Chromosomal abnormalities are changes in the number or structure of chromosomes—the DNA-containing structures that carry most of our genetic information. Some chromosome changes have little or no noticeable effect, while others can affect development, growth, fertility, or health.
These abnormalities can be present from conception, arise during a person’s lifetime, or occur only in some cells. Understanding the type of chromosome change helps explain what it may mean and why its effects can vary so widely.
What are chromosomes?
Chromosomes are long, organized packages of DNA found in the nucleus of most human cells. DNA contains genes, and genes provide instructions that help cells make proteins and carry out biological functions.
Most human cells normally contain 46 chromosomes arranged in 23 pairs. One chromosome in each pair comes from the egg and the other from the sperm. The first 22 pairs are called autosomes. The 23rd pair consists of the sex chromosomes, typically XX or XY, although natural variations in sex-chromosome number and structure also occur.
Chromosomes must be copied and distributed accurately when cells divide. Errors in these processes can produce chromosomal abnormalities.
The two main types of chromosomal abnormalities
Chromosomal abnormalities are broadly divided into numerical abnormalities and structural abnormalities.
Numerical abnormalities
A numerical abnormality means that a cell has an abnormal number of chromosomes.
The most common mechanism is nondisjunction, in which chromosomes fail to separate properly during cell division. If this happens while eggs or sperm are being formed, a resulting reproductive cell may contain an extra chromosome or lack one. After fertilization, this can produce an embryo with an abnormal chromosome number.
Having an extra chromosome is called trisomy. Having only one chromosome from a pair is called monosomy.
Examples include:
- Trisomy 21, commonly called Down syndrome, in which there is an extra copy of chromosome 21.
- Trisomy 18, associated with Edwards syndrome.
- Trisomy 13, associated with Patau syndrome.
- Monosomy X, associated with Turner syndrome, in which some or all cells have only one X chromosome.
Changes involving the sex chromosomes can produce several other chromosome patterns. For example, a person may have an extra X chromosome, as in 47,XXY, a pattern associated with Klinefelter syndrome.
Not every abnormal chromosome number affects every cell in the body. When a person has two or more genetically distinct cell populations arising from the same fertilized egg, the condition is called mosaicism. The proportion and distribution of affected cells can influence how a chromosomal abnormality presents.
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Structural abnormalities
A structural abnormality occurs when part of a chromosome is altered. The chromosome count may remain normal, but the arrangement or amount of genetic material changes.
Important types include:
- Deletion: a segment of a chromosome is missing.
- Duplication: a chromosome segment is present in extra copies.
- Inversion: a chromosome segment breaks off, reverses direction, and is reinserted.
- Translocation: a segment moves to another chromosome. In a reciprocal translocation, segments are exchanged between chromosomes.
- Insertion: a chromosome segment is removed from one location and inserted into another.
The consequences depend largely on which genes are affected and whether genetic material has been lost, gained, or disrupted.
Some structural changes are balanced, meaning that the overall amount of genetic material is essentially unchanged. A person with a balanced rearrangement may have no obvious health problems but can have reproductive difficulties or an increased chance of producing eggs or sperm with an unbalanced chromosome arrangement.
How do chromosomal abnormalities happen?
Chromosomal abnormalities usually arise from errors during cell division or from chromosome breakage and rearrangement.
During the formation of eggs and sperm, chromosomes undergo meiosis, a specialized type of cell division that reduces the chromosome number by half. Errors in chromosome separation can create reproductive cells with too many or too few chromosomes.
Chromosomal changes can also occur after fertilization during ordinary cell divisions. When that happens, only a subset of cells may carry the abnormality, resulting in mosaicism.
Structural changes can arise when chromosomes break and are repaired incorrectly. Some rearrangements are inherited from a parent, while others occur for the first time in an egg, sperm, embryo, or developing individual.
In many cases, there is no identifiable behavior, exposure, or event that caused a particular chromosomal abnormality. A chromosome change should not automatically be interpreted as something a parent did or failed to do.
What effects can chromosomal abnormalities have?
The effects range from essentially none to substantial medical or developmental consequences.
