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.
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FAQs on Chromosomal Abnormalities
1. What are chromosomal aberrations?
Chromosomal aberrations are the genetic disorder which occurred as a response to change in the number of the structural arrangement of chromosomes. Generally, in a lot of cases, chromosomal aberrations are also known as aneuploidies. It includes any kind of change in structure in a different part of the chromosome such as inversion, translocation and insertion and the most common type is trisomy 21. It can be reduced by visiting a doctor and avoiding smoking and alcohol consumption.
2. What are the seven types of abnormalities of chromosomes?
The seven types of abnormalities of chromosomes include – translocations, insertions, deletions, rings, mosaicism, mixoploidy and Aneuploidy.
Chromosomal abnormalities are classified into two categories that are numerical and structural abnormalities. The best example of numerical abnormalities is Down syndrome. Although there are trisomy, monosomy and triploidy as well. Structural abnormalities in the chromosome lead to clinical differences in the individuals. Chromosomal deletion can cause deletion syndrome while chromosomal duplication can cause duplication syndrome. Isochromosomes cause Pallister Killian Syndrome. The condition with ring chromosomes causes Ring chromosome 14 syndrome.
3. What is Down syndrome?
It is a chromosomal disorder which occurs due to an error in cell division, leading to an additional copy of chromosome 21. It tends to hamper the physical and cognitive abilities of persons and also makes them susceptible to various health disorders.
It affects the intellect of an individual. There are characteristic features of an individual with Down syndromes such as a small nose or upward slant eyes. The intellectual disability may range from mild to moderate. They often have higher risks of being affected with leukaemia, Alzheimers and celiac disease. There are also auditory and visual problems in individuals with Down syndrome.
4. State the ways in which the chromosome disorders are diagnosed.
The chromosome disorders can be easily noticed in children with physical or mental abnormalities or delay in growth and development of a child and they are diagnosed in three ways namely:
Microarray which is also known as CGH
Karyotyping
Fluorescence in situ hybridization
Chromosomal disorders can also be diagnosed before birth by the process of CVS which stands for Chorionic Villus Sampling or by amniocentesis in which the cells are grown in the laboratory and then observed under a microscope to check the order of the chromosomes and the picture is known as a karyotype. It helps in identifying the genetic abnormalities in an individual.
5. Does an individual inherit Chromosomal disorder?
There are chances that an individual may inherit chromosomal disorder but it is not often the case. Some chromosome abnormalities are de novo, meaning new, which are not inherited genetically but affect the individual. Since the chromosomal disorder often happens in the fetus, there are very less chances that an individual has inherited the disorder since the abnormalities explained above have various reasons for occurrences. One has to consult a medical expert in order to know whether a chromosomal disorder is inherited or not.
