What Is “Trisomy”?

The best person to answer a question about Down syndrome is a person with Down syndrome. Adam answers this one.

 

This question is a misnomer, “trisomy” just means three chromosomes. Down syndrome occurs when there is a trisomy of chromosome number 21.

There are 3 types of Down syndrome Trisomy 21, Translocation and Mosaicism.

Trisomy 21 – 95% of people with Down syndrome have Trisomy 21, three copies of chromosome 21, instead of two, which occurs during cell division

Translocation – 2-3% of people with Down syndrome have a translocation pattern, Part of chromosome 21 breams off and attaches itself to another chromosome (often the chromosome 14), Two-thirds of translocation occurrences are spontaneous while the other third is inherited from a parent

Mosaicism – 2% of people with Down syndrome have Mosaic Down syndrome, Cell division occurs in one of the early cell divisions after conception, resulting in some cells having three copies of chromosome 21 instead of two, Like their peers, a person with Down syndrome has diverse abilities. An individual’s level of functioning is not dependent upon the determination of one of these types of Down syndrome.

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