Chromosome Abnormalities (Numerical and Structural)
Definition
Chromosome abnormalities are studied in two main groups:
1. Numerical Abnormalities (Aneuploidy)
The number of chromosomes differs from the normal 46:
- Trisomy, one extra chromosome (47). Examples: Down syndrome (trisomy 21), Edwards (18), Patau (13)
- Monosomy, one missing chromosome (45). Example: Turner syndrome (missing X)
These abnormalities can be detected by karyotype and NIPT.
2. Structural Abnormalities
The chromosome number is correct, but the structure is disrupted:
| Type | What happens |
|---|---|
| Deletion | The chromosome breaks and a piece of genetic material is lost |
| Duplication | A section of the chromosome is copied abnormally, resulting in extra genetic material |
| Translocation | A piece that breaks off one chromosome attaches to another chromosome |
| Inversion | The chromosome breaks in two places; the resulting DNA segment is flipped and reinserted |
| Ring chromosome | The chromosome usually breaks in two places at the ends of the p and q arms, then the arms join to form a circular structure, and the genetic material at the ends is often lost |
| Isochromosome | Instead of normal p and q arms, there are two identical arms, with extra genes on one side and missing genes on the other |
Inversion subtypes: Pericentric (includes the centromere) and paracentric (does not include the centromere, within a single arm).
Balanced or Unbalanced?, The most critical distinction
| Balanced | Unbalanced | |
|---|---|---|
| Definition | No genetic material is gained or lost in the cell | Genetic material is gained or lost |
| Effect on the carrier | Usually healthy, shows no symptoms | Usually causes symptoms |
| Reproductive implications | Carries a risk of recurrent pregnancy loss and of a child with an unbalanced chromosome set | , |
This is the main reason a karyotype is requested for couples experiencing recurrent pregnancy loss: One of the parents may be a balanced translocation carrier, completely healthy themselves, yet unbalanced combinations can form in their reproductive cells.