Did you know there are three types of Down syndrome?  Trisomy 21 (nondisjunction), translocation and mosaicism.

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.

TYPICAL CELL DIVISION

TRISOMY 21 (NONDISJUNCTION) CELL DIVISION

MOSAICISM

Mosaicism (or mosaic Down syndrome) is diagnosed when there is a mixture of two types of cells, some containing the usual 46 chromosomes and some containing 47.  Those cells with 47 chromosomes contain an extra chromosome 21.

Mosaicism is the least common form of Down syndrome and accounts for only 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.

MOSAIC CELL DIVISION

TRANSLOCATION

In translocation, which accounts for about 4% of cases of Down syndrome, the total number of chromosomes in the cells remains 46; however, an additional full or partial copy of chromosome 21 attaches to another chromosome, usually chromosome 14. The presence of the extra full or partial chromosome 21 causes the characteristics of Down syndrome.

As usual, this information is from the NDSS website