What are Chromosomes?
Chromosomes are a thread-like formation of protein, and nucleic acid found extensively in most living cells. These chromosomes are responsible for carrying genetic information and tend to play a vital role in formulating the genetic makeup of new generations. Needless to say, any kind of abnormality or disruption in their functioning tends to hamper the healthy genetic formation or transfer of vital genetic data. Keeping this in mind, let’s proceed to find out more about chromosome abnormalities, their types and causes. Doing so, you will further understand the importance of chromosomes in general
What are Chromosomal Abnormalities?
Chromosomal abnormalities are genetic disorders which occur due to a change in chromosomes or owing to the abnormal arrangement of chromosomes. The abnormalities in chromosomes can occur during the early stages of development in the fetus. There are ways to detect such abnormalities but all cannot be detected in prenatal screening or testing. Typically, there are several types of chromosomal abnormalities which can be categorised as either structural or numerical abnormality.
Numerical Abnormalities
In this condition, one of the chromosomes is absent from the pair and such a condition is known as monosomy. Similarly, when an individual has more than two chromosomes in a pair then it is known as trisomy.
The most common types of numerical abnormalities are as follow –
Polyploidy
This particular condition is widely observed in plants. Typically, it occurs when the chromosome count of a set increases owing to the malfunctioning of cytokinesis.
Aneuploidy
It typically arises due to increase or decrease of chromosomes. Notably, it is a result of abnormal segregation of genes. On the other hand, structural abnormalities arise due to breakage or incorrect arrangement of chromosomal segments. For example, ring chromosome abnormality can occur when a particular chromosome breaks twice, and then its broken ends join into a circular chromosome. Likewise, an isochromosome is formed when a chromosome’s arm goes missing, and the remaining one starts to duplicate. This image below shows the common numerical and structural chromosomal aberrations.
Mixoploidy
In this condition, one cell is diploid and the other is polyploid coexisting within the same organism. It is also found in living adults and children. It is considered an essential topic of cytology.
Structural Abnormalities
When the chromosomal segments break and rejoin in the fetus, it results in structural abnormalities. It may result in diseases.
Structural abnormalities can be defined in two ways –
Balanced – A structural chromosome is said to be balanced when the total chromosomal set is present and rearranged.
Unbalanced – A structural chromosome is said to be unbalanced when the material or information is additional or absent.
The common kinds of Structural abnormalities are as follows –
Deletions
Most chromosomal abnormalities occur due to accidents in the egg or the sperm. In this kind of abnormality, a portion of the chromosome is absent or deleted.
Duplications
In this kind of abnormality, a portion of the chromosome is duplicated thereby, resulting in additional genetic material.
Translocations
When a portion of chromosome transfers to another chromosome then it results in translocation. There are two kinds of translocations and they are as follows:
Reciprocal translocation
In this type of translocation, the segments of two different chromosomes are exchanged in the fetus.
Robertsonian translocation
In this type of translocation, a chromosome is entirely attached to another chromosome in the centromere.
Rings
This may happen with or without the loss of genetic material but in this kind of abnormality, the chromosome breaks off in the fetus to form rings.
Inversion
When a chromosome is broken off in the fetus, turns upside down and reattaches to another chromosome, it results in inverted genetic materials.
Dicentric Chromosomes
A chromosome that contains two centromeres instead of one like a normal chromosome is known as a dicentric chromosome. It may be a result of the abnormal fusion of chromosomes with one centromere each.
Chimerism
A chimera is an animal or a human that contains the cells of two more different individuals. It is a rare condition that can also affect animals causing two different colours of eyes. Such an instance is microchimerism, a fetus absorbs the cells of the womb or vice versa.
Isochromosomes
These are the chromosomes with two identical arms. So instead of a normal chromosome with one p arm and one q arm, there is an extra set of p arm and q arm resulting in an additional copy of information or absence of information of genes on missing arms.
It is also to be taken into account that several cancerous cells also develop changes in chromosomal structure. They occur in the somatic cells during the progression of cancerous tumours.
Treatment of Structural Abnormalities in Chromosomes
There is no condensed treatment for structural chromosomal abnormalities, in fact, the chromosomal abnormalities carried by men requires no therapy. However, if an individual is impaired fertility then the said individual can seek medical attention with artificial reproductive techniques along with genetic counselling is advised strongly to the male individual.
Women with balanced structural chromosomal abnormalities are generally fertile.
It is to be noted that with structural abnormalities, there are higher risks of birthing disabled children.
DIY: What is chromosome disorder? Provide suitable examples.
Types of Chromosomal Disorders
Some of the most common examples of chromosomal abnormalities include Down syndrome,
Turner syndrome and Klinefelter syndrome. Details of such disorders are mentioned below.
Down Syndrome
This particular chromosomal abnormality occurs due to an additional chromosome 21, which in turn occurred due to abnormal cell division. In a broader sense, it can be divided into three types – translocation, trisomy 21 and mosaicism.It tends to hamper the physical characteristics and cognitive development in individuals. Also, the most noticeable symptoms of Down syndrome include small height, short neck, low muscle tone, eyes which slant upwards, etc.
