What is a chromosome?
Genes are packaged together on bigger structures called chromosomes. There are 46 chromosomes in total, which are grouped into 23 pairs. The first 22 pairs are numbered largest to smallest (1 - 22) and are called autosomes. The last pair determines gender, and is either XX (female) or XY (male).
What is a chromosome abnormality?
A chromosome abnormality is a change in chromosome number or structure. Chromosome abnormalities can be detected by analyzing the structure and banding patterns of the chromosomes, which is called a chromosome analysis (or karyotype).
What is a ring chromosome?
A ring chromosome typically occurs when a chromosome breaks in two places and the ends of the chromosome arms fuse together to form a circular structure. In rare cases, ring formation occurs when there is only one break or no breaks. A ring chromosome can be derived from any chromosome number.
Ring chromosomes typically result in mental and/or physical abnormalities. This is most likely due to the loss of genetic material in the ends of the chromosome when a ring forms.
Some examples of disorders caused by ring chromosomes include:
- ring chromosome 20 syndrome - associated with epilepsy
- ring chromosome 14 syndrome - associated with mental retardation, skin pigment abnormalities, seizures and abnormal facial features
- ring chromosome 13 syndrome - associated with mental retardation and abnormal facial features
What is a marker chromosome?
A marker chromosome (also called a supernumerary marker chromosome or marker) is an extra piece of chromosomal material that can be derived from any chromosome number. About 70% of the cases with marker chromosomes occur sporadically, while about 30% are inherited.
The significance of a chromosome marker is very variable and depends on what genetic material is contained in the marker.
- Sometimes, a chromosome marker consists of inactive genetic material and therefore, has little or no effect.
- Sometimes, a chromosome marker is inherited from a parent with the same chromosome marker with little or no effect.
- About 30% of people with a marker chromosome have physical and/or mental abnormalities.
It can be difficult to assess the significance of a chromosome marker when diagnosed during pregnancy. Special studies are typically performed to more accurately identify the genetic material contained in the marker. Some markers occur more frequently than others and therefore, are better characterized.
- One common chromosome marker is called inverted duplication of chromosome 15. This is a specific syndrome that is characterized by developmental delay, seizures, and physical abnormalities.
Parental chromosome analyses are also offered to determine if the marker is inherited. Typically, if a parent also has the same chromosome marker and does not have any apparent mental and/or physical abnormalities, the effect is much less.
If the marker is not one of the more common markers and neither parent also has the same chromosome marker, it can be very difficult to predict the significance of a chromosome marker.