How Is Down Syndrome Inherited?
Not all types of Down syndrome are inherited. Parents can transfer Translocation Down syndrome to their children – the only form of Down syndrome that can be inherited and about one-half of all translocation Down syndrome cases are inherited.
- Trisomy 21 causes this condition where there is a chromosomal abnormality that occurs randomly when reproductive cells form in a parent. The abnormality usually takes place in egg cells but it can also be in sperm cells. In case of nondisjunction, there will be an egg or sperm cell with an additional copy of chromosome 21, and the baby will develop this condition if the same cell contributes to the genetic makeup of the baby.
- It is possible to inherit translocation Down syndrome from an unaffected parent. This happens when the parent has a rearrangement of genetic material in any other chromosome and chromosome 21. This is referred to as a balanced translocation in which there is no loss or gain of genetic material and thus no health problems become evident. This translocation can pass on to the baby and cause imbalance. People with an unbalanced translocation associated with chromosome 21 may have Down syndrome.
- Mosaic Down syndrome is rare and cannot be inherited. It is quite like trisomy 21 and occurs randomly when cells divide early in fetal development. It is for this reason that some cells in the body may have two copies of chromosome 21 and others may have three copies, which will cause problems.