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Frequently Asked Questions > FAQ's about Genetics > How are chromosome abnormalities diagnosed?

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In the grand majority of cases, chromosome abnormalities are detected by a karyotype.  We call this the cytogenetic approach to diagnosing chromosome abnormalities.  This approach involves examining a person’s chromosomes under a microscope.  A cytogeneticist looks at the chromosomes to see whether there are any missing or extra pieces.  They also look to see whether the chromosomes are rearranged.

How does a cytogeneticist know whether a piece of chromosome is missing, duplicated, or rearranged?  In a way, a chromosome is like a large phone book.  It contains a great deal of information that is organized in a very specific way.  If we were to remove a hundred pages of the phone book, it would be obvious that a significant section of the phone book was missing.  We would know this by looking at the phone book because some of the tabs are not there. 

Similarly, if we look at a person’s karyotype, we might notice that a band is missing from the short arm of chromosome 1.  This tells us that there is a deletion on chromosome 1.

The Molecular Approach. 

Let’s return to the phone book example.  If we just look at the closed phone book, we do not know which section is missing.  It could be the letter R, S, T, or maybe even all three of these letters.  If we want to know which pages are missing, we have to open up the phone book.


In this phone book, we see that the entry for “Real Estate” is present.  So, we know that this part was not removed from the book.  However, “Record Companies” is missing.  We therefore know that the missing section starts somewhere between “Real Estate” and “Record Companies”.

In the same way, a karyotype only gives us a general idea of which part of the chromosome is missing.  If we want to know the exact part of the chromosome that is missing, we have to look at the chromosome in a different way. We have to use a different technique.

We call this approach the “molecular approach”.  Microarray analysis uses a molecular approach.  Unlike the cytogenetic approach, the molecular approach does not use microscopes or pictures of the chromosomes.  Instead, we look for genetic markers.  Genetic markers are just small stretches of DNA.  They are like the “Real Estate” or “Record Companies” in the phone book example.  We can use specialized techniques to see if a person is missing certain genetic markers.  This helps us to know exactly what part of the chromosome is involved.

It should be noted that, at this point in time, molecular characterization of 1p36 Deletion Syndrome does not allow us to predict what features an individual may or may not have.  It is our hope that, as research advances, we will better understand how the specific genes involved in the abnormality affect the health and development of an individual.

“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.