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Chromosome 18 abnormalities are caused by a change in a person's genetic makeup.  In this section, you will learn more about the genetic basis of chromosome 18 abnormalities.  This information may be helpful in understanding the underlying cause of the condition as well as the implications for other family members.


Chromosome 18 abnormalities are caused by a change in a person's genetic makeup.  In this section, you will learn more about the genetic basis of chromosome 18 abnormalities.  This information may be helpful in understanding the underlying cause of the condition as well as the implications for other family members.


Photograph courtesy of Rick Guidotti of Positive Exposure.
Basic Genetics  Print  

What is a chromosome?

Chromosomes are the structures that hold all of our genes.  Genes are simply the instructions that tell the body how to grow and develop.  For example, genes contain the instructions for visible things, like hair and eye color.  They also contain the instructions for things that we can’t see, such as proteins for digesting food.  Chromosomes are located in the cells of the body.  Chromosomes, and the genes that are located on them, are passed from parent to child through the egg and the sperm. 

Additional details about chromosomes

How many chromosomes do we have?

Below is a picture of a person’s chromosomes.  This picture is called a karyotype.  This picture is taken through a microscope after the white blood cells obtained from a blood sample have been carefully prepared in a lab. 



As you can see, there are 46 chromosomes.  These 46 chromosomes are arranged in pairs.  Therefore, we have 23 pairs of chromosomes.  We inherit one copy of each pair from our mother and the other copy from our father.  The first 22 pairs of chromosomes are arranged according to size.  These are called the autosomes.  The 23rd pair of chromosomes is known as the sex chromosomes.  These chromosomes determine a person’s gender.  Women have two X chromosomes and men have an X and a Y chromosome.  Because the karyotype above has an X and a Y chromosome, we know that it belongs to a male. 

Inheriting Chromosomes

What Is a Karyotype?

How are the chromosomes different?

Each chromosome is different from other chromosomes.  Importantly, each chromosome carries different genes.  For example, chromosome 1 carries a different set of genes than chromosome 5.

Under a microscope, chromosomes even look different from one another.  As you can see in the above karyotype, some chromosomes are larger than other chromosomes.  In addition, each of the chromosomes has a unique banding pattern.  These   bands are used to identify the different sections of each chromosome.  Near the center of each chromosome is a constricted region that divides the chromosome into two arms.  The constricted region is called the centromere.  The shorter arm of each chromosome is called the “p" arm, and the longer arm of the chromosome is called the “q" arm. 

Chromosome Markings and Structure

What is a chromosome abnormality?

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 most cases, chromosome abnormalities are diagnosed by a karyotype.  A geneticist with specialized training looks for any changes in the number or arrangement of the chromosomes.

What Is and Isn't a Chromosome Abnormality?

Diagnosis of Chromosome Abnormalities

How are chromosome abnormalities diagnosed?

In general, most chromosome abnormalities are diagnosed either by karyotype or microarray.  You can read more about karyotypes here.  You can read more about microarrays here.  For a more in-depth discussion about the differences beween karyotypes and other diagnostic techniques, please visit this page.

How common are chromosome abnormalities?

Individually, specific chromosome abnormalities are rare.  There may be only a handful of individuals in the medical literature that have the same chromosome change with the same breakpoints.  Although they are individually rare, chromosome abnormalities are collectively common.  They occur in approximately 1 in every 180 births.

How does a chromosome abnormality affect health and development?

Whenever there is a change in the amount of genetic material, it is possible that this change will impact an individual’s health and development.  This   is because there are extra or missing genes.  This can potentially lead to medical and developmental problems.  The types of problems depend on the specific chromosome change that is present. 

Medical and Developmental Issues Associated with Changes in Chromosome 18

What causes a chromosome abnormality?

In some cases, a parent may carry a chromosome abnormality that is passed to his or her child.  However, in most cases, chromosome abnormalities are de novo events.  This means that the chromosome abnormality was not inherited from a parent.    Rather, it is a change that was present at conception in either the egg or the sperm

Many families wonder why their baby was born with a chromosome abnormality.Although we know that the chromosome change was present in the egg or the sperm, we still do not know why that change happens in the first place.  Research   studies have looked at many different factors, including chemical exposures, geographic location, and ethnicity.  Up to this point in time, none of these studies have shown that these factors cause chromosome abnormalities.

We do know, however, that, as women age, they have a greater likelihood of having a baby with a full extra chromosome.  As discussed above, this is called a trisomy.  Although this risk may increase with age, it is important to understand that the majority of babies born to older mothers will not have an extra chromosome. 

What does this mean for future pregnancies?

As discussed above, there are many different types of chromosome abnormalities.  In addition, some chromosome abnormalities are inherited, and some are de novo.  In general, if the chromosome abnormality is de novo (i.e., not inherited from a parent), the risk for future pregnancies is low.  However, if the chromosome abnormality was inherited from a parent, the risk for recurrence is significantly higher. 

If you have questions regarding future pregnancies, we recommend contacting a genetics provider.  Genetic counselors are health professionals with specialized training and experience in the area of medical genetics and counseling.  Clinical geneticists are physicians who diagnose, treat, and counsel patients and families with genetic conditions.  Both geneticists and genetic counselors can talk with you about your family history and the implications for future pregnancies.  You can locate a genetic counselor through the National Society of Genetic Counselors. You can also search for a list of genetics clinics in your area through the GeneTests Clinic Directory.


What research is going on in this area?

The recent and rapid advances in human genetics and the human genome project have allowed us to ask questions that we couldn’t even contemplate ten years ago.  In the past, we could only characterize the features that individuals with chromosome abnormalities had.  However, now we are trying to understand how missing or duplicated genes contribute to those features.  This is a critical step in developing interventions and therapies for these individuals.

The Chromosome 18 Registry & Research Society and the Chromosome 18 Clinical Research Center at the University of Texas Health Science Center at San Antonio are working together to understand how changes in chromosome 18 can affect an individual’s health and development.  Doing so will allow us to develop better treatments and screening recommendations for these individuals. 

Ongoing research


How does this relate to the Human Genome Project?

As discussed here, chromosomes are composed of 4 different chemical letters: A, T, C, G.  The goal of the Human Genome Project was to determine the sequence of these letters for every single chromosome.  This   was a huge undertaking, given that there are about 3 billion chemical letters contained within our genome. 

In 2001, the first draft of this human genome sequence was completed. Based on the results of this project, we know that humans have between 20,000 and 30,000 genes.  However, even though we know the sequence of the human genome, many questions remain to be answered.  We still do not understand the function of many of these genes.  In addition, we do not completely understand how changes in these genes can affect an individual.

In summary, the Human Genome Project does not provide us with all the answers about chromosome 18.  However, it does provide us with the tools we need to start answering our questions.


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