Approximately 95% of affected individuals have trisomy 21, with a chromosome count of 47. In Down's syndrome due to trisomy 21 there is about a 10 times increased risk of having a Down's child in a subsequent pregnancy, ie increased from 1 in 600 to about 1 in 60.
In approximately 3-5% percent of babies with Down's syndrome a translocation or mosaicism is the cause of the condition. Examination of the parental chromosomes is only necessary if a translocation is found in the baby. The risk of a subsequent child having Down's syndrome is substantially increased if either parent has a translocation.
Chromosomal Basis of Down Syndrome
|Chromosomal Feature||Description||Approximate Percent of Cases|
Occurs in the egg in 95% of cases
Usually occurs with one chromosome 21 attached to chromosome 14,
21, or 22
In 21/21 translocation
The number of affected cells varies among persons
|Partial trisomy||Duplication of a delimited segment of chromosome 21 is present||<1|
- Bull MJ. Down Syndrome.N Engl J Med 2020;382:2344-52.
Last edited 06/2020