Down Syndrome or known as Trisomy 21 (47,XX,+21) is caused by a meiotic nondisjunction event.
With nondisjunction, a gamete,a sperm or egg cell, is produced with an extra copy of chromosome
21.The gamete has 24 chromosomes. When combined with a normal gamete from the other parent, the embryo now
has 47 chromosomes, with three copies of chromosome 21. Trisomy 21 is the cause of approximately 95% of observed Down syndromes,
with 88% coming from nondisjunction in the maternal gamete and 8% coming from nondisjunction in the paternal gamete.
It is named after John Langdon Down. He was a British doctor who first described it in 1866. It is characterized by a combination
of major/minor differences in body structure. Down Syndrome is associated with impairment of cognitive ability and physical
growth,
also a facial appearance. Individuals with trisomy 21, have a lower than average cognitive ability, ranging
from mild to moderate retardation. A small number have mental retardation.
Trisomy 21 is estimated to be at 1 per 800 to 1 per 1,000 births. Some common features
of Down syndrome appear to be in people with a regular set of chromosomes. That includes single transverse palmer crease,
a single instead of a double crease across one or both palms, an almond shape to the eyes caused by an epicanthic fold of
the eyelid, shorter limbs, and protruding tongue.People with down syndrome have mental retardation in the mild (IQ 50-70)
to moderate (IQ 35-50).Also they can have serious abnormalities affecting any part of the body.
