome and how they are different from each other.
How many types of down syndrome exist?
There are three types of down syndrome as enlisted by The National Down Syndrome Society, i.e., Trisomy 21, Mosaic, and Translocation. Usually, it is difficult for people to differentiate between the types as all of them possess similar physical characteristics and behaviour patterns. However, by looking at the type of chromosome, one can differentiate between the three.
How are the three types of down syndrome different from each other?
Trisomy 21 is a rare genetic condition where an extra chromosome is present, i.e., in trisomy 21, there are three copies of chromosome 21 in every cell of the body instead of the usual two. Generally, a person inherits 23 chromosomes from each parent, for a total of 46 chromosomes. Babies with down syndrome, however, end up with three chromosomes at position 21 instead of the usual pair. This type of down syndrome accounts for 95% of the cases, as per the Down Syndrome Society.
The second type is Mosaicism. It is the least common form of down syndrome and accounts for only about 1% of all cases. According to Yale Medicine, Mosaicism is a condition where an individual possesses two or more distinct sets of cells with varying genetic compositions within their body. If the number of abnormal cells surpasses that of normal cells, it can result in an illness that can be traced from the cellular level to the affected tissues, such as the skin, brain, or other organs.
Translocation accounts for about 4% of cases of down syndrome. In this case, there are three 21 chromosomes, just like there are in trisomy 21, but one of the 21 chromosomes is attached to another chromosome instead of being separate.
The Down Syndrome Society states that although the total number of chromosomes in the cells remains constant at 46, down syndrome arises due to the presence of an extra full or partial copy of chromosome 21, which becomes attached to another chromosome, typically chromosome 14. This anomaly is responsible for the distinct characteristics associated with down syndrome.
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