Genetics of Down syndrome
Genetics of Down Syndrome
Down syndrome (pronounced /daʊn/ /ˈsɪndroʊm/), 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, characteristic facial features, and mild to moderate intellectual disability.
Etymology
The condition is named after John Langdon Down, the British doctor who first fully described it in 1866. The term "syndrome" comes from the Greek "σύνδρομον" (pronounced /ˈsɪndrəmən/), meaning "concurrence of symptoms," while "trisomy" refers to the presence of an extra chromosome.
Genetics
Down syndrome is caused by having three copies of the genes on chromosome 21, rather than the usual two. The parent who provides the extra chromosome can be either the mother or the father. There are three types of Down syndrome: trisomy 21 (nondisjunction), translocation, and mosaicism.
Trisomy 21 (Nondisjunction)
Trisomy 21 is the most common type of Down syndrome, accounting for 95% of cases. It occurs when there's an error in cell division called "nondisjunction," which leaves a sperm or egg cell with an extra copy of chromosome 21 before or at conception.
Translocation
Translocation accounts for 4% of cases. In this type, part of chromosome 21 breaks off during cell division and attaches to another chromosome, typically chromosome 14. While the total number of chromosomes in the cells remains 46, the presence of an extra part of chromosome 21 causes the characteristics of Down syndrome.
Mosaicism
Mosaicism is the least common form, accounting for about 1% of cases. In this type, some cells have three copies of chromosome 21, but other cells have the typical two copies.
Related Terms
External links
- Medical encyclopedia article on Genetics of Down syndrome
- Wikipedia's article - Genetics of Down syndrome
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