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Frequently Asked Questions > FAQ's about Genetics > What is and what isn't a chromosome abnormality?

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A chromosome abnormality is a change in the number or the arrangement of the chromosomes.  There are many different types of possible changes.  In some individuals, a piece of a chromosome is missing.  This is called a deletion.  In other individuals, a piece of a chromosome is duplicated.  This is called a duplication.  Sometimes, a piece of one chromosome is broken off and attached to another chromosome.  This is called a translocation.  A   person may have an entire extra copy of a chromosome or a missing chromosome.  These are called trisomy and monosomy, respectively.  There are other types of chromosome abnormalities as well, such as ring chromosomes, insertions, inversions, and isochromosomes (which occur when one half of the chromosome is missing and the other half is duplicated).

In the past, chromosome abnormalities were defined by any chromosome change that was detectable on a routine karyotype.  If you could see a variation from the normal pattern while looking at them under the microscope, this was defined a chromosome abnormality.

Over the past several years, technology has provided us with more tools that can be used to identify chromosome abnormalities.  Now, more sophisticated tests can be used that will detect smaller deletions or duplications that are not identifiable through a routine karyotype. For now, a good definition of a chromosome abnormality is a deletion, duplication or rearrangement of a chromosome region big enough to include two or more genes.

There are quite a variety of chromosome abnormalities. Some chromosome abnormalities, like Down syndrome, involve very large chromosome regions. Down syndrome is the duplication of an entire chromosome 21.  Therefore, all 287 genes located on chromosome 21 are duplicated in these individuals.  Another example of a chromosome abnormality is 1p36 Deletion Syndrome.  1p36 Deletion Syndrome is a deletion of part the short arm of chromosome 1.


What Isn't A Chromosome Abnormality?

There are many different kinds of genetic conditions.  Only a portion of them are chromosome abnormalities. 

Single gene disorders are conditions that are caused by a change in a single gene.  Because there are thousands of genes, there are thousands of single gene disorders. This group of disorders cannot be diagnosed by a karyotype.  In fact, if you were to perform karyotype on someone with a single gene disorder, no abnormalities would be detected. Other types of specialized testing would be required to make a diagnosis.  Some examples of single gene disorders include cystic fibrosis, sickle cell anemia, and Huntingtondisease.

Another group of genetic conditions is caused by a combination of genetic and environmental factors.  These conditions are called multifactorial disorders.  Essentially, they are caused by a combination of inherited and lifestyle factors.  Many common conditions are multifactorial conditions.  Some examples include heart disease and type 2 diabetes.

“The above information is from the Chromosome 18 Registry & Research Society and was reproduced with permission.” Some minor changes may have been made to make the information as applicable as possible to 1p36 Deletion Syndrome.