health

Down syndrome results from genetic errors during the formation of
germ cells (eggs and sperm) or during cell division shortly after the egg
is fertilized by the sperm. The most common form of Down syndrome,
responsible for about 95 percent of cases, occurs when an egg or sperm
carrying two copies of chromosome 21 is involved in conception. The
reason for the extra copy in the abnormal germ cell is a genetic error
called nondisjunction. During the normal process of germ cell formation,
the paired chromosomes in the cell divide so that each daughter cell
has only one member of the pair. In nondisjunction, one daughter cell
gets both members of the chromosome pair and the other cell has none.
If a germ cell carrying two copies of chromosome 21 is fertilized by a
normal germ cell from the other parent, the child will have three copies
of chromosome 21. This genetic error is called a full trisomy 21.
Some children with Down syndrome have some body cells with the
extra copy of chromosome 21 and some body cells without the extra
copy. This condition is called mosaic trisomy 21. It is thought to result
from random errors in cell division during the early stages of fetal development.
Mosaic trisomy 21 accounts for about 2 percent of children
with Down syndrome.
About 3 percent of cases of Down syndrome occur in families. A part
of chromosome 21 may become attached to chromosome 14 either
before or at the moment of conception. This type of genetic error is
called a balanced translocation because there is no extra material from
chromosome 21. A person with this type of translocation looks normal
and develops normally; however, he or she has an increased risk of having
a child with full trisomy 21.
The mother’s age is a risk factor for Down syndrome, rising from
one chance in 1,562 in mothers age twenty-four or younger to one in
nineteen in mothers over age forty-five. Recent studies indicate that the
father’s age is also a factor; men forty-two years and older are at increased
risk of having a child with trisomy 21.

In addition to the physical features mentioned earlier, other indications
of Down syndrome in a newborn include:

• An additional skin fold at the inner corner of the eye
  
• A short neck
  
• White spots on the iris of the eye known as Brushfield spots
  
• A round face
  
• Ears that are smaller than normal
  
• A flattened area at the back of the head
  
• Missing teeth or delayed development of teeth
  
• Protruding tongue and a tendency to breathe through the mouth
  
• An extra-wide space between the big toe and second toe