Did you know that there are three types of Down Syndrome?
TYPES OF DOWN SYNDROME
There are 3 chromosomal patterns that result
in Down syndrome.
1. Trisomy 21 (nondisjunction) is caused by a faulty
cell division that results in the baby having three
chromosomes instead of two. Prior to or at conception,
a pair of #21 chromosomes in either the egg
or the sperm fails to separate properly. The extra
chromosome is replicated in every cell of the body.
percent of all people with Down syndrome have
2. Translocation accounts for only 3% to 4% of all
cases. In translocation a part of chromosome #21
breaks off during cell division and attaches to
another chromosome. The presence of an extra
piece of the 21st chromosome causes the characteristics
of Down syndrome. Unlike Trisomy 21,
which is the result of random error in the early cell
division, translocation may indicate that one of the
parents is carrying chromosomal material that is
arranged in an unusual manner.
Genetic counseling can be sought to ascertain
more information when these circumstances occur.
3. Mosaicism occurs when nondisjunction of chromosome
#21 takes place in one of the initial cell divisions
after fertilization. When this happens, there
is a mixture of two types of cells, some containing
46 chromosomes and some with 47. The cells with
47 chromosomes contain an extra 21st chromosome.
Because of the “mosaic” pattern of the cells,
the term mosaicism is used. This type of Down syndrome
occurs in only one to two percent of all
cases of Down syndrome.
Regardless of the type of Down syndrome a person
may have, a critical portion of the 21st chromosome is
present in all or some of their cells. This additional
genetic material alters the course of development and
causes the characteristics associated with the syndrome.
information from the NADS.org web site