Down syndrome (DS or DNS), also known as trisomy 21, is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. It is typically associated with physical growth delays, mild to moderate intellectual disability, and characteristic facial features.The average IQ of a young adult with Down syndrome is 50, equivalent to the mental ability of an 8- or 9-year-old child, but this can vary widely.
The parents of the affected individual are typically genetically normal. The probability increases from less than 0.1% in 20-year-old mothers to 3% in those age 45. The extra chromosome is believed to occur by chance, with no known behavioural activity or environmental factor that changes the probability.Down syndrome can be identified during pregnancy by prenatal screening followed by diagnostic testing or after birth by direct observation and genetic testing.Since the introduction of screening, pregnancies with the diagnosis are often terminated. Regular screening for health problems common in Down syndrome is recommended throughout the person's life.
There is no cure for Down syndrome.Education and proper care have been shown to improve quality of life. Some children with Down syndrome are educated in typical school classes, while others require more specialised education. Some individuals with Down syndrome graduate from high school, and a few attend post-secondary education.[16] In adulthood, about 20% in the United States do paid work in some capacity,with many requiring a sheltered work environment. Support in financial and legal matters is often needed. Life expectancy is around 50 to 60 years in the developed world with proper health care.
Down syndrome is caused by having three copies of the genes on chromosome 21, rather than the usual two.[2][62] The parents of the affected individual are typically genetically normal.[11] Those who have one child with Down syndrome have about a 1% risk of having a second child with the syndrome, if both parents are found to have normal karyotypes.[60]
The extra chromosome content can arise through several different ways. The most common cause (about 92-95% of cases) is a complete extra copy of chromosome 21, resulting in trisomy 21.
In 1.0 to 2.5% of cases, some of the cells in the body are normal and others have trisomy 21, known as mosaic Down syndrome.The other common mechanisms that can give rise to Down syndrome include: a Robertsonian translocation, isochromosome, or ring chromosome. These contain additional material from chromosome 21 and occur in about 2.5% of cases. An isochromosome results when the two long arms of a chromosome separate together rather than the long and short arm separating together during egg or sperm development.
Trisomy 21 (also known by the karyotype 47,XX,+21 for females and 47,XY,+21 for males)[65] is caused by a failure of the 21st chromosome to separate during egg or sperm development (nondisjunction).[61] As a result, a sperm or egg cell is produced with an extra copy of chromosome 21; this cell thus has 24 chromosomes. When combined with a normal cell from the other parent, the baby has 47 chromosomes, with three copies of chromosome 21.[2][61] About 88% of cases of trisomy 21 result from nonseparation of the chromosomes in the mother, 8% from non separation in the father, and 3% after the egg and sperm have merged.
Source : Wikipedia
19 сен 2024
