Structural chromosome abnormalities occur when there is a change in the structure or parts of a chromosome. The total number of chromosomes is typically 46 total per cell. Structural chromosome abnormalities occur when part of a chromosome is missing, a part of a chromosome is extra, or a part has switched places with another part. Ultimately, this leads to having too much or too little genetic material. This is 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.
The term "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," where part of the #5 chromosome is missing or deleted.
Cri-du-chat or "cat's cry syndrome" is found in approximately 1 in 20,000 to 50,000 live births in the U.S. Cri-du- chat is caused by a deletion of chromosome 5p, which is written "5p-." Babies with Cri-du-chat have a high-pitched cry, poor muscle tone, 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 severe 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, in addition to physical and language therapy, is important for children with Cri-du-chat to reach their full potential.
The term "duplication" simply 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. Since a very small piece of a chromosome can contain many different genes, the extra genes present in a duplication may cause those genes to not function properly. These "extra instructions" 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. One example of a rare genetic disorder of duplication is called Pallister-Killian syndrome, where part of the #12 chromosome is duplicated.
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 severe intellectual disability, poor muscle tone, "coarse" facial features, and a prominent forehead. They tend to have a very thin upper lip, with a thicker lower lip and a short nose. Other health problems include 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.