Day 1 - What is Down syndrome?
Normally, a person has 46 chromosomes. In a person with Down syndrome, they have 47 chromosomes, the extra chromosome comes from a third copy of the 21st chromosome, thus the name trisomy 21 (aka Down syndrome).
There are three types of Down syndrome: trisomy 21 (nondisjunction), mosaicism, and translocation.
Trisomy 21 is the most common type of DS. Prior to or at conception, a pair of 21st chromosomes in either the sperm of the egg fails to separate. As the embryo develops, the extra chromosome is replicated in every cell of the body.
Translocation is the second most common type of DS, occurring in about 4% of people with DS. With translocation, the number of chromosomes in the cells remains 46, but an additional full or partial copy of chromosome 21 attaches to another chromosome, usually chromosome 14. The presence of the extra full or partial chromosome 21 causes the characteristics of Down syndrome.
Mosaic Down syndrome is the most rare, occurring in only about 1% of people with DS. Mosaicism is diagnosed when there is a mixture of two types of cells, some containing the usual 46 chromosomes and some containing 47. Those cells with 47 chromosomes contain an extra chromosome 21.
|Here's my little cutie with t21 chilling on a firetruck!|