A chromosome change can affect the body when it alters the dosage or function of important genes. Losing a chromosome segment can remove genes the body needs. Gaining extra genetic material can cause certain genes to be present in more copies than usual. A rearrangement can also interrupt a gene or change how a gene is regulated.
Possible effects include differences in physical development, intellectual or developmental disabilities, congenital abnormalities, growth differences, problems involving particular organs, and difficulties with fertility or pregnancy.
Some chromosomal abnormalities are associated with an increased risk of miscarriage because the developing embryo has an imbalance of genetic material that is not compatible with normal development. Others are compatible with life but produce a recognizable pattern of health or developmental features.
The same chromosome abnormality can also affect different people differently. The exact genetic change, the amount of material involved, whether the change is present in all cells or only some, and other genetic and environmental factors can all influence the outcome.
Are chromosomal abnormalities inherited?
Some are inherited, but many are not.
A numerical abnormality such as an extra chromosome often results from a chromosome-separation error in the formation of an egg or sperm rather than from a chromosome abnormality inherited from a parent.
Structural abnormalities are more likely than many numerical abnormalities to have a familial component. A parent may carry a balanced translocation or another rearrangement without having obvious health effects. During reproduction, however, that rearrangement can sometimes produce an embryo with missing or extra chromosome material.
Chromosome testing of parents may therefore be recommended in some situations, particularly when a child has a structural rearrangement or when there has been a pattern of certain pregnancy losses.
How are chromosomal abnormalities diagnosed?
Testing depends on the situation and the question being asked.
Karyotyping examines chromosomes under a microscope and can identify many large changes in chromosome number and structure. It is useful when clinicians need to see the overall chromosome arrangement.
Chromosomal microarray can detect many gains and losses of DNA that may be too small to see on a standard karyotype. It does not, however, detect every type of chromosome rearrangement.
More targeted molecular or genetic tests can investigate a specific suspected abnormality. Some tests examine particular chromosome regions, while others can analyze DNA at much greater resolution.
Chromosomal abnormalities can also be investigated during pregnancy. Prenatal screening estimates the chance that a fetus has certain chromosome conditions; it does not by itself establish a diagnosis. Diagnostic prenatal tests, such as chorionic villus sampling or amniocentesis, can obtain fetal or placental cells for chromosome or genetic analysis.
The distinction between screening and diagnosis is important: an abnormal screening result generally calls for appropriate follow-up rather than being treated as proof that a chromosome abnormality is present.
What does a chromosome test result mean?
A chromosome report can contain terminology that is difficult to interpret without genetic context. A result may identify a numerical change, a structural rearrangement, mosaicism, or a finding whose clinical significance is uncertain.
The significance depends on the exact chromosome involved and the specific region or genetic material affected. For structural abnormalities, the difference between a balanced and unbalanced rearrangement can be particularly important.
Some genetic tests also identify a variant of uncertain significance, meaning that available evidence is insufficient to determine whether the finding affects health. Such a result is not the same as a confirmed disease-causing abnormality.
Genetic counseling can help interpret a result in the context of a person’s medical history, family history, reproductive plans, and—when relevant—the results of other family members.
Can chromosomal abnormalities be treated?
Chromosomal abnormalities themselves generally cannot be corrected throughout the body with current medical treatment. Care instead focuses on the effects of the chromosome change.
Depending on the condition, management may involve monitoring for specific medical complications, developmental support, educational services, therapies, treatment of congenital abnormalities, or reproductive counseling.
The appropriate care varies considerably. A chromosome abnormality is a genetic finding, not a single disease with one standard treatment.
Why chromosome abnormalities matter in pregnancy and reproductive health
Chromosome changes are important in reproductive medicine because they can influence fertility, miscarriage risk, fetal development, and the chance of having a child with a chromosome condition.
A person’s age can influence the likelihood of certain chromosome-number abnormalities in eggs, although age is only one factor and does not determine the outcome of an individual pregnancy.
A known parental chromosome rearrangement can also change reproductive risk. In such circumstances, genetic counseling can explain the specific chromosome findings, possible reproductive outcomes, and available testing options.
Chromosome abnormalities are therefore best understood as changes in the organization or amount of genetic material. Their consequences depend not simply on whether a chromosome is “abnormal,” but on which genetic material has changed, how it has changed, and which cells carry the change.