Turner Syndrome
In this case, there is one X chromosome absent in females which decreases the chromosome count to 45 instead of 46. The prevalent symptoms of turner syndrome include sterility and absence of secondary sexual traits.Other than that, heart disorders, high blood pressure, skeletal abnormalities and kidney ailments are other prominent indicators of turner syndrome.
Klinefelter Syndrome
An extra X chromosome in males is the reason behind the occurrence of Klinefelter. Owing to this, the chromosome count stands at 47 instead of the standard 46. This chromosomal aberration tends to manifest in males in the form of absent or delayed puberty, sterility, development of breast, etc. It is also said to affect intellectual development in males.
Test Your Knowledge: Look at the chromosomal abnormalities list below and distinguish them as structural and numerical aberrations.
Translocation.
Rings.
Aneuploidy.
Insertion.
Deletion.
Chimerism.
Mixoploidy.
Polyploidy.
Learn about the chromosomal abnormalities caused in detail straight from our subject experts today. Join our live online classes and improve your understanding of this phenomenon and other vital topics with ease.Also, don’t forget to check out our chromosomal aberration notes online to get a fair idea about the crucial topics on this concept. Get the Vedantu App now to learn more!
FAQs on Chromosomal Abnormalities
1. What are chromosomal abnormalities and how do they impact genetic information in humans?
Chromosomal abnormalities are changes in the number or structure of chromosomes that disrupt the normal transmission of genetic information. These changes can lead to a variety of physical and cognitive conditions by altering or misarranging genes, affecting how traits are inherited and expressed within a person.
2. How are chromosomal abnormalities classified, and what examples fall under structural and numerical types?
Chromosomal abnormalities are mainly classified into numerical and structural types.
- Numerical abnormalities include conditions caused by missing or extra chromosomes, such as trisomy (Down syndrome) and monosomy (Turner syndrome).
- Structural abnormalities involve changes in the chromosome's structure, like deletions, duplications, translocations, and inversions. For example, a deletion may lead to Cri-du-chat syndrome and a translocation can result in certain types of leukemia.
3. What role does non-disjunction play in causing chromosomal abnormalities such as Down syndrome?
Non-disjunction is the failure of chromosomes to separate properly during cell division. This mistake can result in cells with an extra or missing chromosome, causing Down syndrome (trisomy 21), where there's an additional chromosome 21 that leads to developmental changes and health issues.
4. What are the most common syndromes caused by chromosomal abnormalities, and what are their main features?
Some well-known syndromes caused by chromosomal abnormalities include:
- Down syndrome (Trisomy 21): Intellectual disability, distinct facial features, and increased risk of health problems.
- Turner syndrome: Females with only one X chromosome; characterized by short stature, infertility, and heart defects.
- Klinefelter syndrome: Males with an extra X chromosome (XXY); often tall, may have learning difficulties and reduced fertility.
5. Why do structural chromosomal abnormalities often result in health problems, even when no genes are missing?
Even if no genes are lost or gained, structural chromosomal abnormalities change the arrangement of genetic material. This disrupts gene function or regulation, possibly turning genes on/off inappropriately or breaking them, which can lead to health issues such as developmental delays, infertility, or increased cancer risk.
6. How are chromosomal abnormalities diagnosed in a medical setting?
Diagnosis uses specialized tests such as
- Karyotyping – analyzes entire chromosome sets for large changes.
- Fluorescence in situ hybridization (FISH) – detects specific gene or chromosome abnormalities.
- Microarray analysis (CGH) – finds small deletions or duplications. Prenatal diagnoses may use amniocentesis or chorionic villus sampling (CVS) as per CBSE 2025–26 guidelines.
7. What misconceptions do students often have about chromosomal abnormalities and their inheritance?
A common misconception is that all chromosomal abnormalities are inherited. While some can be passed from parents to offspring, many are de novo, meaning they arise spontaneously in the embryo and are not present in the parents.
8. Can environmental factors cause chromosomal abnormalities during fetal development?
Yes, certain environmental factors like radiation, exposure to harmful chemicals, or infections during pregnancy can increase the risk of chromosomal abnormalities by damaging cells during early fetal development, though most cases are due to errors during cell division.
9. What is the significance of balanced versus unbalanced structural chromosomal abnormalities?
A balanced structural abnormality means genetic material is rearranged but none is lost or gained, usually causing no physical symptoms. An unbalanced abnormality involves missing or extra genetic material, often resulting in developmental or health issues.
10. How important is genetic counselling for families affected by chromosomal abnormalities?
Genetic counselling is crucial for families as it helps them understand the causes, risks, and possible outcomes of chromosomal abnormalities. It also provides support for informed decision-making regarding health, pregnancy, and management options as recommended in CBSE 2025–26 curriculum.
