Chromosome 18 Myth Busters

MYTH: Chromosome abnormalities are a small group of distinct conditions

This month marks the first installment of our new series, “Chromosome 18 Myth-Busters”.

Many families, when discussing their child’s chromosome 18 condition with others, have heard the question, “Oh, so is it like Down syndrome?” For our first article, we’re going to bust the myth that there are only a few chromosome conditions and that all chromosome conditions are the same.

Indeed, most people have heard of Down syndrome, also known as trisomy 21. Some have even heard of trisomy 13 and trisomy 18, particularly if they were screened for these conditions during pregnancy. Fewer people are aware that there are more than three chromosome conditions. In reality, the phrase “chromosome abnormalities” is a very nonspecific term that includes hundred, if not thousands, of conditions.

Below is a picture of human chromosomes. It was taken though a microscope after the chromosome have been stained with a special stain and then a lined up by size by a computer for analysis.

As the director of the Research Center, Dr Cody, likes to say, anything you can do to this picture with scissors, glue, and a copy machine can happen to actual chromosomes. This can lead to an almost infinite variety of chromosome abnormalities. In some cases, these extra and missing pieces of chromosomes can lead to miscarriage or stillbirth. However, many of the possible different chromosome changes are viable, meaning that people that have those changes can survive.

To fully understand the vast number of potential chromosome abnormalities, it might help to look them in another way. The image below shows the results from a large database that records reported chromosome abnormalities. Each box represents a different chromosome. Each column within the box represents a person with either a deletion (shown in red) or a duplication (shown in blue) of a part of the chromosome. It may be hard to make out the details in the image here, but hopefully you can see that there are seemingly limitless sizes and combinations of these chromosome abnormalities.

Many of these chromosome changes are unique, meaning that they have only been reported in one individual. Of course, chromosome 18 is no exception. Below is a similar picture of the data for chromosome 18. Again, there are many different duplications and deletions . While some chromosome deleted or duplicated regions overlap with each other, most are unique in terms of where the breakpoint is actually located.

In summary, there are not just four or five or even 23 different chromosome abnormalities. The potential variety of chromosome abnormalities is nearly limitless.

FACT: Chromosome abnormalities are a large and complex group of conditions.

Myth BUSTED!

Do you have questions about anything in this email? Contact Courtney Sebold for more information.

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Courtney Sebold

Genetic Counselor

[email protected]

Chromosome 18 Clinical Research Center

UT Health at San Antonio

Chromosome 18 Registry & Research Society | 210.657.4968