GENETICS
Down syndrome disorders are based on having too many copies of the genes located on chromosome 21. In general, this
leads to an overexpression of the genes.
Understanding the genes involved may help to target medical treatment to
individuals with Down syndrome. It is estimated that chromosome 21 contains 200
to 250 genes.
Recent research has identified a region of the chromosome that contains the main
genes responsible for the pathogenesis of Down syndrome.
The extra chromosomal material can come about in several distinct ways. A
typical human karyotype is designated as 46,XX or 46,XY, indicating 46
chromosomes with an XX arrangement typical of females and 46 chromosomes with an XY arrangement typical of males. In 1–2% of the observed Down syndromes. some of the cells in the body are normal and other cells have trisomy 21, this is called mosaic Down syndrome (46,XX/47,XX,+21).
Trisomy 21
Trisomy 21 (47,XX,+21) is caused by a meiotic nondisjunction event. With nondisjunction, a gamete (i.e., a sperm or egg cell) is produced with an extra copy of chromosome 21; the gamete thus 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. The actual Down syndrome "critical region" encompasses chromosome bands 21q22.1-q22.3. [1]
WHAT CAUSES IT?
Although no one knows for sure why DS occurs and there's no way to prevent the chromosomal error that causes it, scientists do know that women age 35 and older have a significantly higher risk of having a child with the condition. At age 30, for example, a woman has about a 1 in 1,000 chance of conceiving a child with DS. Those odds increase to about 1 in 400 by age 35. By 40 the risk rises to about 1 in 100. [3]
leads to an overexpression of the genes.
Understanding the genes involved may help to target medical treatment to
individuals with Down syndrome. It is estimated that chromosome 21 contains 200
to 250 genes.
Recent research has identified a region of the chromosome that contains the main
genes responsible for the pathogenesis of Down syndrome.
The extra chromosomal material can come about in several distinct ways. A
typical human karyotype is designated as 46,XX or 46,XY, indicating 46
chromosomes with an XX arrangement typical of females and 46 chromosomes with an XY arrangement typical of males. In 1–2% of the observed Down syndromes. some of the cells in the body are normal and other cells have trisomy 21, this is called mosaic Down syndrome (46,XX/47,XX,+21).
Trisomy 21
Trisomy 21 (47,XX,+21) is caused by a meiotic nondisjunction event. With nondisjunction, a gamete (i.e., a sperm or egg cell) is produced with an extra copy of chromosome 21; the gamete thus 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. The actual Down syndrome "critical region" encompasses chromosome bands 21q22.1-q22.3. [1]
WHAT CAUSES IT?
Although no one knows for sure why DS occurs and there's no way to prevent the chromosomal error that causes it, scientists do know that women age 35 and older have a significantly higher risk of having a child with the condition. At age 30, for example, a woman has about a 1 in 1,000 chance of conceiving a child with DS. Those odds increase to about 1 in 400 by age 35. By 40 the risk rises to about 1 in 100. [3]