Chromosomal disorders are primarily caused by changes in the number or structure of chromosomes, often arising as random events during cell division.
Here's a breakdown of the causes:
Numerical Abnormalities
These abnormalities involve having an incorrect number of chromosomes. Normally, humans have 46 chromosomes arranged in 23 pairs.
- Nondisjunction: This is the most common cause of numerical chromosomal disorders. It occurs when chromosomes fail to separate properly during meiosis (the cell division process that creates sperm and egg cells). This results in reproductive cells with either an extra chromosome or a missing chromosome.
- Example: Down syndrome (Trisomy 21) is caused by an extra copy of chromosome 21. Turner syndrome is caused by having only one X chromosome in females.
- Mosaicism: This occurs when nondisjunction happens after fertilization during early cell division. The individual will have some cells with the normal number of chromosomes and some cells with an abnormal number. The severity of the disorder can vary depending on the proportion of cells with the chromosomal abnormality.
Structural Abnormalities
These abnormalities involve alterations in the structure of one or more chromosomes.
- Deletions: A portion of a chromosome is missing.
- Example: Cri-du-chat syndrome, caused by a deletion on chromosome 5.
- Duplications: A portion of a chromosome is duplicated, resulting in extra copies of genes.
- Inversions: A portion of a chromosome breaks off, flips, and reattaches to the same chromosome. If the inversion includes the centromere, it is called a pericentric inversion; if it does not include the centromere, it is called a paracentric inversion.
- Translocations: A portion of one chromosome breaks off and attaches to another chromosome.
- Reciprocal translocation: Segments from two different chromosomes exchange places.
- Robertsonian translocation: An entire chromosome attaches to another at the centromere. This type of translocation only occurs with chromosomes 13, 14, 15, 21, and 22.
- Rings: A chromosome breaks in two places, and the broken ends join to form a circular structure. The broken fragments are usually lost, which can cause specific symptoms.
Inheritance vs. Random Events
While most chromosomal disorders are not inherited and arise as random events during the formation of reproductive cells (sperm and egg), structural abnormalities like translocations can sometimes be inherited from a parent who carries a balanced translocation (a translocation where no genetic material is gained or lost).
In summary, chromosomal disorders stem primarily from errors during cell division (especially nondisjunction) or structural changes to chromosomes, leading to an abnormal number or arrangement of genes. These changes often occur randomly, though some can be inherited.
