Alternative Names
chromosome anomalies; chromosomal disorders; chromosomal conditions
Symptoms & Characteristics
Chromosomes are the structures that hold genes. 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).
Chromosome abnormalities are genetic conditions that are caused by abnormalities in chromosome number or structure. Examples of common chromosome abnormalities include:
- Down syndrome - occurs when there is an extra copy of chromosome 21
- Triploidy - occurs when there is an entire extra set of chromosomes for a total of 69 chromosomes
- Markers - occurs when there is an extra piece of chromosome material which can be derived from any chromosome number
- Ring chromosomes - occurs when the arms of a chromosome fuse together to form a ring
- Chromosome Translocation - occurs when an entire chromosome or a piece of a chromosome becomes attached to or interchanged with another whole chromosome or piece of a chromosome
- Chromosome Inversion - occurs when a chromosome breaks in two places and the resulting piece of genetic material is reversed and re-inserted into the chromosome
- Chromosome Deletion - occurs when a chromosome breaks and some genetic material is lost
- Chromosome Duplication - occurs when part of a chromosome is duplicated resulting in extra genetic material from the duplicated piece
Treatment
The overall treatment, prognosis and management of a chromosome abnormality is specific to the condition and it's symptoms.
How Common Is It?
Some chromosome abnormalities are common, while others are considered rare. Down syndrome is the most common chromosome abnormality and affects about 1 in 800 people worldwide.
Genetics & Inheritance
Most chromosome abnormalities occur sporadically as a result of an error in cell division during the formation of the egg or sperm.
- A particular error called a "non-disjunction event" occurs when chromosomes in the developing egg or sperm fail to separate during cell division. The result is a sperm or egg cell with either an additional chromosome or one that lacks a chromosome. When this sperm or egg cell combines with one from the opposite sex, the resulting embryo will have cells with an extra chromosome (47) or cells that have a missing chromosome (45).
- The chance for a "non-disjunction event" to occur in an egg increases with age of the biological mother (maternal age). As a consequence, so does the risk for Down syndrome and other chromosome trisomies.
In some cases, chromosome abnormalities are inherited (especially when a chromosome translocation is present) or occur post-conception. Chromosome abnormalities that occur post-conception result in chromosomal mosaicism, where some cells in the body have the abnormality and some do not.
The recurrence risk for a chromosome abnormality within a family depends on a number of factors. Some of these factors include maternal age, the type of chromosome abnormality, and whether a parent also has a chromosome abnormality. It is always recommended to see genetic counseling with a trained genetic professional for a complete evaluation, accurate diagnosis, and discussion of the benefits and limitations of testing and recurrence risk.
Genetic Testing
Chromosome abnormalities are typically detected by analyzing the structure and banding patterns of the chromosomes, which is called a chromosome analysis (or karyotype). Other testing techniques, such as FISH analysis may be used when there is a specific chromosome area in question. The availability of testing can be discussed with a genetic counselor and/or a medical geneticist.
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