Down syndrome is primarily caused by a specific error in cell division called nondisjunction, which leads to an extra copy of chromosome 21.
Understanding Nondisjunction
- What it is: Nondisjunction occurs when chromosomes fail to separate properly during the formation of either sperm or egg cells.
- Result: This error results in a reproductive cell (sperm or egg) that carries an extra copy of chromosome 21.
- Consequence: When this cell is involved in fertilization, the resulting embryo has three copies of chromosome 21 instead of the usual two, a condition called Trisomy 21.
Trisomy 21 Explained
| Aspect | Description |
|---|---|
| Normal Chromosomes | Humans typically have two copies of each chromosome, totaling 46. |
| Trisomy 21 | Individuals with Down syndrome have three copies of chromosome 21, totaling 47. |
| Cause | Usually caused by nondisjunction during the formation of a sperm or egg cell prior to, or at the time of conception |
Practical Insights
- Not Inherited: While there are rare cases of inherited Down syndrome, most cases are not inherited. Nondisjunction is a random event.
- Timing: This error usually occurs in the sperm or egg cells before or during conception.
In short, the primary cause of Down Syndrome is Trisomy 21, which is a result of nondisjunction leading to an extra copy of chromosome 21.
