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Structural Abnormalities: Deletions (Cri-du-Chat Syndrome) and Duplications (Pallister-Killian Syndrome)

What are structural chromosome abnormalities?

Structural chromosome abnormalities happen when there are changes to the structure or parts of a chromosome. The typical number of chromosomes is 46 total per cell. Structural chromosome abnormalities occur when part of a chromosome is missing, a part of one is extra, or a part has switched places with another part. This leads to having too much or too little genetic material. It's a cause of some birth defects.

Each chromosome has many segments. These are usually divided into a short arm and a long arm of the chromosome. The short arm, which is the upper half of the chromosome, is known as the "p arm." The long arm, which is the lower half of the chromosome, is the "q arm." The centromere is the center part of a chromosome that appears pinched between the p and q arms.

Normal chromosome including p arm, q arm, and centromere.

What are deletions?

A deletion simply means that a part of a chromosome is missing or deleted. A very small piece of a chromosome can contain many different genes. When genes are missing, there may be errors in the development of a baby, since some of the instructions are missing. One example of a genetic syndrome caused by a deletion is called cri-du-chat or 5p minus syndrome. In this condition, part of the #5 chromosome is missing or deleted.

Normal chromosome with p arm and q arm, compared to chromosome with part of p arm deleted.

What is Cri-du-chat?

Cri-du-chat or "cat's cry syndrome" is found in about 1 in 20,000 to 50,000 live births in the U.S. Cri-du-chat is caused by a deletion of part of the end of chromosome 5p, which is written "5p-." Babies with cri-du-chat have a high-pitched cry, muscle weakness, a small head size, and low birth weight. They also have problems with language and may express themselves by using a small number of words or sign language. Other health problems can be present. These include delays in walking, problems with feeding, hyperactivity, scoliosis, and intellectual disability. Most people with cri-du-chat may have a normal life span, unless they are born with other serious organ defects. Educational intervention at an early age, with physical and language therapy, is important for these children to reach their full potential.

What are duplications?

A duplication means that a part of a chromosome is duplicated, or present in 2 copies. This results in having extra genetic material, even though the total number of chromosomes is usually normal. A very small piece of a chromosome can contain many different genes. So the extra genes in a duplication may make those genes work incorrectly. The extra genetic material can lead to errors in the development of a baby.

One way to think of a duplication is to think of all 46 chromosomes as a cookbook, and each individual chromosome as a recipe. If a deletion is a missing ingredient in the recipe, a duplication is an extra ingredient. An example of a rare genetic disorder of duplication is called Pallister-Killian syndrome, where part of the #12 chromosome is duplicated.

Normal chromosome with p arm and q arm, compared to chromosome with part of p arm duplicated.

What is Pallister-Killian syndrome?

Pallister-Killian syndrome is a result of extra #12 chromosome material. There is usually a mixture of cells (mosaicism), some with extra #12 material, and some that are normal (46 chromosomes without the extra #12 material). Babies with this syndrome have many problems. These include intellectual disability, muscle weakness, unique facial features, seizures, poor feeding, stiff joints, cataracts in adulthood, hearing loss, and heart defects. People with Pallister-Killian syndrome have a shortened life span but may live into their 40s.

Online Medical Reviewer: Chad Haldeman-Englert MD
Online Medical Reviewer: Dan Brennan MD
Online Medical Reviewer: Raymond Kent Turley BSN MSN RN
Date Last Reviewed: 8/1/2025
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