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Types of Down Syndrome

    One may wonder how this condition came to be referred to as Down syndrome. It was named after John Langdon Down. He was a doctor who first describer the condition in 1866. There are three types of Down syndrome namely, trisomy 21, translocation and mosaicism.

    Trisomy 21 ( Nondisjunction)

    The term trisomy refers to three. This is the most common type with up to 95% of babies being born with the condition. It occurs when there are three number 21 chromosomes present in every cell. A normal cell should have two. You see, nondisjunction simply means the failure of a pair of homologous/similar chromosome to separate.

    There are various forms of trisomy e.g. trisomy 13 and trisomy 18. This simply means that there are three copies of the chromosomes number 13 and 18.


    This is the only type of Down syndrome that maybe inherited from the parents. This type of Down syndrome occurs when chromosomal material is rearranged and the extra chromosome 21 may be attached in the egg/ sperm. In some cases, two  number 21 chromosome can be attached to one another. 3-4% of children have translocation Down syndrome

    Additionaly, a person has rearranged chromosomal material, and there is no extra or missing chromosomal material, then they have “balanced translocation”. These individuals are called “balanced translocation carriers”. Parents with balanced translocation may have fertility problems, miscarriages or have higher chances of having a child with health problems. They are at a greater risk of donating too much or too little genetic material to a child.

    When a parent has translocation, it is likely that their relatives may have inherited the translocation. It is advisable that they have their chromosomes studied.

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    We have seen in Trisomy 21, all cells have an extra copy of the chromosome number 21. In Mosaic Down Syndrome, there is a mixture of cells. Some cells will have a normal pair of chromosome 21. This means that the cells have two chromosomes. The other cells will have three copies. In short, some cells have the correct number of chromosomes while others have more. According to www.healthline.com children the Mosaic Down Syndrome have a higher IQ as compared to children with other forms if Down Syndrome. One symptom that I have found exclusive to Mosaic DS is having white spots in the iris of the eye. This I didn’t know. Important to note is, symptoms will vary from one individual to another. The degree will also vary.

    I hadn’t interacted with this type of Down Syndrome and translocation until when I decided to talk about the condition. Anyway, they say learning is a continuous process. There is no cure for Down syndrome but there are various management therapies that are helpful. Children with DS may be in be in need of physical, occupational and speech therapy to help with development.

    Sources: Children’s hospital Philadelphia



    Stanford children’s health


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