Chromosomal abnormalities may be either numerical or structural. The most common type of chromosomal abnormality is known as aneuploidy, an abnormal chromosome number due to an extra or missing chromosome. Most aneuploid patients have trisomy (three copies of a chromosome) instead of monosomy (single copy of a chromosome). Down Syndrome is probably the most well-known example of a chromosomal aneuploidy, caused by an extra copy of chromosome 21 known as trisomy 21. While a trisomy can occur with any chromosome, the condition is rarely viable. The major chromosomal aneuploidies are trisomy 13, trisomy 18, Turner Syndrome (45, X), Klinefelter syndrome (47, XXY), 47XYY, and 47XXX.

Structural chromosomal abnormalities result from breakage and incorrect rejoining of chromosome segments. A range of structural chromosomal abnormalities that result in disease exist. Structural rearrangements are defined as balanced if the complete chromosome set is still present though rearranged, and unbalanced if there is additional or missing information. Unbalanced rearrangements include deletions, duplications, or insertions of a chromosome segment. Ring chromosomes can result when a chromosome undergoes two breaks and the broken ends fuse into a circular chromosome. An isochromosome can form when an arm of the chromosome is missing and the remaining arm duplicated.

Balanced rearrangements included inverted or translocated chromosomal regions. Since the full complement of DNA material is still present, balanced chromosomal rearrangements may go undetected since it may not result in disease. A disease can arise as a result of a balanced rearrangement if the breaks in the chromosomes occur in a gene, resulting in an absent or nonfunctional protein, or if the fusion of chromosomal segments results in a hybrid of two genes producing a new protein product whose function is damaging to the cell. For example, a chimeric gene is observed in many cases of chronic myelogenous leukemia as a result of a translocation between chromosomes 9 and 22. Part of the chimeric gene is made up of a protooncogene, a gene that normally regulates cell proliferation and differentiation. The disruption of the normal function of this gene results in uncontrolled cell growth leading to leukemia.