31 for 21: Day 18

Types of Down syndrome

Did you know there are three types of Down syndrome?

Before Royal was born, I had no idea that there was more than one type of Down syndrome.

Trisomy 21, also known as nondisjunction, which is what Royal was born with and 95% of Down syndrome cases are Trisomy 21.

There is Mosaic Down syndrome, which is rare and makes up 1% of Down syndrome cases.

And then there is Translocation Down syndrome, in one third of cases of translocation, one parent is a carrier of a translocated chromosome, meaning it is hereditary.

The National Down Syndrome Society does an excellent job of explaining the difference between the three types of Down syndrome.

The following information is directly from www.NDSS.org

TRISOMY 21 (NONDISJUNCTION)

Down syndrome is usually caused by an error in cell division called “nondisjunction.”  Nondisjunction results in an embryo with three copies of chromosome 21 instead of the usual two.  Prior to or at conception, a pair of 21st chromosomes in either the sperm or the egg fails to separate.  As the embryo develops, the extra chromosome is replicated in every cell of the body.  This type of Down syndrome, which accounts for 95% of cases, is called trisomy 21.

MOSAICISM 

Mosaicism occurs when nondisjunction of chromosome 21 takes place in one – but not all – of the initial cell divisions after fertilization.  When this occurs, there is a mixture of two types of cells, some containing the usual 46 chromosomes and others containing 47.  Those cells with 47 chromosomes contain an extra chromosome 21.

Mosaicism accounts for about 1% of all cases of Down syndrome.  Research has indicated that individuals with mosaic Down syndrome may have fewer characteristics of Down syndrome than those with other types of Down syndrome.  However, broad generalizations are not possible due to the wide range of abilities people with Down syndrome possess.

TRANSLOCATION 

Translocation accounts for about 4% of all cases of Down syndrome.  In translocation, part of chromosome 21 breaks off during cell division and attaches to another chromosome, typically chromosome 14.  While the total number of chromosomes in the cells remain 46, the presence of an extra part of chromosome 21 causes the characteristics of Down syndrome.

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