Chromosomal abnormalities are any aberration, mutation, or anomaly, such as a missing, extra, or irregular portion of chromosomal DNA. Chromosomal abnormalities tend to occur when there is an error in cell division after mitosis or meiosis. While there are many different types of chromosomal abnormalities, they can be classified into two main groups, numerical or structural anomalies.
Numerical chromosomal abnormalities are called aneuplodidy, meaning an abnormal number of chromosomes. These occur when a person is either missing a chromosome, or has more than two chromosomes in a pair. The most common and well-known example of a numerical chromosomal abnormality is Down Syndrome, medically known as Trisomy 21. In this case, the individual has three copies of chromosome 21.
Structural chromosomal abnormalities are more numerous, and can take several forms. Deletions are cases where a portion of the chromosome is missing, such as Jacobsen syndrome. Duplications result in extra genetic material, such as Charot-Marie-Tooth disease type 1A, which results in progressive loss of muscle tissue. Inversions occur when a portion of the chromosome has broken off, and reattached upside down. Most chromosomal abnormalities are inherited conditions, occurring in either the sperm of egg cell, duplicating the abnormality in every cell in the person's body.