The center (nucleus) of most human cells contains chromosomes that determine many of our characteristics, including eye color and gender. Sometimes these chromosomes incur changes or damage that can cause problems with growth, development, and function. Chromosomal damage is associated with numerous diseases.

Anatomy of Chromosomes

We inherit a set of 23 chromosomes from each parent, for a total of 46. Twenty-two chromosomes are the same in males and females; the twenty-third chromosome determines sex. Chromosomes are numbered (1 to 23) by size.

Chromosomes are comprised of DNA molecules, which are packaged into thread-like structures in a specific sequence. These sequences are important. The chromosomes are broken down into smaller pieces called genes. Genes determine how, why, and where the body makes thousands of different proteins we need to survive.

Chromosomal Abnormalities

Some diseases are caused by a change in a single gene. Others, such as cancer, are complex and may be due to environmental factors and a combination of sequence variations in several genes. Chromosomal abnormalities can occur at any point in an individual's lifetime, including during the formation of eggs and sperm.

There are several types of chromosomal abnormalities. Numerical abnormalities describe gains or losses to the number of chromosomes. Trisomy, an extra single chromosome, is the most common numerical abnormality. Individuals with Down syndrome have an abnormality on chromosome 21 (Down syndrome is also called Trisomy 21). Individuals can also lose a chromosome (monosomy) or gain an extra set of chromosomes (triploid). Turner syndrome and Patau syndrome are both due to numerical chromosomal abnormalities.

Chromosomes can also incur structural changes. They may break, be duplicated or deleted, or a section of the chromosome may be transferred to another section. Klinefelter Syndrome, for example, which affects the male sex organs, is an abnormality of chromosome 23. Structural changes can cause some severe types of mental illness and a form of leukemia.

Other chromosomal disorders include Parkinson's disease, asthma, spinal muscular atrophy, epilepsy, cystic fibrosis, polycystic kidney disease, and Crohn's disease.

Most chromosomal abnormalities are accidents that occur in sperm or egg cells, which means they are present in every cell in an individual's body. Those that occur after birth may be in some cells and not others. Abnormalities usually occur from an error during cell division.

Understanding chromosomal disorders could allow doctors to detect diseases earlier and more accurately. It also offers opportunities to develop customized disease treatments that have fewer side effects.

Liesa Harte, MD, reviewed this article.




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