Chromosomal Abnormalities: What Is It?, Types, & Causes

The genetic material of eukaryotic cells is coiled tightly in linear bundles called chromosomes. Genetic alterations are either inherited from a parent, or they occur de novo, meaning a new variant appears during reproductive cell formation or in embryonic development.

Chromosomal abnormalities can also occur in non-reproductive cells at any stage of life.

Chromosomes are bundles of genetic information comprised of molecular DNA wrapped around proteins. Genes and gene variants (alleles) on chromosomes control protein synthesis and cellular activity.

Chromosomes segregate during asexual mitosis and in sexual reproductive processes like meiosis. Chromosomes condense when the cell divides to keep the strands of DNA from tangling, breaking or partially separating.

Each organism has a specific amount of chromosomes, often coming in homologous pairs. Including sex chromosomes (X and Y chromosomes), humans have a total of 46 chromosomes: one pair of 23 chromosomes inherited from the mother and the other pair of 23 from the father. Dogs have 39 pairs of chromosomes, a rice plant has 12 pairs and fruit flies have four pairs.

Chromosomal abnormalities are changes to the number or structure of chromosomes that can lead to birth defects or other health disorders. Slight alterations to genes on the chromosomes may produce new traits such as bigger claws that may be beneficial to survival. However, they can also have detrimental effects.

Chromosomal abnormalities in the fertilized egg can halt cell growth and trigger spontaneous abortion.

The cause of chromosomal abnormalities is usually attributable to accidents during DNA replication or cell division. Normally, any problems are corrected by enzymes at checkpoints, or the dividing cell is not allowed to proceed to the next phase of the cell cycle.

If mistakes are not noticed or fixed, chromosomal abnormalities can cause cell death, or the abnormalities can be passed along to offspring with potentially dire consequences.

When chromosomes do not segregate properly, cells can end up with missing or extra chromosomes. Chromosomal abnormalities characterized by an atypical number of chromosomes are called aneuploidy.

For instance, trisomy 21 (Down syndrome) is caused by an extra copy of chromosome 21 in the egg or sperm that results in the fertilized egg receiving three copies of chromosome 21. Mosaic trisomy 21 is a rare form of Down syndrome that happens after fertilization.

Trisomy 13 (Patau syndrome) causes severe intellectual and physical disabilities. Trisomy 18 (Edwards syndrome) is even more severe and can threaten children's survival. Trisomy X is an extra copy of the X chromosome in female sex cells. Klinefelter syndrome happens when a male inherits an extra X chromosome from his mother; the XXY condition is sometimes associated with advanced maternal age.

Monosomy occurs when one chromosome is partially or entirely missing. For example, females with Turner syndrome only have one X chromosome instead of two X chromosomes. Cri du chat syndrome results from a deletion of the short arm of chromosome 5.

Damage or changes to the structure of the chromosomes can also lead to health problems and birth defects. Cell functions may cease when large segments of DNA are missing or added to chromosomes.

Chromosomal abnormalities tests are options offered with some home DNA testing kits. Identifying carrier status for mutated genes can aid in early detection and treatment of chromosomal abnormalities and their syndromes.

Types of structural abnormalities include:

• Deletion: A portion
of a chromosome is deleted.
• Duplication: A portion
of a chromosome is doubled or duplicated.
• Inversion: Parts
of the chromosome are mirrored and swapped.
• Translocation: One part of a chromosome is transported to
another chromosome, or an entire chromosome attaches to another chromosome (Robertsonian translocation).
• Ring chromosome: The ends of chromosomes with broken "arms" attach, forming a ring.