Translocation Down syndrome is a specific type of Down syndrome caused by a rearrangement of chromosomal material.
Understanding Translocation Down Syndrome
Unlike the more common Trisomy 21 Down syndrome, where there's an extra copy of chromosome 21 floating freely, translocation Down syndrome involves a different mechanism.
Key Features
- Rearranged Chromosomes: In translocation Down syndrome, a third copy of chromosome 21 material is present, but it's attached to another chromosome instead of being separate.
- This typically means that a piece, or the entire chromosome 21 is attached to another chromosome, often chromosome 14.
- Three Copies of Chromosome 21 Material: Although the extra chromosome 21 material is attached to another chromosome, individuals with translocation Down syndrome still have three copies of the critical region of chromosome 21.
- Different Mechanism, Same Outcome: Despite the different mechanisms at the chromosomal level, the developmental effects of translocation Down syndrome are similar to those seen in trisomy 21.
Comparison with Trisomy 21
| Feature | Translocation Down Syndrome | Trisomy 21 Down Syndrome |
|---|---|---|
| Extra Chromosome 21 | Part or all of it attached to another chromosome | A separate extra chromosome 21 |
| Number of # 21 Chromosomes | Two normal # 21 chromosomes plus the translocation | Three separate #21 chromosomes |
| Total # 21 Material | Three copies of key regions on chromosome #21 | Three separate #21 chromosomes |
| Overall Outcome | Similar physical and developmental effects | Similar physical and developmental effects |
Example
Imagine that the genetic material of a chromosome 21 has been completely attached to chromosome 14. This means that, even though there are only 46 chromosomes present, there are actually three copies of the genetic material from chromosome 21. This is due to the two normal #21 chromosomes plus the extra #21 material attached to chromosome 14.
Practical Insights
- Inheritance: Translocation Down syndrome can be inherited, or can be a new (de novo) event. If a parent carries a balanced translocation involving chromosome 21, they can pass on an unbalanced form to their child resulting in translocation Down syndrome.
- Genetic Testing: Genetic testing, such as karyotyping, can identify a translocation and aid in diagnosis.
- Genetic Counseling: Genetic counseling can help families understand the recurrence risks associated with translocation Down syndrome and help with family planning.
Conclusion
Translocation Down syndrome is a variation of Down syndrome characterized by the attachment of chromosome 21 material to another chromosome. Although the mechanism is distinct from trisomy 21, the underlying genetic cause, which is having three copies of the chromosome 21 material, results in similar phenotypic effects.
