Last Reviewed June 2, 2009
What is a Chromosome Rearrangement?
A chromosome rearrangement is the rearrangement of genetic material within the same chromosome (called a chromosome inversion) or between chromosomes (called a chromosome translocation). Chromosome rearrangements can be detected by analyzing the structure and banding patterns of the chromosomes, which is called a chromosome analysis (or karyotype).
What is a Chromosome Translocation?
A chromosome translocation is a chromosome abnormality caused by the rearrangement of genetic material between different numbered (also called non-homologous) chromosomes.
Chromosome translocations can be characterized as balanced or unbalanced.
- A balanced translocation involves an apparently even exchange of genetic material resulting in no loss or gain of total genetic information. A balanced translocation does not typically result in physical and/or mental abnormalities.
- An unbalanced translocation involves an uneven exchange of genetic material resulting in a gain or loss of total genetic information. An unbalanced translocation is more likely to result in physical and/or mental abnormalities.
There are two main types of translocations: reciprocal and Robertsonian.
- Reciprocal translocations typically involve an exchange of genetic material between non-homologous chromosomes and are found in about 1 in 600 newborns.
- Robertsonian translocations involve the fusion of acrocentric chromosomes near the centromere with the loss of the p (short) arm. Acrocentric autosomal chromosomes include chromosomes 13, 14, 15, 21, and 22. The chromosome analysis of a person with a balanced Robertsonian translocation would reveal 45 chromosomes since two of the chromosomes have fused together. The most common Robertsonian translocation is found is about 1 in 1300 people and involves chromosomes 13 and 14.
Although people with balanced chromosome translocations do not usually have mental and/or physical abnormalities, they may be at increased risk to have pregnancy losses, stillbirths, and children with chromosome abnormalities.
For example, people who carry a balanced Robertsonian translocation involving chromosome 21 have a higher chance to have a child with Down syndrome.
What is a Chromosome Inversion?
A chromosome inversion is a chromosome abnormality caused when a segment of genetic material is broken away from the chromosome, inverted from end to end, and re-inserted into the chromosome at the same breakage sites.
Chromosome inversions can be characterized as balanced or unbalanced.
- A balanced inversion involves no net gain or loss of total genetic information. A balanced inversion does not typically result in physical and/or mental abnormalities. However, in some cases, gene disruptions at the breakage site can cause genetic conditions.
- An unbalanced inversion involves a gain or loss of total genetic information. A unbalanced inversion almost always results in physical and/or mental abnormalities.
There are two type of inversions: paracentric and pericentric.
- Paracentric inversions do not include the centromere and both breaks occur in one arm of the chromosome.
- Pericentric inversions include the centromere and there is a break point in each arm.
Although people with balanced inversions do not usually have mental and/or physical abnormalities, they may be at increased risk to have pregnancy losses, stillbirths, and children with chromosome abnormalities.
For example, people with balanced paracentric inversions have a higher risk to have non-viable pregnancies (pregnancies that miscarry).