As technology has advanced, the answer to this question has changed. 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 18q-. 18q- is the deletion of part the long arm of chromosome 18. This deletion can involve as many as 350 genes.
Although many chromosome abnormalities involve large regions, other abnormalities involve smaller regions. These are often called microdeletion syndromes. Sometimes these are big enough to be seen with a microscope. Usually, more specialized testing is required to identify these chromosome abnormalities. Typically, these abnormalities involve between 2 and 50 genes. Examples are Williams syndrome (a microdeletion of chromosome 7) and 22q11 deletion syndrome (a microdeletion of chromosome 22).
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.